PAGES Magazine articles

Allison W. Jacobel1, R.F. Anderson2, B.A.A. Hoogakker3 and S.L. Jaccard4

Proxy-based reconstructions of oceanic dissolved oxygen and carbon concentrations have helped to refine our understanding of past ocean-atmosphere carbon partitioning, consistently indicating lower dissolved oxygen in the deep Pacific Ocean during the last ice age. Better quantitative and spatio-temporally resolved estimates of these parameters are critical for closing the carbon budget and elucidating the relative importance of the mechanisms and feedbacks driving past carbon exchange among Earth's carbon reservoirs.

Over at least the last 800 kyr BP, glacial-interglacial cycles have been characterized by variations in atmospheric CO2 concentrations of 50-100 ppm (Bereiter et al. 2015 and references therein). The glacial drawdown of atmospheric CO2 is thought to be largely a consequence of changes in southern high-latitude surface-ocean processes including increased CO2 solubility, enhanced water-column stratification, decreased air-sea gas exchange, improved efficiency of the biological pump, and heightened ocean alkalinity (e.g. Sigman et al. 2010). One way to tease apart these mechanisms is to reconstruct relative changes in the carbon storage of the ocean's various water masses, especially those which have biogeochemical signatures strongly influenced by processes occurring in the Southern Ocean that are propogated to other basins. Not only do these reconstructions help provide insight into the relative importance of different air-sea carbon exchange mechanisms, they also help us to balance the global carbon budget by quantifying how much additional carbon was stored in the glacial ocean.

Figure 1: Modern distribution of oxygen in the Pacific Ocean. South-to-north transect from 150°W and west-to-east transect along the equator. Data from the PACIFICA dataset (Suzuki et al. 2013). Figure composed using Ocean Data View with in situ data gridded using data-interpolating varational analysis (Schlitzer 2018).

Until recently, consensus on the significance of glacial respired carbon storage in the Pacific Ocean was hampered by conflicting interpretations procured in part from 14C-derived water-mass ventilation ages, taken to be indicative of carbon accumulation in the abyssal ocean. Numerous caveats to the ventilation age approach have been identified in recent years (e.g. Zhao and Keigwin 2018), suggesting that direct reconstructions of water-mass carbon and oxygen concentrations are a more straightforward way to avoid these interpretive challenges. Indeed, proxy reconstructions focused on dissolved oxygen and respired carbon consistently demonstrate the key role of respired carbon storage in the Pacific Ocean and its role in glacial atmospheric CO2 minima (e.g. Anderson et al. 2019; Hoogakker et al. 2018; Jaccard and Galbraith 2012; Jacobel et al. 2017). With this consensus, subsequent work has turned towards quantifying the total storage, parsing its distribution among the basin’s various water masses, and investigating the role of mechanisms driving the observed changes. Important questions remain about whether the relative distributions of carbon and oxygen that characterize the modern ocean (Fig. 1) were maintained during the last ice age despite overall higher carbon storage and lower oxygen availability.

Dissolved carbon and oxygen in the ocean

One of the early approaches to reconstructing respired carbon in bottom waters utilized the δ13C of epibenthic foraminifera tests, in which progressive δ13C depletion along a water-mass trajectory is interpreted as indicative of increased respiration of organic carbon (e.g. Curry and Oppo 2005). Unfortunately, air-sea disequilibrium in water-mass source regions can complicate the interpretation of this signal (Galbraith et al. 2015). Oxygen equilibrates an order of magnitude faster than carbon, and because of the tight stoichiometric relationship between oxygen and respired carbon (imparted by the consumption of oxygen and concomitant release of carbon during microbial respiration), the deviation of dissolved oxygen concentrations from saturation levels is the most direct and quantitative paleoceanographic variable for reconstructing changes in deep-sea carbon storage (Sigman et al. 2010). While the air-sea equilibration of oxygen in water-mass source areas may be incomplete (Ito et al. 2004), any undersaturation at the time of water-mass formation would lead to an underestimation of respiratory CO2 storage (for more detail see Anderson et al. 2019). This results in paleoceanographic estimates that reflect a conservative assessment of changes to respired carbon sequestration through time.

Proxy toolbox

Although reconstructing dissolved oxygen concentrations is an advantageous way to quantify respired carbon storage, finding simple proxies, especially quantitative ones, has been challenging. Early reconstructions examined sedimentary enrichments in redox-sensitive metals including Cd, Cr, Mn, Mo, Re, and most notably U. One of the reasons for the popularization of authigenic U (aU) as a bottom-water oxygen proxy is because it is a product of U/Th series measurements made to quantify mass fluxes to the seafloor using the 230Th-normalization method. Under reducing (i.e. anoxic) conditions soluble U(VI) is transformed to insoluble U(IV) and is precipitated from porewaters, likely by iron-reducing microbes (McManus et al. 2005). If the co-occurring flux of organic carbon to the sediments can be established (to control for the typically confounding effect of microbial respiration on porewater oxygen concentrations), changes in the sedimentary abundance of aU can be interpreted qualitatively as indicative of changes in bottom-water oxygen availability. Because the post-depositional re-introduction of oxygen can remove aU from sediments, careful attention must be paid to avoid interpreting diagenetic artifacts introduced by down-core diffusion of oxygen and subsequent aU loss (e.g. Jacobel et al. 2017).

Recently, two proxies for quantitatively reconstructing bottom-water oxygen have evolved in parallel. The first is grounded in observations made in the 1980s (McCorkle et al. 1985) suggesting that the carbon isotope gradient in porewaters is, at least in part, related to bottom-water oxygen concentrations. The proxy was recently calibrated by Hoogakker et al. (2015) and empirically relates the carbon isotope gradient (Δδ13C) between epifaunal benthic foraminifera and infaunal benthic species to the bottom-water oxygen concentration. The second proxy, most recently detailed by Anderson et al. (2019), makes use of the semi-quantitative relationship between oxygen availability and the remineralization of organic matter (for example the C37 alkenone biomarker) by oxic respiration. As with aU, both the Δδ13C and biomarker preservation proxies may respond to the rain of labile organic carbon to the site. Thus, reconstructions of bottom-water oxygen content should always be presented alongside, and interpreted in tandem with, a diagenesis-resistant, flux-normalized proxy for organic carbon flux such as biogenic (or excess) barium or opal flux.

Figure 2: Schematic representation of the distribution of oxygen during the Last Glacial Maximum in the Pacific Ocean based qualitatively on model results and paleo reconstructions. Additional spatial and quantitative constraints are needed to refine this conceptual picture.

Findings

Numerous records of aU, Δδ13C, and C37 biomarker preservation have been measured at Pacific Ocean sediment core sites, especially in the equatorial Pacific, where sites provide good coverage of the basin’s deep water masses. Where multiple proxies have been measured on co-located samples, general agreement on the sense and timing of bottom-water oxygen changes has been found (Anderson et al. 2019), despite some site-specific data limitations due to changes in the rate of organic carbon fluxes, or post-depositional diagenesis. Importantly, lower oxygen concentrations during the last ice age have been found for all deep equatorial Pacific sites below ~1 km (Anderson et al. 2019), suggesting that the entire deep Pacific below the depth of the modern oxygen minimum zone experienced increased respired carbon storage (see schematic in Fig. 2). Data from the high latitude North and South Pacific Oceans are consistent with findings from the equatorial Pacific (e.g. Jaccard and Galbraith 2012; Jaccard et al. 2009). The latest conservative estimates based on the biomarker preservation proxy (Anderson et al. 2019) suggest that CO2 storage during the last ice age may have been up to ~850 PgC greater than at present. This estimate is based on the assumption that the magnitude of carbon storage in the deep equatorial Pacific during the last ice age is representative of 50% of the ocean's volume – an extrapolation that, while reasonable, reflects the scarcity of quantitative estimates of ocean carbon storage. Although additional data are needed to better constrain the uncertainties associated with this estimate, deep ocean carbon storage of this magnitude is sufficient to close the glacial carbon budget by accounting for both estimates of atmospheric CO2 drawdown and estimates of carbon loss from the terrestrial biosphere.

