Planktonic foraminiferal Mg/Ca is one of the most widely applied sea surface temperature proxies. While the influence of salinity on Mg/Ca has led the accuracy of Mg/Ca‐temperatures to be questioned, the effect of seawater carbonate chemistry (pH) is seldom accounted for down‐core. Using published data sets, we review controls on Mg/Ca in laboratory cultures of planktonic foraminifera Globigerinoides ruber (white), Trilobatus sacculifer, Globigerina bulloides, and Orbulina universa. All are characterized by a temperature sensitivity of ~6% per °C and a salinity sensitivity of ~4% per salinity unit; all except T. sacculifer are sensitive to carbonate chemistry (−5 to −9% per 0.1 pH units). We demonstrate the down‐core manifestation of these sensitivities using data spanning the last deglaciation and an Earth System Model forced with last glacial maximum conditions. While the effects of salinity are relatively minor, seawater carbonate chemistry exerts a large bias on Mg/Ca temperature if unaccounted for; however, as local pH changes beyond the effect of atmospheric CO2 are relatively small across most of the ocean (less than ±0.05 pH units, 2σ), atmospheric CO2 can be used to accurately correct Mg/Ca temperatures. We present protocols to correct Mg/Ca for pH down‐core using either atmospheric CO2 or (preferably) boron isotopes in a new software package “MgCaRB.”

Abstract. 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 those of the present day. As such, the study of early Eocene climate provides insight into how a super-warm Earth system behaves and offers an opportunity to 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 (PETM) and early Eocene climatic optimum (EECO). 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 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.

Past greenhouse periods with elevated atmospheric CO were characterized by globally warmer sea-surface temperatures (SST). However, the extent to which the high latitudes warmed to a greater degree than the tropics (polar amplification) remains poorly constrained, in particular because there are only a few temperature reconstructions from the tropics. Consequently, the relationship between increased CO, the degree of tropical warming, and the resulting latitudinal SST gradient is not well known. Here, we present coupled clumped isotope (Δ)-Mg/Ca measurements of foraminifera from a set of globally distributed sites in the tropics and midlatitudes. Δ is insensitive to seawater chemistry and therefore provides a robust constraint on tropical SST. Crucially, coupling these data with Mg/Ca measurements allows the precise reconstruction of Mg/Ca throughout the Eocene, enabling the reinterpretation of all planktonic foraminifera Mg/Ca data. The combined dataset constrains the range in Eocene tropical SST to 30-36 °C (from sites in all basins). We compare these accurate tropical SST to deep-ocean temperatures, serving as a minimum constraint on high-latitude SST. This results in a robust conservative reconstruction of the early Eocene latitudinal gradient, which was reduced by at least 32 ± 10% compared with present day, demonstrating greater polar amplification than captured by most climate models.

Foraminifera Mg/Ca paleothermometry forms the basis of a substantial portion of ocean temperature reconstruction over the last 5 Ma. Furthermore, coupled Mg/Ca-oxygen isotope (δ 18 O) measurements of benthic foraminifera can constrain eustatic sea level (ESL) independent of paleoshoreline derived approaches. However, this technique suffers from uncertainty regarding the secular variation of the Mg/Ca seawater ratio (Mg/Ca sw) on timescales of millions of years. Here we present coupled seawater-test Mg/Ca-temperature laboratory calibrations of Globigerinoides ruber in order to test the widely held assumptions that (1) seawater-test Mg/Ca co-vary linearly, and (2) the Mg/Ca-temperature sensitivity remains constant with changing Mg/Ca sw. We find a nonlinear Mg/Ca test-Mg/Ca sw relationship and a substantial lowering of the Mg/Ca-temperature sensitivity at lower than modern Mg/Ca sw from 9.0%°C-1 at Mg/Ca sw = 5.2 mol mol-1 to 7.5±0.9%°C-1 at 3.4 mol mol-1. Using our calibrations to more accurately calculate the offset between Mg/Ca and biomarkerderived paleotemperatures for four sites, we derive a Pliocene Mg/Ca sw ratio of ~4.3 mol mol-1. This

[1] The Mg/Ca ratio of foraminifera tests is increasingly being utilized as a paleotemperature proxy. Deep time (pre-Pleistocene) Mg/Ca paleothermometry is complicated by the fact that the Mg/Ca ratio of seawater (Mg/Ca sw ) has undergone considerable secular variation over the Cenozoic. Previous studies have corrected for this by assuming an invariant Mg distribution coefficient (D Mg ) with Mg/Ca sw . More recent laboratory culturing has shown that this is not the case, demonstrating that a power relationship best describes the variation in test Mg/Ca (Mg/Ca test ) with Mg/Ca sw . Therefore, previous corrections are likely to have led to inaccurate temperature reconstructions. Here, we show how the systematics of such a correction should be applied and demonstrate why this provides good evidence that the Mg/Ca ratio of Paleogene seawater was lower than previously implied by foraminiferal constraints, in agreement with the majority of the proxy Mg/Ca sw evidence. We also demonstrate how it is indirectly possible to constrain the value of H, the power component of a Mg/Ca test -Mg/Ca sw calibration, potentially enabling the appropriate correction of results derived from species where this relationship has not been calibrated. However, this technique should not be treated as a substitute for culturing. The previous erroneous assumptions regarding both (1) the relationship between Mg/Ca test and Mg/Ca sw and (2) the Mg/Ca ratio of seawater at a given time in the past may counteract each other to differing extents. As a result, previous absolute pre-Pleistocene paleotemperature estimates derived from Mg/Ca ratios in foraminifera should be treated with caution, although relative temperature changes over short (<1 Ma timescales) are likely to be reliable.