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In previous model-data comparisons, the centennial to millennial scale variance of local climate (e.g., SST) reconstructed from proxies was significantly higher than that simulated by climate models. One possible explanation is the lack of long-term feedback mechanisms, e.g. from the representation of changes in ice-sheets in models. Additionally, proxy records are short, and sparse, and the climate signal is significantly modified during the processes of encoding, archiving, and recovery.Here we introduce new methods to infer the climate variability of the past from proxy data and compare them to new transient model simulations of the last deglaciation. This will allow us to estimate the amplitude of climate variability and to evaluate whether climate models are capable of capturing changes in climate variability between different climate states (e.g. glacial vs. interglacial periods), which is also relevant for the accuracy of future projections. We compare the variability of marine d18O reconstructed from proxies with that simulated by a state-of-the-art Earth System Model.From the proxy side, our analysis is based on a new dataset of marine oxygen isotope data from planktonic foraminifera compiled for the PALMOD project. We use new methods to first calculate power-spectra for the LGM, transition and Holocene and then to correct these spectra by fitting a Bayesian model describing the effects of bioturbation and measurement error on the reconstructed climate signal. From the model side we use marine d18O variability calculated using temperature and salinity from transient model simulations of the last deglaciation, performed within the PALMOD project, that include changes in the ice sheets.This combination of new data and methods will allow us to investigate the effect of different ice-sheet configurations and physical parametrizations in the model on their ability to characterise long-timescale climate variability and its dependence on climate state.

The climate of the past two thousand years (2k) provides context for current and future changes, and as such is vital for developing our understanding of the modern climate system. Building on previous phases of the PAGES 2k network, Phase 4 of the PAGES 2k Network paves the way for a new level of understanding of the global water cycle, including enhanced science-policy integration. Previous PAGES 2k network phases emphasised temperature reconstructions, fundamentally improving our understanding of global climate changes over the Common Era. These reconstructions received widespread recognition and were featured in the Summary for Policymakers of the IPCC’s Sixth Assessment Report. Integration of this data with state-of-the-art Earth systems models, proxy system models and data assimilation yielded a more comprehensive understanding of the associated physical drivers and climate dynamics. Phase 4 challenges our community to turn its focus towards hydroclimate. Our aim is to reconstruct hydroclimatic variability over the Common Era, from local to global spatial scales, at sub-annual to multi-centennial time scales, developing a process-level understanding of past hydroclimate events and variability. Our multi-faceted approach includes (1) developing new hydroclimate syntheses that are well-suited for data-model comparisons, (2) improving the interoperability and scope of existing data and model products, and (3) facilitating the translation of our science into evidence-based policy outcomes. In this presentation, we report on our activities and progress to date, particularly highlighting the early stages of our data synthesis efforts.

Paleoclimate proxy records from the North Atlantic region reveal substantially greater multi-centennial temperature variability during the Last Glacial Maximum (LGM) compared to the current interglacial. As there was no obvious change in external forcing, causes for the increased variability remain unknown. Here we provide a mechanism for enhanced multi-centennial North Atlantic climate variability during the LGM based on experiments with the coupled climate model CESM. The model simulates an internal mode of multi-centennial variability, which is associated with variations in the Atlantic meridional overturning circulation. In accordance with high-resolution proxy records from the glacial North Atlantic, this mode induces highest surface temperature variability in subpolar and mid latitudes and almost no variance in low latitudes. Greenland surface air temperature varies by up to 4°C, which is in line with multi-centennial variability reconstructed from ice cores. We show that this mode is based on a salt-oscillator mechanism and emerges only under full LGM climate forcing. Moderate deviations from full-glacial boundary conditions lead to its disappearance. We further argue that the multi-centennial mode has to be distinguished from millennial-scale Dansgaard-Oeschger oscillations.

Marine zooplankton time series are crucial to understand the dynamics of pelagic ecosystems. However, most observational time series are only a few decades long, which limits our understanding of long-term zooplankton dynamics, renders attribution of observed trends to global change ambiguous, and hampers prediction of future response to environmental change. Planktonic foraminifera are calcifying marine zooplankton that have the unique potential to substantially extend our view on plankton dynamics because their skeletal remains are preserved for millions of years in deep-sea sediments. Thus, linking sedimentary and modern time series offers great potential to study zooplankton dynamics across time scales not accessible by direct observations. However, this link is rarely made and the potential of planktonic foraminifera for advancing our understanding of zooplankton dynamics remains underexploited. This underutilization of this potential to bridge time scales is mainly because of the lack of collaboration between biologists, who have mostly focused on other (zoo)plankton, and micropalaeontologists, who have focussed too narrowly on fossil foraminifera. With this food for thought article, we aim to highlight the unique potential of planktonic foraminifera to bridge the gap between biology and geology. We strongly believe that such collaboration has large benefits to both scientific communities.