Future work

Advances in proxy development and interpretation represent significant progress towards the goal of quantitatively reconstructing respired carbon storage in abyssal Pacific water masses and in those of the other ocean basins during the last ice age and other past climate intervals. As paleoceanographers push these reconstructions to become more robust and spatio-temporally resolved, their ability to provide insight into past changes increases, and their role in validating models of glacial-interglacial change that include biogeochemical cycles (e.g. Khatiwala et al. 2019; Yamamoto et al. 2019) is enhanced. As the global oceans experience increasing impacts from anthropogenic climate forcing, improving biogeochemical models of ocean responses is ever more crucial. Using past variability in climate states as experimental realizations for testing hypotheses about mechanisms of change is critical for improving predictions and targeting preventative and mitigating action.

affiliations

1Department of Earth, Environmental and Planetary Sciences, and Institute at Brown for Environment and Society, Brown University, Providence, RI, USA

2Lamont-Doherty Earth Observatory, and Department of Earth and Environmental Sciences, Columbia University, New York, NY, USA

3The Lyell Centre, Heroit-Watt University, Edinburgh, UK

4Insitute of Geological Sciences, and Oeschger Center for Climate Change Research, University of Bern, Switzerland

contact

Allison Jacobel: jacobelbrown.edu

references

Anderson RF et al. (2019) Global Biogeochem Cy 33: 301-317

Bereiter B et al. (2015) Geophys Res Lett 42: 542-549

Curry WB, Oppo DW (2005) Paleoceanography 20: PA1017

Galbraith ED et al. (2015) Global Biogeochem Cy 109: 38-48

Hoogakker BAA et al. (2015) Nat Geosci 8: 40-43

Hoogakker BAA et al. (2018) Nature 562: 410-413

Ito T et al. (2004) Geophys Res Lett 31: L17305

Jaccard SL et al. (2009) Earth Planet Sc Lett 277: 156-165

Jaccard SL, Galbraith ED (2012) Nat Geosci 5: 151-156

Jacobel AW et al. (2017) Nat Commun 8: 1727

Khatiwala S et al. (2019) Sci Adv 5: eaaw4981

McCorkle DC et al. (1985) Earth Planet Sc Lett 74: 13-26

McManus J et al. (2005) Geochim Cosmochim Ac 69: 95-108

Schlitzer R (2018) Ocean Data View

Sigman DM et al. (2010) Nature 466: 47-55

Suzuki T et al. (2013) PACIFICA Data Synthesis Project

Yamamoto A et al. (2019) Clim Past 15: 981-996

Zhao N, Keigwin LD (2018) Nat Commun 9: 3077

Tianyu Chen1 and Laura F. Robinson2

The history of ocean ventilation helps to resolve timing and pathways of carbon transfer between the ocean and the atmosphere. Radiocarbon records reveal climate-linked, abrupt changes in the deep-ocean ventilation during the last deglaciation.

Radiocarbon as an ocean-ventilation proxy

The transport of surface seawater to depths, known as "ventilation", represents a fundamental aspect of our climate system as it is tightly linked to the dynamics of overturning circulation and the global carbon cycle. Proxy-based reconstruction of past ventilation makes it possible to examine the interaction between the ocean and the atmosphere on long timescales when instrument records are not available. Among various proxies, 14C is one of the most sensitive for characterizing past deep-ocean ventilation. In the ocean interior, 14C is exclusively supplied through ventilation of the ocean from surface to deep, and it decays away with a half-life of 5730 years. Today, a large portion of 14C in the abyssal ocean (Fig. 1) is derived from the formation of North Atlantic Deep Water (NADW), with less than a third from overturning circulation in the Southern Ocean and biological remineralization (Broecker and Peng 1982). Since there is little deep convection in the present North Pacific, it contains the least ventilated waters at ~2 km depth (where the ventilation age is defined as the 14C age difference between the sample and the contemporaneous atmosphere) resulting from the slow transit of bottom water from the Southern Hemisphere. Along with the 14C-aging of the deep-water masses, there is a concomitant increase in the dissolved inorganic carbon inventory (~0.14 μmol/kg/year; Fig. 1) associated with the organic carbon respiration and calcite dissolution. As such, 14C is widely used as a semi-quantitative chronometer of ocean ventilation providing critical constraints on circulation dynamics and carbon storage in the past.

The reconstruction of past deep-water 14C signatures is not straightforward, as an independent age is required to correct for in-situ 14C decay in paleoarchives since their formation. Most deep-ocean radiocarbon records have been reconstructed using the fossilized remains of benthic foraminifera extracted from sediment cores (e.g. Skinner et al. 2010). Deep-sea scleractinian corals form a complementary emerging new archive that provides well-constrained, absolute-dated 14C records of the sub-surface ocean (e.g. Frank et al. 2004; Burke and Robinson 2012; Chen et al. 2015; Hines et al. 2015) since the Last Glacial Maximum (LGM).

Millennial-scale changes in ventilation

The low atmosphere CO2 concentration during the LGM is thought to be largely caused by coupled oceanic changes in ventilation and the biological pump. Indeed, compiled radiocarbon data suggest an ~700 14C-year increase in the average residence time of carbon in the deep ocean (> 1 km) during the LGM compared to the modern ocean, which allows the ocean to store more carbon (Skinner et al. 2017). So how did the deep ocean switch from an isolated, poorly ventilated LGM mode to the opposite in the Holocene? The key lies in the timing and pathways of the dissipation of 14C-depleted signatures of the deep ocean during the last deglaciation.

Figure 1: Modern distribution of Δ14C and total carbon in deep waters of the global ocean (defined as those with neutral density >27.6 kg/m3). Note that nuclear bomb testing since the 1950s has dramatically increased the level of 14C in the atmosphere and subsequently deep waters in the North Atlantic. Figure was made with Ocean Data View, and the data are from GLODAP 2019.

During the early deglaciation when the Northern Hemisphere was experiencing a cold stadial (Heinrich Stadial 1, HS1), the atmospheric CO2 concentration was increasing accompanied by a Δ14C decrease (Fig. 2; Reimer et al. 2013), implying mixing from a 14C-depleted CO2 source. Radiocarbon data from the upper and lower circumpolar deep waters (UCDW and LCDW) highlight converging trends in ventilation ages, indicative of increased Southern Ocean deep convection and air-sea exchange efficiency. This would have brought well-ventilated waters to depths and released 14C-depleted CO2 to the atmosphere (Anderson et al. 2009; Skinner et al. 2010; Burke and Robinson 2012; Chen et al. 2015).

