Productivity changes across the mid-Pleistocene climate transition

We use benthic foraminiferal accumulation rates as a proxy for productivity changes during the mid-Pleistocene climate transition (MPT) (~1.2 Ma to 0.4 Ma). Our data are chosen to test the hypothesis that longer-term cooling and the onset of 100 kyr cyclicity are linked to atmospheric CO2 draw-down associated with an increase in primary productivity. To this end, we have constructed records from a global array of seven sites spanning major ocean basins and representing different hydrographic regimes (e.g., high and low latitudes, upwelling versus the deep western warm pools). We compare our data to published productivity proxy records from each site to identify limitations and uncertainties in the reconstructions. Results indicate that there is evidence for productivity increases during the onset of the MPT (1.2–1.0 Ma), but the changes are not globally synchronous and likely reflect regional hydrographic variability. On the orbital scale, productivity maxima tend to occur more closely related to glacial than interglacial intervals overall, but the relationships are not consistent. High interglacial productivity characterizes low latitude sites some of the time. In the obliquity band, high interglacial productivity in the eastern equatorial Pacific coincides with low interglacial productivity in the Southern Ocean, supporting a high to low latitude link via intermediate water circulation distribution of nutrients. On the regional scale, our records contribute new evidence for changes in Northern Hemisphere frontal systems during the MPT and for a close link between surface ocean production of organic matter and consumption on the ocean floor in the western tropical Atlantic. Pyrite counts at the two Southern Ocean sites provide supporting evidence for sluggish thermohaline overturn during the mid-point of the MPT at ~900 ka. Taken together, our records do not show a globally synchronous productivity signal that would support the biological pump as a driver for potential CO2-induced climate cooling during the MPT. Instead, we document complex regional variations in the carbon cycle, reflecting a combination of both biological and physical processes both on the longer as well as on the orbital time-scale

The Mg/Ca–temperature relationship in brachiopod shells: calibrating a potential palaeoseasonality proxy

Brachiopods are long-lived, long-ranging, extant organisms, of which some groups precipitate a relatively diagenetically stable low magnesium calcite shell. Previous work has suggested that the incorporation of Mg into brachiopod calcite may be controlled by temperature (Brand et al., 2013). Here we build upon this work by using laser ablation sampling to define the intra-shell variations in two modern brachiopod species,Terebratulina retusa (Linnaeus, 1758) and Liothyrella neozelanica (Thomson, 1918). We studied three T. retusa shells collected live from the Firth of Lorne, Scotland, which witnessed annual temperature variations on the order of 7 °C, in addition to four L. neozelanica shells, which were dredged from a water depth transect (168–1488 m) off the north coast of New Zealand. The comparison of intra-shell Mg/Ca profiles with shell δ<sup>18</sup>O confirms a temperature control on brachiopod Mg/Ca and supports the use of brachiopod Mg/Ca as a palaeoseasonality indicator. Our preliminary temperature calibrations are Mg/Ca = 1.76 ± 0.27 e<sup>(0.16 ± 0.03)T</sup>, R<sup>2</sup> = 0.75, for T. retusa and Mg/Ca = 0.49 ± 1.27 e<sup>(0.2 ± 0.11)T</sup>, R<sup>2</sup> = 0.32, for L. neozelanica (errors are 95% confidence intervals)

Carbon cycle feedbacks during the Oligocene-Miocene transient glaciation

Ice sheet models suggest that once formed, the large, high- altitude East Antarctic Ice Sheet was relatively self-stabilizing, due to its cold upper surface. The ice sheet hysteresis problem results from an inability to reconcile this expectation with geological evidence for episodes of ice sheet retreat. A classic example of this problem is manifested in benthic foraminiferal oxygen isotope records across the Oligocene-Miocene boundary (ca. 23 Ma), which display a transient ~1‰ excursion to higher values. The inferred increase and subsequent decrease in ice volume has been linked to advance and retreat of the Antarctic ice sheet across the continental shelf. However, oxygen isotope records alone do not provide unambiguous records of temperature and ice volume, hindering assessment of the driving mechanism for these variations. Here we present new benthic foraminiferal Mg/Ca, Li/Ca, and U/Ca records across the Oligocene- Miocene boundary from Ocean Drilling Program Sites 926 and 929. Our records demonstrate that Atlantic bottom-water temperatures varied cyclically, with the main cooling and warming steps followed by ice growth and decay respectively. We suggest that enhanced organic carbon burial acted as a positive feedback as climate cooled. Several lines of evidence suggest that the deglaciation was associated with an input of carbon to the ocean-atmosphere system, culminating in a previously unidentified seafloor dissolution event. We suggest that one of the initial sources of carbon was organic matter oxidation in ocean sediments. This study demonstrates that carbon cycle feedbacks should be considered when evaluating the stability of ancient ice sheets

Multi-elemental composition of authigenic carbonates in benthic foraminifera from the eastern Bering Sea continental margin (International Ocean Discovery Program Site U1343)