Reconstructions of global hydroclimate during the Common Era (CE; the past ∼2000 years) are important for providing context for current and future global environmental change. Stable isotope ratios in water are quantitative indicators of hydroclimate on regional to global scales, and these signals are encoded in a wide range of natural geologic archives. Here we present the Iso2k database, a global compilation of previously published datasets from a variety of natural archives that record the stable oxygen (δ18O) or hydrogen (δ2H) isotopic compositions of environmental waters, which reflect hydroclimate changes over the CE. The Iso2k database contains 759 isotope records from the terrestrial and marine realms, including glacier and ground ice (210); speleothems (68); corals, sclerosponges, and mollusks (143); wood (81); lake sediments and other terrestrial sediments (e.g., loess) (158); and marine sediments (99). Individual datasets have temporal resolutions ranging from sub-annual to centennial and include chronological data where available. A fundamental feature of the database is its comprehensive metadata, which will assist both experts and nonexperts in the interpretation of each record and in data synthesis. Key metadata fields have standardized vocabularies to facilitate comparisons across diverse archives and with climate-model-simulated fields. This is the first global-scale collection of water isotope proxy records from multiple types of geological and biological archives. It is suitable for evaluating hydroclimate processes through time and space using large-scale synthesis, model–data intercomparison and (paleo)data assimilation. The Iso2k database is available for download at https://doi.org/10.25921/57j8-vs18 (Konecky and McKay, 2020) and is also accessible via the NOAA/WDS Paleo Data landing page: https://www.ncdc.noaa.gov/paleo/study/29593 (last access: 30 July 2020).

The Last Glacial Maximum (23,000 – 19,000 years ago; LGM) is the most recent time when Earth’s climate was fundamentally different from today. The LGM hence remains a prime target to evaluate climate models outside current boundary conditions. Evaluation of paleoclimate simulations is usually done using proxy-based reconstructions. However, such reconstructions are indirect and associated with marked uncertainty, which often renders model-data comparison equivocal. Here we take a different approach and use macro-ecological patterns preserved in fossil marine zooplankton to evaluate simulations of LGM near-surface ocean temperature. We utilise the distance-decay pattern in planktonic foraminifera to evaluate modelled temperature gradients. Distance decay emerges because of differences in habitat preferences among species that cause the compositional similarity between assemblages to decrease the further apart they are from each other in environmental space. Distance decay is a fundamental concept in ecology and is observed in many different taxa and ecosystems, including planktonic foraminifera that show a monotonous decrease in similarity with increasing difference in temperature. Because the ecological niches of planktonic foraminifera are unlikely to have changed since the LGM, the distance-decay relationship based on simulated LGM temperatures and LGM assemblages should in principle be identical to the modern distance decay pattern. Thus we can use fossil planktonic foraminifera species assemblages to evaluate climate model simulations based on ecological principles. Our analysis is based on an extended new LGM planktonic foraminifera database (2,085 assemblages from 647 unique sites) and a suite of 10 simulations from state-of-the-art climate models (PMIP3 and 4). We find markedly different planktonic foraminifera distributions during the LGM, primarily due to the equatorward expansion of polar assemblages at the expense of transitional assemblages. The distance-decay pattern that emerges when the LGM assemblages are combined with simulated ocean temperatures is different from the modern pattern. All simulations suggest large thermal gradients between regions where the planktonic foraminifera indicate no, or only weak, gradients. This pattern arises from the pronounced shift to polar species assemblages in the North Atlantic where the simulations predict only moderate cooling. In general, the models predict spatially rather uniform cooling, whereas the microfossil evidence suggests more pronounced regional differences in the temperature change. The difference between reconstructions and the simulations reaches up to 10 K in the North Atlantic. Importantly, simulations with a reduced AMOC and hence lower North Atlantic near sea surface temperatures, yield a distance-decay pattern that is much more similar to the modern pattern. The planktonic foraminifera assemblages thus question the view of the LGM ocean as an equilibrium response to external forcing.

