Description: The response of the marine carbon cycle to changes in atmospheric CO2 concentrations will be determined, in part, by the relative response of calcifying and non-calcifying organisms to global change. Planktonic foraminifera are responsible for a quarter or more of global carbonate production, therefore understanding the sensitivity of calcification in these organisms to environmental change is critical. Despite this, there remains little consensus as to whether, or to what extent, chemical and physical factors affect foraminiferal calcification. To address this, we directly test the effect of multiple controls on calcification in culture experiments and core-top measurements of Globigerinoides ruber. We find that two factors, body size and the carbonate system, strongly influence calcification intensity in life, but that exposure to corrosive bottom waters can overprint this signal post mortem. Using a simple model for the addition of calcite through ontogeny, we show that variable body size between and within datasets could complicate studies that examine environmental controls on foraminiferal shell weight. In addition, we suggest that size could ultimately play a role in determining whether calcification will increase or decrease with acidification. Our models highlight that knowledge of the specific morphological and physiological mechanisms driving ontogenetic change in calcification in different species will be critical in predicting the response of foraminiferal calcification to future change in atmospheric pCO2.

Description: The process by which foraminifera precipitate calcite from seawater has received much attention, in part because a mechanistic basis for empirical calibrations between shell chemistry and environmental parameters is desirable given their widespread application in palaeoceanography. The incorporation of fluorescent membrane-impermeable molecules into the shell demonstrates that seawater, transported by vacuolisation, is present at the site of calcification. However, recent discussion has focused on whether the calcium required for chamber formation is sourced predominantly by transmembrane Ca transport (TMT), with seawater vacuolisation playing a passive role, or vice versa. This debate has arisen in part because of the need to explain the low Mg/Ca ratio of most foraminifera compared to inorganic calcite. Here, we present trace element data of Operculina ammonoides and Globigerinoides ruber, a high-Mg shallow benthic, and low-Mg planktonic species respectively, cultured under variable seawater carbonate and elemental chemistries. We find that Mg incorporation in high and low-Mg species is characterised by an opposite response to the carbonate system, demonstrating that the negative relationship between Mg/Ca and pH or [ CO32- ] in several low-Mg foraminifera is not an intrinsic feature of foraminiferal (or inorganic) calcite precipitation. Therefore, any biomineralisation model must be able to explain why the mechanism by which seawater Mg/Ca is reduced is impacted by the carbonate system. Moreover, we show that trace element incorporation in G. ruber is consistent with Rayleigh fractionation from unmodified seawater except for Mg-removal, but in very poor agreement with a biomineralisation site [Ca] substantially elevated above that of seawater as required by the TMT hypothesis. In addition, any biomineralisation model must explain the nonlinear relationship between seawater and shell Mg/Ca, and the large number of seawater vacuoles observed in some species. Although there are important inter-species differences in biomineralisation, evident from the observed range of shell Mg/Ca ratios, we argue that these differences are mechanistically related to the degree of Mg exclusion prior to chamber formation. Indeed, whilst our data for both low-Mg and high-Mg species are consistent with biomineralisation via ions sourced through seawater vacuolisation, it is difficult to reconcile many of these observations with a model based on significant transmembrane Ca transport.

Description: Past warm periods provide an opportunity to evaluate climate models under extreme forcing scenarios, in particular high ( 〉  800 ppmv) atmospheric CO2 concentrations. Although a post hoc intercomparison of Eocene ( ∼  50  Ma) climate model simulations and geological data has been carried out previously, models of past high-CO2 periods have never been evaluated in a consistent framework. Here, we present an experimental design for climate model simulations of three warm periods within the early Eocene and the latest Paleocene (the EECO, PETM, and pre-PETM). Together with the CMIP6 pre-industrial control and abrupt 4 ×  CO2 simulations, and additional sensitivity studies, these form the first phase of DeepMIP – the Deep-time Model Intercomparison Project, itself a group within the wider Paleoclimate Modelling Intercomparison Project (PMIP). The experimental design specifies and provides guidance on boundary conditions associated with palaeogeography, greenhouse gases, astronomical configuration, solar constant, land surface processes, and aerosols. Initial conditions, simulation length, and output variables are also specified. Finally, we explain how the geological data sets, which will be used to evaluate the simulations, will be developed.