Bering Sea sediments represent exceptional archives, offering the potential to study past climates and biogeochemistry at a high resolution. However, abundant hydrocarbons of microbial origin, especially along the eastern Bering Sea continental margin, can hinder the applicability of palaeoceanographic proxies based on calcareous foraminifera, due to the formation of authigenic carbonates. Nonetheless, authigenic carbonates may also bear unique opportunities to reconstruct changes in the sedimentary redox environment. Here we use a suite of visual and geochemical evidence from single-specimens of the shallow infaunal benthic foraminiferal species Elphidium batialis Saidova (1961), recovered from International Ocean Discovery Program (IODP) Site U1343 in the eastern Bering Sea, to investigate the influence of authigenic carbonates on the foraminiferal trace metal composition. Our results demonstrate that foraminiferal calcite tests act as a nucleation template for secondary carbonate precipitation, altering their geochemistry where organoclastic sulphate reduction and anaerobic oxidation of methane cause the formation of low- and high-Mg calcite, respectively. The authigenic carbonates can occur as encrusting on the outside and/or inside of foraminiferal tests, in the form of recrystallization of the test wall, or as banding along natural laminations within the foraminiferal test walls. In addition to Mg, authigenic carbonates are enriched in U/Ca, Mn/Ca, Fe/Ca, and Sr/Ca, depending on the redox environment that they were formed in. Our results demonstrate that site-specific U/Ca thresholds are a promising tool to distinguish between diagenetically altered and pristine foraminiferal samples, important for palaeoceanographic reconstructions utilising the primary foraminiferal geochemistry. Consistent with previous studies, U/Mn ratios of foraminifera at IODP Site U1343 increase according to their degree of diagenetic alteration, suggesting a potential response of authigenic U/Mn to the microbial activity in turn linked to the sedimentary redox environment

Temperature and carbonate ion effects on Mg/Ca and Sr/Ca ratios in benthic foraminifera : aragonitic species Hoeglundina elegans

Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 21 (2006): PA1007, doi:10.1029/2005PA001158.Core top samples from Atlantic (Little Bahama Banks (LBB)) and Pacific (Hawaii and Indonesia) depth transects have been analyzed in order to assess the influence of bottom water temperature (BWT) and aragonite saturation levels on Mg/Ca and Sr/Ca ratios in the aragonitic benthic foraminifer Hoeglundina elegans. Both the Mg/Ca and Sr/Ca ratios in H. elegans tests show a general decrease with increasing water depth. Although at each site the decreasing trends are consistent with the in situ temperature profile, Mg/Ca and Sr/Ca ratios in LBB are substantially higher than in Indonesia and Hawaii at comparable water depths with a greater difference observed with increasing water depth. Because we find no significant difference between results obtained on “live” and “dead” specimens, we propose that these differences are due to primary effects on the metal uptake during test formation. Evaluation of the water column properties at each site suggests that in situ CO3 ion concentrations play an important role in determining the H. elegans Mg/Ca and Sr/Ca ratios. The CO3 ion effect is limited, however, only to aragonite saturation levels ([ΔCO3]aragonite) below 15 μmol kg−1. Above this level, temperature exerts a dominant effect. Accordingly, we propose that Mg/Ca and Sr/Ca in H. elegans tests can be used to reconstruct thermocline temperatures only in waters oversaturated with respect to the mineral aragonite using the following relationships: Mg/Ca = (0.034 ± 0.002)BWT + (0.96 ± 0.03) and Sr/Ca = (0.060 ± 0.002)BWT + (1.53 ± 0.03) (for [ΔCO3]aragonite > 15 μmol kg−1). The standard error associated with these equations is about ±1.1°C. Reconstruction of deeper water temperatures is complicated because in undersaturated waters, changes in Mg/Ca and Sr/Ca ratios reflect a combination of changes in [CO3] and BWT. Overall, we find that Sr/Ca, rather than Mg/Ca, in H. elegans may be a more accurate proxy for reconstructing paleotemperatures.Yair Rosenthal acknowledges the support of Amtzia Genin and the Hebrew University, Forchheimer Fellowship, during his sabbatical in the Inter-University Institute in Eilat, Israel. This project has been funded by NSF Awards OCE 0220922 to Y.R. and OCE 0220776 to D.W.O. and B.K.L

Cenozoic benthic foraminiferal Mg/Ca and Li/Ca records: toward unlocking temperatures and saturation states

The sensitivities of benthic foraminiferal Mg/Ca and Li/Ca to bottom water temperature and carbonate saturation state have recently been assessed. Here we present a new approach that uses paired Mg/Ca and Li/Ca records to calculate simultaneous changes in temperature and saturation state. Using previously published records, we first use this approach to document a cooling of deep ocean waters associated with the establishment of the Antarctic ice sheet at the Eocene-Oligocene climate transition. We then apply this approach to new records of the Middle Miocene Climate Transition from ODP Site 761 to estimate variations in bottom water temperature and the oxygen isotopic composition of seawater. We estimate that the oxygen isotopic composition of seawater varied by ∼1‰ between the deglacial extreme of the Miocene Climatic Optimum and the glacial maximum following the Middle Miocene Climate Transition, indicating large amplitude variations in ice volume. However, the longer-term change between 15.3 and 12.5 Ma is marked by a ∼1°C cooling of deep waters, and an increase in the oxygen isotopic composition of seawater of ∼0.6‰. We find that bottom water saturation state increased in the lead up to the Middle Miocene Climate Transition and decreased shortly after. This supports decreasing pCO2 as a driver for global cooling and ice sheet expansion, in agreement with existing boron isotope and leaf stomatal index CO2 records but in contrast to the published alkenone CO2 records