Understanding the climate of the past two millennia (2k) remains vital for developing our wider comprehension of the climate system, including the nature and scale of recent and future anthropogenic change. Phase 4 of the PAGES 2k network will build on previous phases and take us to a new level of understanding and science-policy integration.During previous phases, PAGES 2k members compiled global networks of proxy measurements, extending records beyond the instrumental period by more than an order of magnitude, reconstructing past climate and developing new knowledge of past variability and processes. Through data-model integration with state-of-the-art Earth systems models, proxy system modelling and data assimilation, we took key steps towards a more comprehensive understanding of climate dynamics.Phase 4 will take us even further, challenging our community to turn its focus primarily towards the hydroclimate of the Common Era: performing new reconstructions and improving the interoperability, extent and scope of our data and model products. In doing so, we also seek to facilitate the translation of our science into evidence-based policy outcomes. Our overarching aim is to reconstruct hydroclimate variability over the Common Era from local to global spatial scales, at sub-annual to multi-centennial time scales. We propose to achieve this through new community-led data curation efforts and the development of new data-driven tools and practices to maximise the interoperability of convenient, efficient and widespread model/data products. We will aim for a process-level understanding of past hydroclimate events and variability by evaluating and constraining Earth system models and through data assimilation.Our coordination team places a strong emphasis on respect and inclusion, aiming to foster a diverse and equitable community. Through a ‘hub and spoke’ structure, our team will provide a facilitation, coordination and support role (the hub) for Pages 2k working groups (the spokes). We are actively seeking participation of those engaging in climate policy and climate services. Welcome to Phase 4! We warmly invite your collaborations and contributions!

Planktonic foraminifera, marine protists and calcifiers, are globally responsible for about one quarter of the global pelagic calcite flux, but their contribution to it in the subarctic and Arctic realm has not been fully resolved. With ongoing warming and sea ice loss, the Arctic Ocean and its marginal seas are becoming more hospitable to pelagic calcifiers like planktonic foraminifera, resulting in modifications of the regional carbonate cycle and the composition of the seafloor sediment. To quantify planktonic foraminifera Arctic calcite production and loss in the upper water layer, we compile and analyze data on the dominant planktonic foraminifera species of the Arctic realm, Neogloboquadrina pachyderma, from vertical profiles in the upper water column. Using summertime observations of shell concentrations, sizes and weights of this species across the Arctic, we calculate the base of the productive zone of N. pachyderma to be regionally highly variable, on average located at 125 m, at maximum at 300 m. The calcite flux immediately below the productive zone (production flux) is on average 8 mg CaCO3 m-2 d-1, and we observe that this flux is attenuated until at least 300 m below the base of the productive zone. Across the Arctic realm, the summer production flux of N. pachyderma calcite varies by more than two orders of magnitude and the estimated mean export flux below the twilight zone is sufficient to account for about a quarter of the total pelagic carbonate flux in the region. These results indicate that estimates of the Arctic pelagic carbonate budget will have to account for large regional differences in production flux of the major pelagic calcifiers and confirm that substantial attenuation of the production flux occurs in the twilight zone.

An increasing number of climate model simulations is becoming available for the transition from the Last Glacial Maximum to the Holocene. Assessing the simulations’ reliability requires benchmarking against environmental proxy records. To date, no established method exists to compare these two data sources in space and time over a period with changing background conditions. Here, we develop a new algorithm to rank simulations according to their deviation from reconstructed magnitudes and temporal patterns of orbital- as well as millennial-scale temperature variations. The use of proxy forward modeling avoids the need to reconstruct gridded or regional mean temperatures from sparse and uncertain proxy data. First, we test the reliability and robustness of our algorithm in idealized experiments with prescribed deglacial temperature histories. We quantify the influence of limited temporal resolution, chronological uncertainties, and non-climatic processes by constructing noisy pseudo-proxies. While model-data comparison results become less reliable with increasing uncertainties, we find that the algorithm discriminates well between simulations under realistic non-climatic noise levels. To obtain reliable and robust rankings, we advise spatial averaging of the results for individual proxy records. Second, we demonstrate our method by quantifying the deviations between an ensemble of transient deglacial simulations and a global compilation of sea surface temperature reconstructions. The ranking of the simulations differs substantially between the considered regions and timescales. We attribute this diversity in the rankings to more regionally confined temperature variations in reconstructions than in simulations, which could be the result of uncertainties in boundary conditions, shortcomings in models, or regionally varying characteristics of reconstructions such as recording seasons and depths. Future work towards disentangling these potential reasons can leverage the flexible design of our algorithm and its demonstrated ability to identify varying levels of model-data agreement.