Description: Accurate estimates of past global mean surface temperature (GMST) help to contextualise future climate change and are required to estimate the sensitivity of the climate system to CO2 forcing through Earth's history. Previous GMST estimates for the latest Paleocene and early Eocene (∼57 to 48 million years ago) span a wide range (∼9 to 23 ∘C higher than pre-industrial) and prevent an accurate assessment of climate sensitivity during this extreme greenhouse climate interval. Using the most recent data compilations, we employ a multi-method experimental framework to calculate GMST during the three DeepMIP target intervals: (1) the latest Paleocene (∼57 Ma), (2) the Paleocene–Eocene Thermal Maximum (PETM; 56 Ma), and (3) the early Eocene Climatic Optimum (EECO; 53.3 to 49.1 Ma). Using six different methodologies, we find that the average GMST estimate (66 % confidence) during the latest Paleocene, PETM, and EECO was 26.3 ∘C (22.3 to 28.3 ∘C), 31.6 ∘C (27.2 to 34.5 ∘C), and 27.0 ∘C (23.2 to 29.7 ∘C), respectively. GMST estimates from the EECO are ∼10 to 16 ∘C warmer than pre-industrial, higher than the estimate given by the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report (9 to 14 ∘C higher than pre-industrial). Leveraging the large “signal” associated with these extreme warm climates, we combine estimates of GMST and CO2 from the latest Paleocene, PETM, and EECO to calculate gross estimates of the average climate sensitivity between the early Paleogene and today. We demonstrate that “bulk” equilibrium climate sensitivity (ECS; 66 % confidence) during the latest Paleocene, PETM, and EECO is 4.5 ∘C (2.4 to 6.8 ∘C), 3.6 ∘C (2.3 to 4.7 ∘C), and 3.1 ∘C (1.8 to 4.4 ∘C) per doubling of CO2. These values are generally similar to those assessed by the IPCC (1.5 to 4.5 ∘C per doubling CO2) but appear incompatible with low ECS values (〈1.5 per doubling CO2).

Description: The early Eocene (56 to 48 million years ago) is inferred to have been the most recent time that Earth's atmospheric CO2 concentrations exceeded 1000 ppm. Global mean temperatures were also substantially warmer than present day. As such, study of early Eocene climate provides insight into how a super-warm Earth system behaves and offers an opportunity to 10 evaluate climate models under conditions of high greenhouse gas forcing. The Deep Time Model Intercomparison Project (DeepMIP) is a systematic model-model and model-data intercomparison of three early Paleogene time slices: latest Paleocene, Paleocene-Eocene thermal maximum and early Eocene climatic optimum. A previous article outlined the model experimental design for climate model simulations. In this article, we outline the methodologies to be used for the compilation and analysis of climate proxy data, primarily proxies for temperature and CO2. This paper establishes the protocols for a concerted and 15 coordinated effort to compile the climate proxy records across a wide geographic range. The resulting climate "atlas" will be used to constrain and evaluate climate models for the three selected time intervals, and provide insights into the mechanisms that control these warm climate states. We provide version 0.1 of this database, in anticipation that this will be expanded in subsequent publications.

Description: Paleotemperature proxy data form the cornerstone of paleoclimate research and are integral to understanding the evolution of the Earth system across the Phanerozoic Eon. Here, we present PhanSST, a database containing over 150,000 data points from five proxy systems that can be used to estimate past sea surface temperature. The geochemical data have a near-global spatial distribution and temporally span most of the Phanerozoic. Each proxy value is associated with consistent and queryable metadata fields, including information about the location, age, and taxonomy of the organism from which the data derive. To promote transparency and reproducibility, we include all available published data, regardless of interpreted preservation state or vital effects. However, we also provide expert-assigned diagenetic assessments, ecological and environmental flags, and other proxy-specific fields, which facilitate informed and responsible reuse of the database. The data are quality control checked and the foraminiferal taxonomy has been updated. PhanSST will serve as a valuable resource to the paleoclimate community and has myriad applications, including evolutionary, geochemical, diagenetic, and proxy calibration studies.

Description: Acesta excavata (Fabricius, 1779) is a slow growing bivalve from the Limidae family and is often found associated with cold-water coral reefs along the European continental margin. Here we present the compositional variability of frequently used proxy elemental ratios (Mg/Ca, Sr/Ca, Na/Ca) measured by laser-ablation mass spectrometry (LA-ICP-MS) and compare it to in-situ recorded instrumental seawater parameters such as temperature and salinity. Shell Mg/Ca measured in the fibrous calcitic shell section was overall not correlated with seawater temperature or salinity; however, some samples show significant correlations with temperature with a sensitivity that was found to be unusually high in comparison to other marine organisms. Mg/Ca and Sr/Ca measured in the fibrous calcitic shell section display significant negative correlations with the linear extension rate of the shell, which indicates strong vital effects in these bivalves. Multiple linear regression analysis indicates that up to 79% of elemental variability is explicable with temperature and salinity as independent predictor values. Yet, the overall results clearly show that the application of Element/Ca (E/Ca) ratios in these bivalves to reconstruct past changes in temperature and salinity is likely to be complicated due to strong vital effects and the effects of organic material embedded in the shell. Therefore, we suggest to apply additional techniques, such as clumped isotopes, in order to exactly determine and quantify the underlying vital effects and possibly account for these. We found differences in the chemical composition between the two calcitic shell layers that are possibly explainable through differences of the crystal morphology. Sr/Ca ratios also appear to be partly controlled by the amount of magnesium, because the small magnesium ions bend the crystal lattice which increases the space for strontium incorporation. Oxidative cleaning with H2O2 did not significantly change the Mg/Ca and Sr/Ca composition of the shell. Na/Ca ratios decreased after the oxidative cleaning, which is most likely a leaching effect and not caused by the removal of organic matter.