Meanwhile, low-latitude North Atlantic deep waters at ~2 km remained poorly ventilated throughout HS1, some 700-800 years "older" than modern times (Fig. 2). This result is consistent with greatly reduced Atlantic Meridional Overturning Circulation (AMOC) during HS1 (McManus et al. 2004) that reduced the supply of 14C-enriched waters to the deep North Atlantic. In fact, the deep North Atlantic might have accumulated carbon during HS1 due to reduced ventilation (Menviel et al. 2018). Similar processes probably occurred during the Younger Dryas (YD) albeit with a smaller magnitude. Overall, larger 14C depth gradients are observed in the low-latitude North Atlantic during cold HS1 and YD compared to the Bølling-Allerød (B-A) warm event (Fig. 2), closely linked with changing production of the well-ventilated North Atlantic deep waters (McManus et al. 2004).

Centennial abrupt changes in ventilation

A particularly exciting aspect of the data being collected from deep-sea corals is the potential to reveal oceanic changes that occurred on centennial timescales. The transitions from HS1 to the B-A and from the YD to the Holocene are marked by abrupt increases in 14C of the low-latitude North Atlantic waters (Fig. 2), synchronous with warmings recorded in the Greenland ice cores and rapid increases in atmospheric CO2 concentrations (Chen et al. 2015). The coral 14C records from the Atlantic and the Southern Oceans also converge during these two transitions. This convergence likely reflects "flushing" events resulting from the abrupt resumption of the AMOC, which released a large amount of respired carbon from the deep ocean to the atmosphere (Chen et al. 2015).

Figure 2: Ventilation age evolution of deep waters compared with other climate records. (A) Δ14C of the atmosphere (IntCal13, Reimer et al. 2013). (B) Sedimentary 231Pa/230Th ratios (an AMOC-strength index) from the subtropical North Atlantic (McManus et al. 2004). (C) Ventilation age reconstructed from deep-sea corals (UCDW and low latitude Atlantic; Burke and Robinson 2012; Chen et al. 2015) and benthic foraminifera (LCDW; Skinner et al. 2010).

At the same time, boron-based pH reconstructions from Southern Ocean deep-sea corals further imply that these strong centennial ventilation events might be associated with delayed sea-ice advances at the beginning of the interstadial, allowing more efficient air-sea exchange (Rae et al. 2018). Multiple causes have to be invoked to account for the carbon release at the end of HS1 and the YD. Atmospheric δ13C records, for example, do not show the change expected from release of respired CO2 during the above two intervals, indicating increasing SST and/or increased land biosphere could also have played a role (Bauska et al. 2016). Moreover, the increased supply of nutrients to the surface is expected to stimulate primary productivity, enhancing the loss of alkalinity from surface waters and facilitating carbon release by increased overturning (Bronselaer et al. 2016). These puzzles are being addressed through increasing the resolution of data at these and other important oceanographic locations.

Concluding remarks

The upwelled 14C-depleted signatures from the abyssal ocean should eventually dissipate in the upper ocean and the atmosphere. Many uncertainties, unfortunately, still remain regarding intermediate water ventilation, with some 14C reconstructions closely tracking the evolution of atmospheric 14C while others showed extreme 14C depletions. Some studies further suggest that geological carbon addition to the water column could be an important component of the deglacial carbon cycle (e.g. Stott et al. 2019). In addition, consensus has not been reached regarding the magnitude and timing of 14C variability in the deep Pacific during the last deglaciation. Thus, the timing and pathways of deglacial oceanic carbon release have not yet been fully resolved. A growing understanding of 14C geochemistry in foraminifera and emerging deep-sea coral studies from the Pacific will provide a fresh look at these issues. Overall, 14C reconstructions of the deep ocean yield powerful constraints on ocean dynamics and carbon cycle during the last deglaciation.

affiliations

1School of Earth Sciences and Engineering, Nanjing University, China
2School of Earth Sciences, University of Bristol, UK

contact

Tianyu Chen: tianyuchennju.edu.cn

references

Anderson RF et al. (2009) Science 323: 1443-1448

Bauska TK et al. (2016) Proc Natl Acad Sci 113: 3465-3470

Broecker W, Peng T (1982) Tracers in the sea, Palisades, New York: 690 pp

Bronselaer B et al. (2016) Global Biogeochem Cy 30: 844-858

Burke A, Robinson LF (2012) Science 335: 557-561

Chen T et al. (2015) Science 349: 1537-1541

Frank N et al. (2004) Earth Planet Sc Lett 219: 297-309

Hines SK et al. (2015) Earth Planet Sc Lett 432: 46-58

McManus JF et al. (2004) Nature 428: 834-837

Menviel L et al. (2018) Nat Commun 9: 2503

Rae JWB et al. (2018) Nature 562: 569-573

Reimer PJ et al. (2013) Radiocarbon 55: 1869-1887

Skinner LC et al. (2010) Science 328: 1147-1151

Skinner LC et al. (2017) Nat Commun 8: 16010

Stott LD et al. (2019) Environ Res Lett 14: 025007

Carlye D. Peterson1, G. Gebbie2, L.E. Lisiecki3, J. Lynch-Stieglitz4, D. Oppo5, J. Muglia6, J. Repschläger7 and A. Schmittner8

How does deep-ocean circulation influence atmospheric CO2 across deglacial transitions? Although biogeochemical and physical processes complicate interpretation of foraminiferal stable carbon isotope data, these complications can be addressed with expanded data compilations, multiproxy approaches, and model-data assimilation efforts.

The transition from glacial to interglacial climate involves carbon redistribution between the atmosphere, terrestrial biosphere, and ocean reservoirs. During repeated glaciations of the past ~1 million years (Myr), about 100 ppm of CO2 from the atmosphere was temporarily sequestered in the terrestrial biosphere and ocean. Although terrestrial biosphere carbon storage may have increased or decreased (see Jeltsch-Thömmes et al. 2019 and references therein) between the Last Glacial Maximum (LGM, ~20,000 years (20 kyr) before present) and the preindustrial period, the vast, deep-ocean reservoir most likely controls glacial-interglacial carbon cycling and, hence, atmospheric CO2 variations. On these timescales, ocean circulation and biological productivity influence carbon distribution in the deep ocean and regulate glacial carbon sequestration and deglacial CO2 outgassing. However, the details of these ocean changes and their role in modulating deep-ocean carbon storage remain poorly understood. Compilations of global benthic stable carbon isotopes (δ13C), such as those synthesized by the PAGES Ocean Circulation and Carbon Cycling (OC3, pastglobalchanges.org/oc3) working group, can help decipher these processes.

Ambiguity in proxy reconstructions

Past changes in ocean circulation and carbon storage have been reconstructed using the spatial distribution of stable carbon isotopes of dissolved inorganic carbon (DIC) inferred from stable carbon isotope records of benthic foraminifera Cibicides wuellerstorfi (and related genera) (δ13Ccib). These records are influenced by numerous fractionation processes including surface-ocean thermodynamic fractionation, air-sea gas exchange, and biological productivity (see Mackensen and Schmiedl 2019). These processes set the unique δ13C source properties of modern North Atlantic Deep Water (NADW) and its glacial counterpart. Processes that drive the low δ13C signature of the deep ocean include diabatic and turbulent mixing of water masses with different carbon isotope signatures during circulation and the degradation of surface-produced organic matter (remineralization) that subsequently sinks into the deep ocean. The low end-member δ13C signature of modern southern-sourced waters, such as Antarctic Bottom Water (AABW), is achieved through a combination of cold, dense waters forming under sea ice that are isolated from the atmosphere, water-mass mixing (see Talley 2013), and organic-matter remineralization during deep-water formation and transit. Quantifying different fractionation influences is one of the challenges to inferring past changes in deep-ocean circulation and carbon storage from δ13Ccib records.