Biodiversity is expected to change in response to future global warming. However, it is difficult to predict how species will track the ongoing climate change. Here we use the fossil record of planktonic foraminifera to assess how biodiversity responded to climate change with a magnitude comparable to future anthropogenic warming. We compiled time series of planktonic foraminifera assemblages, covering the time from the last ice age across the deglaciation to the current warm period. Planktonic foraminifera assemblages shifted immediately when temperature began to rise at the end of the last ice age and continued to change until approximately 5,000 years ago, even though global temperature remained relatively stable during the last 11,000 years. The biotic response was largest in the mid latitudes and dominated by range expansion, which resulted in the emergence of new assemblages without analogues in the glacial ocean. Our results indicate that the plankton response to global warming was spatially heterogeneous and did not track temperature change uniformly over the past 24,000 years. Climate change led to the establishment of new assemblages and possibly new ecological interactions, which suggests that current anthropogenic warming may lead to new, different plankton community composition.

The development of larger and longer lasting northern hemisphere ice sheets during the mid‐Pleistocene Transition (MPT) coincided with global cooling. Here, we show that surface waters of the north‐eastern Atlantic actually warmed across this interval (∼1.2–0.8 Ma), which we argue reflects an increase in the north‐eastward transport of heat and moisture via the North Atlantic Current (NAC) into the Nordic Seas (the Atlantic Inflow). We suggest that simultaneous cooling and warming along the north‐western and south‐eastern margins (respectively) of the NAC during Marine Isotope Stage 28 (∼995 ka) reflected the increasing persistence of northern ice sheets as Atlantic Inflow increased. This resulted in a diachronous shift from ∼40 to ∼100 kyr cyclicity across the North East (NE) Atlantic as the growing influence of northern ice sheets spread southwards; to the north‐west of the NAC the first “100 kyr” cycle preceded Termination 12 (∼960 ka), while on the south‐eastern margin of the NAC the transition occurred ∼100 kyr later. Exploratory climate model simulations suggest that increasing Atlantic Inflow at this time could have accelerated ice sheet growth because pre‐existing moderately sized ice sheets allowed the positive effect of increasing precipitation to outpace melting. In addition, we propose that the dependence of post‐MPT ice sheets on moisture transport via the Atlantic Inflow may ultimately contribute to their apparent vulnerability to insolation forcing once a critical size threshold is crossed and high latitude ice sheets become starved of a vital moisture source.

Stable oxygen isotopes (δ18O) of planktonic foraminifera are one of the most used tools to reconstruct environmental conditions of the water column. Since different species live and calcify at different depths in the water column, the δ18O of sedimentary foraminifera reflects to a large degree the vertical habitat and interspecies δ18O differences and can thus potentially provide information on the vertical structure of the water column. To fully unlock the potential of foraminifera as recorders of past surface water properties, it is necessary to understand how and under what conditions the environmental signal is incorporated into the calcite shells of individual species. Deep-dwelling species play a particularly important role in this context, since their calcification depth reaches below the surface mixed layer. Here we report δ18O measurements made on four deep-dwelling Globorotalia species collected with stratified plankton tows in the Eastern North Atlantic. Size and crust effects on the δ18O signal were evaluated showing that a larger size increases the δ18O of Globorotalia inflata and Globorotalia hirsuta, and a crust effect is reflected in a higher δ18O in Globorotalia truncatulinoides. The great majority of the δ18O values can be explained without invoking disequilibrium calcification. When interpreted in this way the data imply depth-integrated calcification with progressive addition of calcite with depth to about 300 m for G. inflata and to about 500 m for G. hirsuta. In Globorotalia scitula, despite a strong subsurface maximum in abundance, the vertical δ18O profile is flat and appears dominated by a surface layer signal. In G. truncatulinoides, the δ18O profile follows equilibrium for each depth, implying a constant habitat during growth at each depth layer. The δ18O values are more consistent with the predictions of the Shackleton (1974) paleotemperature equation, except in G. scitula, which shows values more consistent with the Kim and O’Neil (1997) prediction. In all cases, we observe a difference between the level where most of the specimens were present and the depth where most of their shell appears to calcify.