Description: Hyrrokkin sarcophaga is a parasitic foraminifera that is commonly found in cold-water coral reefs where it infests the file clam Acesta excavata and the scleractinian coral Desmophyllum pertusum (formerly known as Lophelia pertusa). Here, we present measurements of the trace element and isotopic composition of these parasitic foraminifera, analyzed by inductively coupled optical emission spectrometry (ICP-OES), electron probe microanalysis (EPMA) and mass spectrometry (gas-source MS and inductively-coupledplasma MS). Our results reveal that the geochemical signature of H. sarcophaga depends on the host organism it infests. Sr / Ca ratios are 1.1 mmol mol 1 higher in H. sarcophaga that infest D. pertusum, which could be an indication that dissolved host carbonate material is utilized in shell calcification, given that the aragonite of D. pertusum has a naturally higher Sr concentration compared to the calcite of A. excavata. Similarly, we measure 3.1 parts per thousand lower delta C-13 and 0.25 parts per thousand lower delta O-18 values in H. sarcophaga that lived on D. pertusum, which might be caused by the direct uptake of the host's carbonate material with a more negative isotopic composition or different pH regimes in these foraminifera (pH can exert a control on the extent of CO2 hydration/hydroxylation) due to the uptake of body fluids of the host. We also observe higher Mn / Ca ratios in foraminifera that lived on A. excavata but did not penetrate the host shell compared to specimen that penetrated the shell, which could be interpreted as a change in food source, changes in the calcification rate, Rayleigh fractionation or changing oxygen conditions. While our measurements provide an interesting insight into the calcification process of this unusual foraminifera, these data also indicate that the geochemistry of this parasitic foraminifera is unlikely to be a reliable indicator of paleoenvironmental conditions using Sr / Ca, Mn / Ca, delta O-18 or delta C-13 unless the host organism is known and its geochemical composition can be accounted for.

Description: Here we present a comprehensive attempt to correlate aragonitic Na∕Ca ratios from Desmophyllum pertusum (formerly known as Lophelia pertusa), Madrepora oculata and a caryophylliid cold-water coral (CWC) species with different seawater parameters such as temperature, salinity and pH. Living CWC specimens were collected from 16 different locations and analyzed for their Na∕Ca ratios using solution-based inductively coupled plasma-optical emission spectrometry (ICP-OES) measurements. The results reveal no apparent correlation with salinity (30.1–40.57 g kg−1) but a significant inverse correlation with temperature (−0.31±0.04  mmolmol−1∘C−1). Other marine aragonitic organisms such as Mytilus edulis (inner aragonitic shell portion) and Porites sp. exhibit similar results highlighting the consistency of the calculated CWC regressions. Corresponding Na∕Mg ratios show a similar temperature sensitivity to Na∕Ca ratios, but the combination of two ratios appears to reduce the impact of vital effects and domain-dependent geochemical variation. The high degree of scatter and elemental heterogeneities between the different skeletal features in both Na∕Ca and Na∕Mg, however, limit the use of these ratios as a proxy and/or make a high number of samples necessary. Additionally, we explore two models to explain the observed temperature sensitivity of Na∕Ca ratios for an open and semi-enclosed calcifying space based on temperature-sensitive Na- and Ca-pumping enzymes and transport proteins that change the composition of the calcifying fluid and consequently the skeletal Na∕Ca ratio.

Description: Rationale Potassium (K) is a major component of several silicate minerals and seawater, and, therefore, constraining past changes in the potassium cycle is a promising way of tracing large‐scale geological processes on Earth. However, [K] measurement using inductively coupled plasma mass spectrometry (ICP‐MS) is challenging due to an ArH + interference, which may be of a similar magnitude to the K + ion beam in samples with 〈0.1% m/m [K]. Methods In this work, we investigated the effect of the ArH + interference on K/Ca data quality by comparing results from laser‐ablation (LA)‐ICP‐MS measured in medium and high mass resolution modes and validating our LA results via solution ICP‐optical emission spectroscopy (OES) and solution ICP‐MS measurements. To do so, we used a wide range of geological reference materials, with a particular focus on marine carbonates, which are potential archives of past changes in the K cycle but are typically characterised by [K] 〈 200 μg/g. In addition, we examine the degree to which trace‐element data quality is driven by downhole fractionation during LA‐ICP‐MS measurements. Results Our results show that medium mass resolution (MR) mode is sufficiently capable of minimising the effect of the ArH + interference on K + . However, the rate of downhole fractionation for Na and K varies between different samples as a result of their differing bulk composition, resulting in matrix‐specific inaccuracy. We show how this can be accounted for via downhole fractionation corrections, resulting in an accuracy of better than 1% and a long‐term reproducibility (intermediate precision) of 〈6% (relative standard deviation) in JCp‐1NP using LA‐ICP‐MS in MR mode. Conclusion Our [K] measurement protocol is demonstrably precise and accurate and applicable to a wide range of materials. The measurement of K/Ca in relatively low‐[K] marine carbonates is presented here as a key example of a new application opened up by these advances.