Figure 1: Observed δ13Ccib difference between the Holocene and LGM in a zonally averaged cross section through the Atlantic Ocean integrating both the eastern and western basins (circles; δ13Ccib record core sites) that constrain the model simulation of seawater δ13CDIC (contours) from Muglia et al. (2018).

One way to better constrain influences on δ13Ccib paleorecords is to increase the spatial coverage of high-resolution, well-dated δ13Ccib records. Even in relatively well-sampled regions such as the Atlantic Ocean, we must interpolate and extrapolate δ13Ccib values between core sites to capture spatial variations across ocean basins, which results in large uncertainties. To understand the temporal evolution of δ13Ccib, we rely on timeseries of δ13Ccib. High-resolution age models reduce the uncertainty in δ13Ccib timeseries, but δ13Ccib records with "high" temporal resolution (better than 3 kyr) are generally restricted to regions with high sedimentation rates and good carbonate preservation (e.g. the Atlantic Ocean). Thus, existing compilations are strongly dominated by regions with more favorable sedimentation regimes. Including low-resolution δ13Ccib records presents a trade-off between temporal and spatial resolution that is likely reasonable for characterizing the LGM and late Holocene time periods (Peterson et al. 2014).

Identifying locations of CO2 degassing can be challenging using δ13Ccib records because the strong air-sea exchange process "erases" the deep-ocean signature. However, independent nutrient proxies can help separate air-sea signals from recently upwelled deep waters (Lynch-Stieglitz et al. 2019). As such, multiproxy records allow us to estimate water-mass δ13C signatures at the time of their formation (i.e. "preformed" δ13CDIC), as well as their origin and transit (Oppo et al. 2018). Therefore, multiproxy records combined with well-dated, high-resolution δ13Ccib records from numerous locations across the seafloor allow us to reconstruct water-mass properties and explore the ocean circulation and carbon-cycling signals.

Additionally, systematic and regional deviations between late Holocene δ13Ccib and nearby seawater δ13CDIC estimates are found in more than 1700 δ13Ccib records of varying temporal resolutions (Schmittner et al. 2017). This compilation suggests that the carbonate ion content of seawater has a small (<15%) influence on δ13Ccib records. Although δ13Ccib and δ13CDIC lack a perfect one-to-one relationship, previous δ13Ccib interpretations likely hold (Schmittner et al. 2017). The effect of deep-ocean carbonate ion variations on glacial-interglacial δ13Ccib records remains to be evaluated.

Figure 2: Zonally averaged cross section of difference in Atlantic stream function (Sv) between the Holocene and the LGM from the δ13Ccib data-constrained numerical simulations of Muglia et al. (2018). The numerical simulations indicate that a shoaled and weaker AMOC at the LGM results in the closest match with existing datasets of seawater δ13CDIC, i.e. δ13Ccib (Fig. 1).

Deglacial ocean circulation changes

Classical interpretations of glacial Atlantic Ocean δ13Ccib records propose a shoaled boundary between northern-sourced NADW and southern-sourced AABW at 2000 m water depth (Curry and Oppo 2005). This interpretation has since been tested with additional δ13Ccib records, with an expanded spatial distribution, and new model-data comparisons (e.g. Hesse et al. 2011). Recently, Oppo et al. (2018) argued that western North Atlantic waters shoaled by about 500 m during the glacial onset, consistent with the prevailing hypotheses that NADW shoaled while AABW expanded (Curry and Oppo 2005). However, it remains challenging to constrain ocean circulation changes in water-mass formation regions principally because the locations of deep-water formation shifted over time. Furthermore, changes in the source properties of water masses could explain changes previously attributed to carbon and nutrient storage change (Repschläger et al. 2015).

Modeling studies constrained by glacial-age δ13Ccib records indicate a shallow and weak Atlantic Meridional Overturning Circulation (AMOC) and enhanced Southern Ocean iron fertilization (Menviel et al. 2016; Muglia et al. 2018), although it remains to be determined how well δ13Ccib spatial variability constrains changes in AMOC strength versus depth. Additionally, model results indicate enhanced glacial Antarctic Intermediate Water (AAIW) formation in the Southern Hemisphere and a more closed circulation between NADW and AAIW (e.g. Ferrari et al. 2014). In the Atlantic Ocean, zonally averaged profiles of the difference between Holocene and LGM model-simulated seawater δ13CDIC and the δ13Ccib records used to constrain the model indicate a reduced deglacial vertical δ13Ccib gradient coinciding with reduced deep-ocean carbon storage in the same model runs (Fig. 1; Muglia et al. 2018). This is in agreement with results from fewer but higher-resolution deglacial δ13Ccib timeseries (Peterson and Lisiecki 2018). Complementary to this model run that best fits the δ13Ccib records (Fig. 1), the zonally averaged Atlantic stream function (Sverdrup, Sv) difference between the Holocene and LGM time periods indicates a deglacial strengthening of AMOC at intermediate depths (surface to ~1000 m) throughout the Atlantic Ocean (Fig. 2).

Two deglacial depth transects of δ13Ccib records from the Southwest Atlantic and Pacific oceans suggest that the depth of glacial NADW was shallower than the sill depth of the Drake Passage (approximately 2500 m), reducing the contribution of NADW to AABW formation (Sikes et al. 2017). Hence, glacial AABW may have been derived from Pacific Deep Water and Indian Deep Water (Sikes et al. 2017). Expanded abyssal AABW may have resulted from expanded sea ice (Ferrari et al. 2014), reduced basal melting of ice shelves (Miller et al. 2012) or reduced southward meridional water-vapor transport (Muglia et al. 2018). A strong LGM δ13Ccib gradient in the southeastern Atlantic, associated with the lowest δ13Ccib values in the glacial ocean, may indicate a more isolated version of Circumpolar Deep Water (CDW) distinct from the CDW that filled the Pacific and Indian oceans (Ullermann et al. 2016; Williams et al. 2019).

Conclusion

For more than 50 years, paleoceanographers have sought to characterize the link between deep-ocean circulation and CO2 cycling. To gain a better understanding of these processes, the traditional interpretation of δ13Ccib records should be re-evaluated and updated as we expand our understanding of the climate system and global carbon cycle. Collegial and interdisciplinary collaboration can foster new ideas and insights into the comparison between paleorecords and modeling approaches. By archiving our hard-earned, high-resolution multiproxy paleorecords, densely sampled age models, and model simulations in public databases online, we can improve reconstructions of ocean circulation and carbon-cycle changes based upon more complete paleodata compilations. Certainly, interpretations would benefit from improved age models and additional paleorecords from the Southern, Indian, and Pacific oceans. Together, we can synthesize our work to improve our understanding of carbon-cycle dynamics between global reservoirs and within the ocean, as well as changes in biological, physical, and chemical processes, for the past, present, and future. Community collaboration could help us extract more clues about the deglacial carbon cycle from the data we have already generated.