Global mean surface temperature is a fundamental measure for climate evolution in both past and present and a key quantity to evaluate climate simulations. However, for paleoclimate periods, its calculation hinges on proxy data distributed sparsely and inhomogeneously in both space and time. Thus, large sets of different proxy records need to be combined in order to obtain global mean temperature reconstructions, but there is no widely accepted method to perform this task. Building on the work of [1], we suggest and evaluate an algorithm to reconstruct spatially averaged surface temperatures on centennial to orbital timescales. As the most abundant archive for continuous temperature reconstructions, we focus on marine sediment records as input data. Our implementation is applicable to any compilation of sea-surface temperature reconstructions and capable of calculating global, hemispherical and regional temperature. Major steps of the reconstruction algorithm are interpolation to a common timescale, zonal normalization and calculation of spatially weighted sums, including uncertainty propagation via Monte Carlo methods. We assess the applicability of the algorithm by employing it to the PalMod130k marine palaeoclimate data synthesis [2] and to pseudo-proxy data generated from transient simulations of the last glacial cycle. Our results suggest that the algorithm is capable of calculating average temperatures mostly consistent with expectations, however capturing centennial-scale variability is limited due to the low spatio-temporal distribution of the input data. This underlines the importance of both increasing the amount, resolution and age control of proxy data as well as extending the algorithm such that it also incorporates other types of paleoclimate archives. References:[1] C. W. Snyder, “Evolution of global temperature over the past two million years,” Nature, vol. 538, no. 7624, pp. 226–228, 2016[2] L. Jonkers, O. Cartapanis, M. Langner, N. McKay, S. Mulitza, A. Strack, and M. Kucera, “Integrating palaeoclimate time series with rich metadata for uncertainty modelling: Strategy and documentation of the PALMOD 130k marine palaeoclimate data synthesis,” Earth System Science Data, vol. 12, no. 2, pp. 1053–1081, 2020

Neogloboquadrina pachyderma is the dominant species of planktonic foraminifera found in polar waters and is therefore invaluable for paleoceanographic studies of the high latitudes. However, the geochemistry of this species is complicated due to the development of a thick calcite crust in its final growth stage and at greater depths within the water column. We analyzed the in situ Mg/Ca and δ18O in discrete calcite zones using LA‐ICP‐MS, EPMA and SIMS within modern N. pachyderma shells from the highly dynamic Fram Strait and the seasonally isothermal/isohaline Irminger Sea. Here we compare shell geochemistry to the measured temperature, salinity and δ18Osw in which the shells calcified to better understand the controls on N. pachyderma geochemical heterogeneity. We present a relationship between Mg/Ca and temperature in N. pachyderma lamellar calcite that is significantly different than published equations for shells that contained both crust and lamellar calcite. We also document highly variable SIMS δ18O results (up to a 3.3‰ range in single shells) on plankton tow samples which we hypothesize is due to the granular texture of shell walls. Finally, we document that the δ18O of the crust and lamellar calcite of N. pachyderma from an isothermal/isohaline environment are indistinguishable from each other, indicating that shifts in N. pachyderma δ18O are primarily controlled by changes in environmental temperature and/or salinity rather than differences in the sensitivities of the two calcite types to environmental conditions.

A comprehensive database of paleoclimate records is needed to place recent warming into the longer-term context of natural climate variability. We present a global compilation of quality-controlled, published, temperature-sensitive proxy records extending back 12,000 years through the Holocene. Data were compiled from 679 sites where time series cover at least 4000 years, are resolved at sub-millennial scale (median spacing of 400 years or finer) and have at least one age control point every 3000 years, with cut-off values slackened in data-sparse regions. The data derive from lake sediment (51%), marine sediment (31%), peat (11%), glacier ice (3%), and other natural archives. The database contains 1319 records, including 157 from the Southern Hemisphere. The multi-proxy database comprises paleotemperature time series based on ecological assemblages, as well as biophysical and geochemical indicators that reflect mean annual or seasonal temperatures, as encoded in the database. This database can be used to reconstruct the spatiotemporal evolution of Holocene temperature at global to regional scales, and is publicly available in Linked Paleo Data (LiPD) format., Measurement(s)climateTechnology Type(s)digital curationFactor Type(s)temporal interval • geographic location • proxy typeSample Characteristic - Environmentclimate systemSample Characteristic - LocationEarth (planet) Machine-accessible metadata file describing the reported data: https://www.ncdc.noaa.gov/paleo/study/27330

Planktonic foraminifera precipitate calcareous shells, which after the death of the organisms are exported from the sea surface to the sea floor, where they are preserved on geologically relevant timescales. The export flux of planktonic foraminifera shells constitutes globally up to a half, and in the studied region off Cap Blanc (Atlantic Ocean) about one third, of the marine pelagic calcite flux. Given their importance for the marine calcite budget and for the pelagic carbonate counter pump, which counteracts the biological pump in terms of oceanic capacity for intake of CO2, it is crucial to gain an understanding of factors modulating the export flux of planktonic foraminifera calcite. In principle, variability in the export flux of planktonic foraminifera calcite could depend within one species on i) shell flux, ii) shell size and iii) calcification intensity, and where shell size and calcification intensity differ among species also on the species composition of the deposited assemblage. To assess the importance of these aspects in modulating the export flux of planktonic foraminifera calcite, we investigated two annual time series (from 1990-1991 and 2007-2008) from sediment traps moored in the Cap Blanc upwelling area. We assessed the predictability of foraminifera calcite flux variability on seasonal and interannual time scales, by determining the variability in species-specific shell fluxes, shell sizes and weights with bi-weekly resolution. We find a remarkable discrepancy in the contribution of the controlling factors between seasonal and interannual scales. On the seasonal time scale, 80% of the variability of the calcite flux is explained by shell flux. On the inter-annual time scale, on the other hand, variations in shell size and calcification intensity are key to explain the calcite flux, since the time series from 2007-2008 yielded 58% larger and 11% heavier specimens. These results imply that for the global estimate of planktonic foraminifera calcite flux, shell flux is likely the most relevant predictor. However, a prediction of the temporal evolution of the calcite flux will likely require estimates of changes in shell size and calcification intensity of the involved foraminifera species.

Understanding the response of marine ecosystems to climate change requires knowledge of processes that operate over long time scales. Over the last decades, abundant data have been generated on the change in the composition of marine microplankton assemblages across the last deglaciation. These data were used to reconstruct various aspects of the ocean and climate system during this climatic upheaval; however, their potential to evaluate biotic response to climatic forcing has been rarely explored. Here, we compiled records of plankton response to the last deglaciation covering the entire North Atlantic Ocean. The records comprise assemblage composition data of marine zooplankton (planktonic foraminifera) and phytoplankton (coccolithophores, diatoms and dinoflagellates) covering the last 24 ka with a resolution of at least 1 ka. The comparability of the data is ensured by using either published age models or a combination of radiocarbon ages and correlated oxygen isotope data. We use these records to first determine the shape of the major compositional change in each record by principle components analyses and quantification of compositional turnover. The mean global response of the plankton to the deglaciation was then evaluated by an Empirical Orthogonal Function analysis of the main biotic trends across all sites. A preliminary analysis was run solely on the zooplankton data set as the phytoplankton data set is still work in progress. We find that the dominant response of the zooplankton consists of synchronous unidirectional shifts initiated between 16-17 ka BP, and progressing into the Holocene. When regressed on the global ocean temperature and CO2 trends, we can see a proportionate response to the forcing during the last glacial maximum, the deglaciation and the early Holocene. In contrast, the late Holocene is characterised by continued compositional change, which does not appear related to environmental forcing. We speculate that this decoupling indicates the existence of a multi-millennial delay in community change following the climatic forcing, likely due to biotic interactions acting on communities that have been newly assembled or geographically displaced due to abiotic forcing. We will present a similar analysis for marine phytoplankton and discuss the consequences of the observations for the understanding of community variability on millennial time scales.

La reconstrucción de la temperatura de la superficie del mar (TSM) ha estado durante mucho tiempo en el centro de la investigación paleoceanográfica. Los estudios en el Mediterráneo no han sido una excepción, ya que la reconstrucción cuantitativa de TSM en esta cuenca semicerrada es crucial para comprender el cambio climático pasado en la región. Muchos de estos métodos se basaron en foraminíferos planctónicos, tanto en su geoquímica de caparazón como en la composición de los ensamblajes (por ejemplo, funciones de transferencia). Comprender y modelar las relaciones entre el censo actual y las variables ambientales es la base para transformar los datos fósiles en estimaciones cuantitativas de estas variables. Aunque globalmente, los conjuntos de foraminíferos parecen estar determinados principalmente por la temperatura, en cuencas marginales como el Mediterráneo, In this study we attempt to determine which environmental parameters may control the variability of planktonic foraminifer assemblages in the modern Mediterranean. For this purpose, census counts of planktonic foraminifera assemblages from Mediterranean coretops (ForCenS data base) have been integrated with monthly estimates of SST, chlorophyll concentration, and vertical gradients of various parameters as proxies for water column stratification/mixing (WOA 1998). Redundancy Analysis (RDA) was used to evaluating the explanatory power and the collinearity among tested environmental parameters and a forward selection of variables was carried out to identify those explaining independently the largest share of the variance in the composition of planktonic foraminifera assemblages.Se identificaron nueve variables significativas. Tres de ellos corresponden a TSM, mientras que los otros seis se distribuyen entre las concentraciones de clorofila superficial (2) y los gradientes térmicos verticales (4). Las variables más explicativas son la TSM de junio (R 2 0.43) y el gradiente térmico vertical de diciembre (R 2 0.15).