affiliations

1Department of Earth Sciences, University of California Riverside, USA
2Department of Physical Oceanography, Woods Hole Oceanographic Institution, MA, USA
3Department of Earth Science, University of California Santa Barbara, USA
4School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, USA
5Department of Geology and Geophysics, Woods Hole Oceanographic Institution, MA, USA
6Center for the Study of Marine Systems (CESIMAR), Centro Nacional Patagónico, National Scientific and Technical Research Council (CONICET), Puerto Madryn, Argentina
7Max Planck Institute for Chemistry, Mainz, Germany
8College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, USA

contact

Carlye Peterson: carlye.petersongmail.com

references

Curry WB, Oppo DW (2005) Paleoceanography 20: PA1017

Ferrari R et al. (2014) P Natl Acad Sci 111 8753-8758

Hesse T et al. (2011) Paleoceanography 26: PA3220

Jeltsch-Thömmes A et al. (2019) Clim Past 15: 849-879

Lynch-Stieglitz J et al. (2019) Earth Planet Sc Lett 506: 466-475

Mackensen A, Schmiedl G (2019) Earth-Sci Rev 197: 102893

Menviel L et al. (2016) Paleoceanography 32: 2-17

Miller MD et al. (2012) Paleoceanography 27: PA3207

Muglia J et al. (2018) Earth Planet Sc Lett 496: 47-56

Oppo DW et al. (2018) Paleoceanogr Paleocl 33: 1013-1034

Peterson CD et al. (2014) Paleoceanography 29: 549-563

Peterson CD, Lisiecki LE (2018) Clim Past 14: 1229-1252

Repschläger J et al. (2015) Earth Planet Sc Lett 425: 256-267

Schmittner A et al. (2017) Paleoceanography 32: 512-530

Sikes EL et al. (2017) Paleoceanography 32: 1000-1017

Talley LD (2013) Oceanography 26: 80-97

Ullermann J et al. (2016) Paleoceanography 31: 639-650

Williams TJ et al. (2019) Paleoceanogr Paleocl 34: 833-852

Julia Gottschalk1, X. Zhang2 and A. Burke3

This Past Global Changes Magazine issue celebrates achievements in our understanding of the mechanisms governing changes in ocean circulation and the global carbon cycle in the past, and the complex interplay between them. This issue also emphasizes the current challenges and open questions in the field, to motivate research into improving our understanding of the fundamental links between ocean circulation, carbon cycling, and climate in the past, as well as in assessing the implications for the future.

This year, we mourn the loss of Wallace (Wally) Smith Broecker (1931-2019), professor of Geology at Columbia University. Wally was truly a "conveyor belt" of ideas and remarkable accomplishments in the fields of chemical oceanography and paleoclimatology, and his intellectual legacy has nurtured and will continue to nurture generations to come. His memoir (Broecker 2012) is a highly recommended read, as it also provides an overview of how (paleo-)ocean sciences evolved during his career.

Since the last assessment report of the Intergovernmental Panel on Climate Change (IPCC) in 2013 (Stocker et al. 2013), atmospheric CO2 levels have risen as a result of human activities by a magnitude similar to past millennial CO2 changes (e.g. Bereiter et al. 2015), though at a much faster rate. In this year's Special Report on the Ocean and Cryosphere in a Changing Climate (ipcc.ch/srocc; Pörtner et al. 2019), IPCC has, for the first time, highlighted a recognizable weakening of the Atlantic Meridional Overturning Circulation relative to 1850-1900. These observations emphasize the ongoing dramatic changes in the Earth's climate system today. Studying the fingerprints and records of past climate changes in diverse geological archives provides our best opportunity to document and assess the fundamentals of Earth's climate system under a variety of (extreme) climate boundary conditions.

Figure 1: Our understanding of processes and feedbacks between different components of the Earth system has remarkably expanded over the last decades, here illustrated by conceptual visualizations of global ocean circulation over time: from the "Great Ocean Conveyor Belt" (left; Broecker 1987), to more complex ocean circulation dynamics (middle) with different deep water formation sites (yellow dots; Rahmstorf 2002) and (right) to an ocean with different water masses that interact and interfere with each other in a complex manner (Talley 2013). Different colors depict different types of water masses (see aforementioned references).

The advent of stable isotope analyses of calcareous marine fossils (e.g. foraminifera) and ice-core drilling in the 1950s has provided crucial insights into past ocean and climate variability. Nearly 70 years later, the field of paleoceanography is an amalgamation of different techniques applied to study climate variability on various timescales based on many archives. However, overarching questions that motivated Earth scientists decades ago remain of great interest to the paleocommunity: What leads to changes in ocean circulation and how has it varied in the past? What drives the variations in greenhouse gas concentrations in the atmosphere?

To address these questions, the Ocean Circulation and Carbon Cycling working group (OC3, pastglobalchanges.org/oc3), which launched in 2014, has worked towards a global synthesis of stable isotope data from foraminifera and their comparison with numerical model simulations (e.g. Muglia et al. 2018). This effort is geared towards assessing deglacial circulation dynamics of different water masses globally and their role in changing ocean carbon storage over time. While a core-top (i.e. Late Holocene) data compilation has been completed and successfully compared against pre-industrial ocean observations (e.g. Schmittner et al. 2017), comprehensive regional and global data products for the last glacial-interglacial transition will be released in the near future.

In addition to using traditional stable-isotope proxies, paleoceanographers, geochemists, and Earth-system modelers distill information from paleodata constraints of past ocean and carbon-cycle dynamics that are derived from a number of proxies, many of which are discussed in this issue. Furthermore, in order to understand climate thresholds and sensitivities, it is important to study a range of climate boundary conditions in the past. Key intervals discussed in this magazine include the last deglaciation, the mid-Pleistocene transition, the Pliocene, and the Paleocene-Eocene Thermal Maximum. However, accurately estimating variations of global geochemical ocean inventories on a variety of timescales, and quantitatively determining the implications for ocean circulation, climate, and the global carbon cycle, remain major challenges. As some authors highlight, community-wide efforts of synthesizing regional data into a global framework and combining different proxy approaches will be important for addressing these challenges, as well as developments of numerical models and model-data assimilation efforts. This issue therefore aims to motivate future scientific research on these crucial aspects in our quest to improve our understanding of mechanisms behind past ocean circulation and global carbon-cycle dynamics.

affiliationS

1Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA

2Lanzhou University, China, and Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, Bremerhaven, Germany

3School of Earth and Environmental Sciences, University of St. Andrews, UK

contact

Julia Gottschalk: jgottschldeo.columbia.edu

references

Bereiter B et al. (2015) Geophys Res Lett 42: 542-549

Broecker WS (1987) Nat Hist 97: 74-82

Broecker WS (2012) Geochemical Perspect 1: 221-343

Muglia J et al. (2018) Earth Planet Sc Lett 496: 47-56

Pörtner H-O et al. (in press) IPCC Special Report on the Ocean and Cryosphere in a Changing Climate

Rahmstorf S (2002) Nature 419: 207-214

Schmittner A et al. (2017) Paleoceanography 32: 512-530

Stocker TF et al. (2013) Climate Change 2013: The Physical Science Basis. Cambridge University Press, 1535 pp.

Talley LD (2013) Oceanography 26: 80-97

Ray Bradley1, M. Grosjean2, T.F. Stocker3 and H. Wanner2

After a long illness, Bruno Messerli passed away during a clear winter night in February 2019, surrounded by his wife Béatrice and his whole family. Bruno was an outstanding scientist, colleague and friend, full of enthusiasm, ideas and congeniality, and always ready to take the lead. We will miss him dearly.

After studying geography, geology and history and completing his doctoral thesis on the geomorphology of the Sierra Nevada in Andalusia, Spain, Bruno began his career with an impressive habilitation thesis on the Pleistocene glaciation of mountain ranges around the Mediterranean. This was the start of Bruno’s broad and fruitful fieldwork in high-altitude mountains – in Africa (the Tassili, Tibesti, Aïr and Hoggar mountains in the Sahara, Semien mountains and Mount Kenya in East Africa), the arid Andes of South America and the Himalayas. For decades, his scientific work was driven by the question of whether or not the highest mountains in the most arid zones of the world were glaciated during the Last Glacial Maximum, or if the lack of moisture prevented widespread glaciation, despite extremely low temperatures. He investigated how glaciation in the mountains related to paleolakes and water resources in the nearby lowlands.