Reconstructions of global hydroclimate during the Common Era (CE; the past ~ 2000 years) are important for providing context for current and future global environmental change. Stable isotope ratios in water are quantitative indicators of hydroclimate on regional to global scales, and these signals are encoded in a wide range of natural geologic archives. Here we present the Iso2k database, a global compilation of previously published datasets from a variety of natural archives that record the stable oxygen (δ18O) or hydrogen (δ2H) isotopic composition of environmental waters, which reflect hydroclimate changes over the CE. The Iso2k database contains 756 isotope records from the terrestrial and marine realms, including: glacier and ground ice (205); speleothems (68); corals, sclerosponges, and mollusks (145); wood (81); lake sediments and other terrestrial sediments (e.g., loess) (158); and marine sediments (99). Individual datasets have temporal resolutions ranging from sub-annual to centennial, and include chronological data where available. A fundamental feature of the database is its comprehensive metadata, which will assist both experts and non-experts in the interpretation of each record and in data synthesis. Key metadata fields have standardized vocabularies to facilitate comparisons across diverse archives and with climate model simulated fields. This is the first global-scale collection of water isotope proxy records from multiple types of geological and biological archives. It is suitable for evaluating hydroclimate processes through time and space using large-scale synthesis, model-data intercomparison and (paleo)data assimilation. The Iso2k database is available for download at: https://doi.org/10.6084/m9.figshare.11553162 (McKay and Konecky, 2020).

Temperature appears to be the best predictor of species composition of planktonic foraminifera communities, making it possible to use their fossil assemblages to reconstruct sea surface temperature (SST) variation in the past. However, the role of other environmental factors potentially modulating the spatial and vertical distribution of planktonic foraminifera species is poorly understood. This is especially relevant for environmental factors affecting the subsurface habitat. If such factors play a role, changes in the abundance of deeper dwelling species may not solely reflect SST variation. In order to constrain the effect of subsurface parameters on species composition, we here characterize the vertical distribution of living planktonic foraminifera community across the subtropical South Atlantic Ocean, where SST variability is small but the subsurface water mass structure changes dramatically. Four planktonic foraminifera communities could be identified across the top 700 m of the E–W transect. Gyre and Agulhas Leakage faunas were predominantly composed of Globigerinoides ruber, Globigerinoides tenellus, Trilobatus sacculifer, Globoturborotalita rubescens, Globigerinella calida, Tenuitella iota and Globigerinita glutinata, and only differed in terms of relative abundances (community composition). Upwelling fauna was dominated by Neogloboquadrina pachyderma, Neogloboquadrina incompta, Globorotalia crassaformis and Globorotalia inflata. Thermocline fauna was dominated by Tenuitella fleisheri, Globorotalia truncatulinoides and Globorotalia scitula in the western side, and by G. scitula in the eastern side of the basin. The largest part of the standing stock was consistently found in the surface layer, but SST was not the main predictor of species composition, neither for the total fauna at each station nor in analyses considering each depth layer separately. Instead, we identified a consistent vertical pattern in parameters controlling species composition at different depths, in which the parameters appear to reflect different aspects of the pelagic habitat. Whereas productivity appears to dominate in the mixed layer (0–60 m), physical-chemical parameters are important at depth immediately below (60–100 m), followed by parameters related to the degradation of organic matter (100–300 m), and parameters describing the dissolved oxygen availability (> 300 m). These results indicate that the seemingly straightforward relationship between assemblage composition and SST in sedimentary assemblages reflects vertically and seasonally integrated processes that are only indirectly linked to SST. This also implies that fossil assemblages of planktonic foraminifera should also contain a signature of subsurface processes, which could be used for paleoceanographic reconstructions.