Chantal Camenisch1, S. White2, M. Bauch3, Q. Pei4 and C. Rohr1

1st CRIAS workshop, Bern, Switzerland, 1-2 October 2018

The recently founded PAGES working group Climate Reconstruction and Impacts from the Archives of Societies (CRIAS) held its first workshop on methods and interdisciplinary communication from 1-2 October 2018 in Bern, Switzerland. CRIAS focuses on the methods of historical climatology, a discipline which deals with three different fields:

1. The reconstruction of climate and weather on the basis of archives of society that contain man-made sources such as chronicles, account books or even pictures (Fig. 1 is an example of an illumination in a Bernese medieval chronicle).

2. The impacts of climate and weather on past societies.

3. The history of climate science and perceptions.

After the opening remarks from steering committee member Sam White, the program began with a panel on the state of the field in Historical Climatology, with a focus on Central Europe and China. The presenters Andrea Kiss, Rudolf Brázdil, Xiuqi Fang and Jie Fei gave insight into their research based on the archives of society on drought in medieval Hungary, on the Central European temperature and precipitation series, the unique Chinese historical records, which go back for more than 2000 years, and on the water-level changes of Lake Nansi during the Qing dynasty. This panel gave participants the possibility to compare the rich historiographic tradition of China with that of parts of Europe, which will be one of the goals of this working group in the next years.

The focus of the second panel was on the narrative sources used in Historical Climatology. Qing Pei gave a comprehensive introduction into weather-sensitive Chinese sources, their context of origin, and their content. Chantal Camenisch and Lukas Heinzmann presented results from their recent research, which includes climate impacts on society in Rouen, France, from 1315 to 1715, and weather conditions and climate impacts recorded in an extended and detailed diary written by a monk in the Einsiedeln, Switzerland, monastery during the Late Maunder Minimum.

Figure 1: Avalanche killing Bernese and Fribourgese mercenaries at the Gotthard massive in 1478. Bern, Burgerbibliothek, Mss.h.h.I.3, p. 917 – Diebold Schilling, Amtliche Berner Chronik, Bd. 3 (e-codices.ch/de/list/one/bbb/Mss-hh-I0003)

The third panel was dedicated to phenological observations in documentary sources and the production of climate indices. Because the source density in Europe in the early 14th century is considerably less dense compared to later centuries, Martin Bauch and Thomas Labbé proposed new ways of using climate indices for the reconstruction of this climatological key period. Melanie Salvisberg presented an index-based flood reconstruction of the Gürbe river, Switzerland, and flood impacts in the Gürbe valley. Based on the harvest length reported in manorial accounts (account rolls or books in the feudal system), Kathleen Pribyl reconstructed summer precipitation in East Anglia. The last paper of this panel, presented by Christian Pfister and Thomas Labbé, was dedicated to the longest available homogenized grape harvest series from Beaune, France, (1354-2018) and the temperature reconstruction based on that evidence.

New frontiers in Historical Climatology was the topic of the last panel of the workshop. These new frontiers were found in Southern India, where Gemma Ives reconstructed monsoons from 1730 to 1920. Marie-Michèle Ouellet-Bernier presented a sea-ice-cover reconstruction from Nunatsiavut in Labrador which was based on the reports of Moravian missionaries from 1750 to 1950. Dagomar Degroot’s presentation focused on the rich information of Dutch ship logbooks. Finally, David Nash talked about precipitation series from South Africa.

Dominik Collet, Rüdiger Glaser, Michael Kahle, Heli Huhtamaa, and Chaochao Gao lead a roundtable focusing on the importance of interdisciplinary collaboration and ways to combine data from the archives of society and data from those of nature. The last part of the workshop comprised discussions in three break-out groups on “preserving, classifying and disseminating data”, “climate history as global history”, and “comparing Chinese and Central European historical climatology”. The aim of the workshop was to determine the future outline of the working group in a broader frame and to bring together Historical Climatologists from different continents. Both aims were achieved in Bern, thanks in part to the financial and administrative support from the Oeschger Centre of Climate Change Research and PAGES.

affiliations

1Oeschger Centre for Climate Change Research and Section of Economic, Social and Environmental History, University of Bern, Switzerland

2Department of History, Ohio State University, Columbus, USA

3Leibniz Institute for the History and Culture of Eastern Europe, Leipzig, Germany

4Department of Social Sciences, Education University of Hong Kong, China

contact

Chantal Camenisch, chantal.camenischhist.unibe.ch

Juan José Gómez-Navarro1, P. Ludwig2, N. Zeiher3, S. Talento4, U. Parveen5 and S. Wagner6

2nd PALEOLINK workshop, Murcia, Spain, 6-8 February 2019

Past climate changes and variations are assessed with proxy reconstructions based on various archives and climate modeling approaches. However, combining both proxy and modeling approaches still includes profound temporal- and spatial-scale gaps. Empirical climate reconstructions are most skillful on a local-to-regional scale covering time periods up to millennia and more, albeit they exhibit a coarse temporal resolution. In contrast, results from comprehensive General Circulation Models (GCM) or Earth system models, which have high temporal resolution, are only representative on regional- to large-scale spatial scales. Thus, innovative and integrated efforts are necessary to bridge the gap between the scales and to bring data and models to a common basis for comparison of past climatic and environmental changes. Therefore, Regional Climate Models (RCM) may be helpful to overcome this spatial and temporal mismatch, but are currently seldom used in the paleo perspective (Fig. 1).

Figure 1: Schematic of GCM/RCM-proxy data comparison for temperature in the Pyrenees: (A) part of the GCM model domain (orography shaded). Black box marks RCM domain, red cross marks grid point for time series data in (D); (B) RCM model domain (orography shaded). Blue box marks area averaged over the Pyrenees for RCM data in (D), black arrow illustrates location of (C) tree rings used as proxy. (D) Synopsis of GCM, RCM, and proxy data time series. Figure taken from Ludwig et al. (2018) with permission.

To address these issues, leaders of the PAGES 2k Network project PALEOLINK organized a workshop in the scenic town of Murcia, as a follow up to the PALEOLINK kick-off meeting at the European Geosciences Union (EGU) General Assembly in Vienna in April 2018. The workshop brought together 22 scientists from different countries – most of them early-career scientists working in the fields of global and regional climate modeling, as well as proxy reconstruction based on different archives and statistical techniques.

The workshop consisted of two sections: a series of oral talks with participants presenting their work and fields of interest, followed by sufficient time for questions, and a series of breakout groups running in parallel.

During the first section, talks were organized around four main topics, ranging from climate reconstructions, using (regional) climate and forward modeling, to model-data integration. In the second section, several breakout groups were created to address specific open issues and future directions applicable to the entire working group. These groups were not previously defined, but were proposed in situ based on the previous discussions, with the aim to condense ideas stemming and emerging from the preceding talks.

In the first round, four topics were addressed including i) statistical reconstruction methods of hydroclimate variables, ii) identification of variables/regions where the added value in regional paleoclimatic model simulations is most noticeable, iii) regional oceanographic models, and iv) regional glacial and interglacial concepts and models. In the second round, workshop participants were encouraged to change groups, thereby sharing their experiences and expertise in order to co-develop strategies and synergetic structures between the different groups. The workshop concluded with summarizing the main results and defining strategies for workshop products related to scientific papers and research initiatives led by enthusiastic group leaders, and coordinating the goals and tasks within the various groups.

An open follow-up meeting took place in a splinter meeting at EGU 2019, where attendants and interested new colleagues in the field of paleoclimatic and paleoenvironmental research had the opportunity to be involved in post-workshop activities. In the future, we plan to aim for additional meetings in the form of online webinars and in-person meetings at larger conferences. In particular, the PALEOLINK leaders are co-conveners of a session at the 20th INQUA congress in July 2019 in Dublin. The group is completely open to input and active participation from the paleoclimatic and paleoenvironmental community interested in addressing issues in the context of the link between the different paleoclimatic spatial and temporal scales.

affiliationS

1Department of Physics, University of Murcia, Spain

2Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Germany

3Department of Geography, Friedrich Alexander University, Erlangen, Germany

4Department of Geography, Justus Liebig University, Giessen, Germany

5Center for the Study of Regional Development, Jawaharlal Nehru University, New Delhi, India

6Institute of Coastal Research, Helmholtz-Zentrum Geesthacht, Germany

contact

Juan José Gómez-Navarro: jjgomeznavarroum.es

reference

Ludwig P et al. (2018) Ann. N.Y. Acad. Sci. 1436: 54-69

Yassine Ait Brahim1, N. Kaushal2 and L. Comas-Bru3

3rd SISAL workshop, Agadir, Morocco, 8-12 October 2018

PAGES’ SISAL (Speleothem Isotope Synthesis and Analysis) working group was set up to create a database of speleothem δ18O and δ13C records and to synthesize these records for targeting climate questions such as investigating long-term drivers of the global monsoon and for data-model comparisons (Comas-Bru et al. 2017). The first version of the database (SISAL_v1; Atsawawaranunt et al. 2018a; Fig. 1), with 371 speleothem records and 10 composites from 174 cave systems, has been made available online with an accompanying paper describing its structure (Atsawawaranunt et al. 2018b).

Figure 1: Speleothem records in SISAL_v1 (Atsawawaranunt et al. 2018a,b) for the key periods relevant to the studies that the workshop focused on: (A) the last two millennia, (B) the mid-Holocene, and (C) the Last Glacial Maximum. In each, "kyr BP" refers to thousands of years before present, where present is 1950 CE.

Building on this foundation, the 3rd SISAL workshop was held from 8-12 October 2018 at Ibn Zohr University in Agadir, Morocco. Twenty-four participants, including 12 early-career researchers, came to Agadir from 14 countries. The SISAL members gave presentations about their SISAL-related activities on the first day. The discussions involved: (i) the current status of the database; (ii) an update by the age modeling group, which aims to provide a common denominator for age-uncertainty envelopes through the construction of a set of new age-depth models termed “SISAL chronologies” employing a range of commonly used techniques – a critical addition to the next version of the database (SISAL_v2); (iii) progress and feedback on the regional review papers that are part of a special issue in the journal Quaternary (mdpi.com/journal/quaternary/special_issues/speleothem_records_climate); and (iv) preliminary work on the first scientific SISAL paper and three additional papers using the SISAL database.

On the following three days, the analyses and discussions were centered around the first scientific SISAL paper (“Evaluating model outputs using integrated global speleothem records of climate change since the last glacial”) and the three additional papers currently in progress: “Hydrological records of the evolution of regional monsoons during the Holocene and the Last Interglacial”, “The Holocene from the speleothems’ view: Global trends and teleconnections”, and “The MCA/LIA as reflected by speleothems”. Participants separated into small groups to work on these analyses with daily feedback from the larger group. By the end of the workshop there were paper outlines with clearly defined timelines, preliminary analyses, and further steps to be taken. Data gaps of key climate events from different regions necessary to make these papers robust were identified and have been prioritized. In accordance with the co-authorship agreement created to encourage inclusivity, all researchers are welcome to contribute intellectually to these papers.

Two talks were given by participants at the workshop. On Day 1, Mike Rogerson presented an idea of drip water synthesis. On Day 3, Colin Prentice gave an invited talk titled “Stable carbon isotope ratios in plants and the atmosphere, from inter-annual variability to glacial-interglacial cycles”, reflecting on the idea of interpreting global patterns in speleothem δ13C records from the SISAL database.

During the workshop, it was decided that SISAL_v2, which will incorporate records identified as missing during the workshop and the SISAL chronologies, will be released in late 2019. In the closing session, participants discussed locations for the next workshop to be held in October 2019, a SISAL session and poster presentations at EGU 2019, potential collaborations with other PAGES working groups, funding sources, and the perspectives of SISAL’s future (e.g. the extension of the SISAL database with the addition of more records and/or, for example, trace element and drip water measurements). Finally, informal SISAL meetings at the INQUA 2019 and EGU 2019 conferences were scheduled.

The last day of the workshop was spent exploring the Wintimdouine cave (30.68°N, -9.34°W, 1400 m.a.s.l) and its associated geology. Located 70 km northeast of Agadir city, the Wintimdouine cave (i.e. "the spring of lakes” in the Moroccan Berber language) is developed within the karst system of Tasroukht in the Western High Atlas Mountains. It includes the longest known underground river in Africa with 19 km explored so far. Participants had the chance to enter the cave and examined various beautiful forms of speleothems.

Those re­searchers with data to add to the database are encouraged to contact the regional coordina­tor for the geographic area of their stalagmite record. The deadline for submission for the second version of the database is 30 June 2019. For more information about SISAL and how to get involved, go to pastglobalchanges.org/sisal

acknowledgements

We thank PAGES, Ibn Zohr University, University of Reading, and the Institute of Research for Development for their financial support. We also thank the Scientific Association of Speleology of Agadir for their assistance during the field trip.

affiliationS

1Institute of Global Environmental Change, Xi’an Jiaotong University, China

2Asian School of the Environment, Nanyang Technological University, Singapore

3School of Archaeology, Geography & Environmental Sciences, University of Reading, UK

contact

Yassin Ait Brahim: aitbrahimxjtu.edu.cn

references

Atsawawaranunt K et al. (2018a) SISAL database Version 1.0. University of Reading. Dataset

Atsawawaranunt K et al. (2018b) Earth Syst Sci Data 10: 1687-1713

Comas-Bru L et al. (2017) PAGES Magazine 25: 129

Thomas Hoffmann1 and Veerle Vanacker2

3rd GloSS workshop, Koblenz, Germany, 18-20 October 2018

The PAGES Global Soil and Sediment transfers in the Anthropocene (GloSS) working group aims to build a comprehensive global database on soil and sediment transfers in the Anthropocene, to identify hotspots of soil erosion and sediment deposition in response to human impacts, and locate data-poor regions as strategic foci for future work (Fig. 1).

Figure 1: Screenshot of the GloSS wiki. The colored points indicate the number of datasets entered in the wiki as of 20 March 2019. The wiki is an open platform to enter data on past and present soil and sediment transfer. For more information see: pagesgloss.colorado.edu/wiki.

The third workshop of the GloSS working group was hosted by the Federal Institute of Hydrology (Koblenz) and supported by the UNESCO-IHP International Center for Water Resources and Global Change (ICWRGC). The workshop aimed to synthesize the results from the regional task forces, discuss progress on the GloSS database and identify ways to motivate the GloSS members and the broader scientific community working with human impacts on soils and sediments to contribute to the compilation of the GloSS database. A total of 19 participants from different disciplines (geomorphology, geology, soil science, ecology, (paleo)limnology, and hydrology) and 10 countries from four continents contributed to the workshop.