The ocean—the Earth’s largest ecosystem—is increasingly affected by anthropogenic climate change1,2. Large and globally consistent shifts have been detected in species phenology, range extension and community composition in marine ecosystems3–5. However, despite evidence for ongoing change, it remains unknown whether marine ecosystems have entered an Anthropocene6 state beyond the natural decadal to centennial variability. This is because most observational time series lack a long-term baseline, and the few time series that extend back into the pre-industrial era have limited spatial coverage7,8. Here we use the unique potential of the sedimentary record of planktonic foraminifera—ubiquitous marine zooplankton—to provide a global pre-industrial baseline for the composition of modern species communities. We use a global compilation of 3,774 seafloor-derived planktonic foraminifera communities of pre-industrial age9 and compare these with communities from sediment-trap time series that have sampled plankton flux since ad 1978 (33 sites, 87 observation years). We find that the Anthropocene assemblages differ from their pre-industrial counterparts in proportion to the historical change in temperature. We observe community changes towards warmer or cooler compositions that are consistent with historical changes in temperature in 85% of the cases. These observations not only confirm the existing evidence for changes in marine zooplankton communities in historical times, but also demonstrate that Anthropocene communities of a globally distributed zooplankton group systematically differ from their unperturbed pre-industrial state. Seafloor-derived planktonic foraminifera communities of pre-industrial age are compared with communities from sediment-trap time series and show that Anthropocene communities of a globally distributed zooplankton group differ from their unperturbed pre-industrial state.

Neogloboquadrina pachyderma is the dominant species in the polar regions. In the northern high latitude ocean, it makes up more than 90 % of the total planktonic foraminifera assemblages, making it the dominant pelagic calcifier and carrier of paleoceanographic proxies. To assess the reaction of this species to future climate change and to be able to interpret the paleoecological signal contained in its shells, its habitat depth must be known. Previous work showed that N. pachyderma in the northern polar regions has a highly variable depth habitat, ranging from the surface mixed layer to several hundreds of metres below the surface, and the origin of this variability remained unclear. In order to investigate the factors controlling the habitat depth of N. pachyderma, we compiled new and existing population density profiles from 104 stratified plankton tow hauls collected in the Arctic and the North Atlantic Oceans during 14 oceanographic expeditions. For each vertical profile, the Depth Habitat (DH) was calculated as the abundance-weighted mean depth of occurrence. We then tested to what degree environmental factors (mixed layer depth, sea surface temperature, sea surface salinity, Chlorophyll a concentration and sea ice concentration) and ecological factors (synchronised reproduction and daily vertical migration) can predict the observed DH variability and compared the observed DH behaviour with simulations by a numerical model predicting planktonic foraminifera distribution. Our data show that the DH of N. pachyderma varies between 25 m and 280 m (average ~ 100 m). In contrast with the model simulations, which indicate that DH is associated with the depth of chlorophyll maximum, our analysis indicates that the presence of sea-ice together with the concentration of chlorophyll at the surface have the strongest influence on the vertical habitat of this species. N. pachyderma occurs deeper when sea-ice and chlorophyll concentrations are low, suggesting a time transgressive response to the evolution of (near) surface conditions during the annual cycle. Since only surface parameters appear to affect the vertical habitat of N. pachyderma, light or light-dependant processes might influence the ecology of this species. Our results can be used to improve predictions of the response of the species to climate change and thus to refine paleoclimatic reconstructions.

The ocean-the Earth's largest ecosystem-is increasingly affected by anthropogenic climate change

[1] Past water column stratification can be assessed through comparison of theδ18O of different planktonic foraminiferal species. The underlying assumption is that different species form their shellssimultaneously, but at different depths in the water column. We evaluate thisassumption using a sediment trap time-series of Neogloboquadrinapachyderma(s) and Globigerina bulloides from the NWNorth Atlantic. We determined fluxes, δ18O and δ13C of shells from two size fractionsto assess size-related effects on shell chemistry and to better constrain the underlying causes of isotopic differences between foraminifera in deep-sea sediments. Our data indicate that in the subpolar North Atlantic differences in the seasonality of the shell flux, and not in depth habitat or test size, determine the interspecies Δδ18O. N. pachyderma(s) preferentially forms from early spring to late summer, whereas the flux ofG. bulloides peaks later in the season and is sustained until autumn.Likewise, seasonality influences large and small specimens differently, with large shells settling earlier in the season. [2] The similarity of the seasonal δ18O patterns between the two species indicatesthat theycalcify in an overlapping depth zone close to the surface. However, their δ13C patterns are markedly different (>1 ‰). Bothspecies have a seasonally variable offset from δ13CDIC that appears to be governed primarily by temperature, with larger offsets associated with higher temperatures. The variable offset from δ13CDIC implies that seasonality of the flux affects the fossil δ13C signal, which has implications for reconstruction of the past oceanic carbon cycle.