The first day was dedicated to reports by the regional taskforce leaders that highlighted the progress of the regional working groups in terms of data compilation. The keynotes were given by Rajiv Sinha and Sohini Bhattacharjee (Indian Institute of Technology, Kanpur, India), Juan Restrepo (EAFIT, University of Medellin, Colombia), Duncan Cook (Australian Catholic University, Melbourne), Allan James (University of South Carolina, USA), Dongfeng Li (National University of Singapore), Aleksey Sidorchuk (Moscow State University, Russia), and Gert Verstreaten (Leuven University, Belgium). Stephan Dietrich (ICWRGC, Germany) gave a presentation of the global GEMStat water quality database (gemstat.org), which is hosted at the ICWRGC in Koblenz, and Jean Phillipe Jenny (INRA Thonon, France) presented results from a European database on lake sediments.

All keynotes presented a wealth of studies and data that provide the backbone for the GloSS database. However, it was noted by the keynote speakers that the focus of the GloSS working group on the full sediment pathway including hillslope and river systems (channels and floodplains), as well as lakes and deltas, is a major challenge (obstacle) to the compilation of the database. In contrast to other databases that have been compiled by the PAGES community, GloSS deals with various sedimentological archives and proxies over various temporal and spatial scales, with varying sensitivity to human disturbances. It was further noted that colleagues from the scientific community hesitate to contribute their data to the GloSS community if the benefit is not fully clarified.

During the second day, participants discussed the major shortcomings of the GloSS database and developed a strategy to increase the number of contributions from the scientific community to the GloSS working group. First, the strategy includes a statement on the publication policy of the GloSS working group, indicating that all publications derived from the GloSS working group results and the GloSS database should be published by the key authors and the GloSS consortium. Each scientist who provides information and data that support the population of the database will be a part of the GloSS consortium. Second, the strategy includes the compilation of a special issue in the journal Anthropocene. The special issue will include a mixture of i) regional synthesis papers that highlight the specific histories of human disturbance on soils and sediments on various continents, ii) large-scale/global compilations, and iii) parameter-specific databases that are of relevance for the GloSS community. The special issue will be completed by a paper on the conceptual framework of the GloSS database with a focus on the global synthesis.

The third day was dedicated to a city field trip along the Rhine River in Koblenz. Participants learned about the long-term history of soil erosion and sediment transport in the Rhine basin and the present-day sediment issues related to the management of the waterways in Germany. One focus was on the sediment-monitoring activities of the Federal Institute of Hydrology and the Water and Shipping Authority in Germany and the sediment budget analysis of the Rhine, which highlights the functioning of the heavily exploited Rhine waterway in the Anthropocene.

This group officially ended as a PAGES working group in 2018, but it is still active. Find it here: pastglobalchanges.org/science/wg/former/gloss/intro.

affiliationS

1German Federal Institute for Hydrology, Koblenz, Germany
2Earth and Life Institute, Catholic University of Louvain, Belgium

contact

Thomas Hoffmann: Thomas.Hoffmannbafg.de

Sandy P. Harrison1, M.-J. Gaillard2 and B.D. Stocker3

Sitges, Spain, 24-28 September 2018

Members of the PAGES LandCover6k working group (LandCover6k), the Paleoclimate Modeling Intercomparison Project (PMIP), and representatives of the carbon-cycle modeling community and PAGES’ PEOPLE 3000 working group met in Sitges, Spain, to co-design paleo simulations to evaluate the impact of land-use and anthropogenic land-cover change on climate and the carbon cycle over the Holocene.

The impact of anthropogenic land-cover change due to land use (LULC) on biogeochemical cycles and climate is still uncertain. Climate-model simulations indicate that LULC impacts on temperature and precipitation are large, both in and beyond the regions where these changes occur (Smith et al. 2016). However, the LULC changes used to drive these simulations are unrealistic (Gaillard et al. 2010). Furthermore, the currently available LULC scenarios (HYDE: Klein Goldewijk et al. 2017; KK10: Kaplan et al. 2011) are inconsistent with the constraints imposed by carbon budgeting (Stocker et al. 2017). Creating more realistic LULC scenarios, using paleovegetation reconstructions and archaeological data, is the central goal of LandCover6k.

The group has made considerable progress towards LULC reconstructions for key regions and times (Morrison et al. 2018). It is now time to test and use these reconstructions. Discussions at the Sitges workshop focused on how to incorporate LandCover6k information into LULC scenarios, to design biogeochemical model simulations to test the reliability of these reconstructions, and to design Earth system model simulations to provide a realistic assessment of the impact of LULC changes on climate and the carbon cycle over the Holocene.

The first day of the workshop focused on overview presentations of the various initiatives relevant to this goal and the research interests of different communities (for presentations see the LandCover6k homepage: pastglobalchanges.org/landcover6k). Discussions around the presentations ensured there was common understanding of the terminology and clarified the data needs of each community. Subsequent breakout group discussions addressed co-operative activities from several different perspectives. Key issues were (a) how information from the LandCover6k project could be used to improve the HYDE and KK10 scenarios or (b) as input to and/or validation of climate model simulations, (c) how HYDE and KK10 scenarios could be used as input for climate model and/or carbon-cycle simulations, and (d) whether existing products fulfilled the minimum and/or desirable requirements for climate and carbon-cycle modeling experiments.

Figure 1: Schematic showing how LandCover6k fast-track products will feed into the production of new land use-land cover (LULC) scenarios. Offline simulations with biogeochemical models will provide a validation of the realism and impact of the LandCover6k products. The ultimate goal is to produce new LULC scenarios as forcing for Earth system model simulations.

Positive outcomes of the workshop were (a) agreement on data collection priorities to maximize the usefulness of LandCover6k products in the short, medium and long term, and (b) the design of protocols for model simulations to test LULC impacts on climate and biogeochemical cycle (Fig. 1). Some data syntheses need to be fast-tracked to test whether available information has discernible impacts on LULC scenarios, including new estimates of population growth/decline through time from archaeological ¹⁴C dates (Crombé and Robinson 2014), maps of the initial date of the introduction of agriculture from archaeological studies and related land conversion inferred from pollen-based vegetation maps, and information about the type of crops and grazing animals.

Short-term (six-month) goals encompass products needed as input to model simulations designed for inclusion in the next IPCC report. The intermediate (12-month) goals are LandCover6k products that contribute directly to this report (e.g. comparisons between pollen-based LULC reconstructions based on different methodologies, evaluation of the HYDE and KK10 LULC scenarios using the pollen-based reconstructions), while the longer-term goals include LandCover6k products that will be ready by the end of the second phase of LandCover6k. A more complete description of the experimental protocols will be published as a joint-authored paper, to help enable modeling groups to run the LandCover6k-PMIP co-designed simulations.

affiliationS

1School of Archaeology, Geography and Environmental Science, University of Reading, UK

2Department of Biology and Environmental Science, Linnaeus University, Kalmar, Sweden

3Centre for Ecological Research and Forestry Applications, Bellaterra, Spain

contact

Sandy P. Harrison: s.p.harrisonreading.ac.uk

references

Crombé P, Robinson E (2014) J Arch Sci 52: 558-566

Gaillard MJ et al. (2010) Clim Past 6: 483-499

Kaplan JO et al. (2011) Holocene 21: 775-791

Klein Goldewijk K et al. (2017) Earth Syst Sci Data 9: 927-953

Morrison KD et al. (2018) Past Global Changes 26: 8-9

Smith MC et al. (2016) Clim Past 12: 923-941

Stocker BD et al. (2017) Proc Natl Acad Sci 114: 1492-1497