Cenozoic climate changes: A review based on time series analysis of marine benthic δ18O records

The climate during the Cenozoic era changed in several steps from ice-free poles and warm conditions to ice-covered poles and cold conditions. Since the 1950s, a body of information on ice-volume and temperature changes has been built up predominantly on the basis of measurements of the oxygen isotopic composition of shells of benthic foraminifera collected from marine sediment cores. The statistical methodology of time series analysis has also evolved, allowing more information to be extracted from these records. Here we provide a comprehensive view of Cenozoic climate evolution by means of a coherent and systematic application of time-series analytical tools to each record from a compilation spanning the interval from 4 to 61 Myr ago. We quantitatively describe several prominent features of the oxygen isotope record, taking into account the various sources of uncertainty (including measurement, proxy noise, and dating errors). The estimated transition times and amplitudes allow us to assess causal climatological-tectonic influences on the following known features of the Cenozoic oxygen isotopic record: Paleocene-Eocene Thermal Maximum, Eocene-Oligocene Transition, Oligocene-Miocene Boundary, and the Middle Miocene Climate Optimum. We further describe and causally interpret the following features: Paleocene-Eocene warming trend; the two-step, long-term Eocene cooling; and the changes within the most recent interval (Miocene-Pliocene). We review the scope and methods of constructing Cenozoic stacks of benthic oxygen isotope records and present two new latitudinal stacks, which capture besides global ice volume also bottom-water temperatures at low (less than 30◦) and high latitudes. This review concludes with an identification of future directions for data collection, statistical method development, and climate modeling

Arctic Ocean benthic foraminifera Mg/Ca ratios and global Mg/Ca-temperature calibrations: New constraints at low temperatures

We explore the use of Mg/Ca ratios in six Arctic Ocean benthic foraminifera species as bottom water palaeothermometers and expand published Mg/Ca-temperature calibrations to the coldest bottom temperatures (<1 °C). Foraminifera were analyzed in surface sediments at 27 sites in the Chukchi Sea, East Siberian Sea, Laptev Sea, Lomonosov Ridge and Petermann Fjord. The sites span water depths of 52–1157 m and bottom water temperatures (BWT) of −1.8 to +0.9 °C. Benthic foraminifera were alive at time of collection, determined from Rose Bengal (RB) staining. Three infaunal and three epifaunal species were abundant enough for Mg/Ca analysis. As predicted by theory and empirical evidence, cold water Arctic Ocean benthic species produce low Mg/Ca ratios, the exception being the porcelaneous species Quinqueloculina arctica. Our new data provide important constraints at the cold end (<1 °C) when added to existing global datasets. The refined calibrations based on the new and published global data appear best supported for the infaunal species Nonionella labradorica (Mg/Ca = 1.325 ± 0.01 × e^(0.065 ± 0.01 × BWT), r2 = 0.9), Cassidulina neoteretis (Mg/Ca = 1.009 ± 0.02 × e^(0.042 ± 0.01 × BWT), r2 = 0.6) and Elphidium clavatum (Mg/Ca = 0.816 ± 0.06 + 0.125 ± 0.05 × BWT, r2 = 0.4). The latter is based on the new Arctic data only. This suggests that Arctic Ocean infaunal taxa are suitable for capturing at least relative and probably semi-quantitative past changes in BWT. Arctic Oridorsalis tener Mg/Ca data are combined with existing O. umbonatus Mg/Ca data from well saturated core-tops from other regions to produce a temperature calibration with minimal influence of bottom water carbonate saturation state (Mg/Ca = 1.317 ± 0.03 × e^(0.102 ± 0.01 BWT), r2 = 0.7). The same approach for Cibicidoides wuellerstorfi yields Mg/Ca = 1.043 ± 0.03 × e^(0.118 ± 0.1 BWT), r2 = 0.4. Mg/Ca ratios of the porcelaneous epifaunal species Q. arctica show a clear positive relationship between Mg/Ca and Δ[CO32−] indicating that this species is not suitable for Mg/Ca-palaeothermometry at low temperatures, but may be useful in reconstructing carbonate system parameters through time

Cenozoic seawater Sr/Ca evolution

Records of seawater chemistry help constrain temporal variations in geochemical processes that impact the global carbon cycle and climate through Earth’s history. Here we reconstruct Cenozoic seawater Sr/Ca (Sr/Casw) using fossil Conus and turritellid gastropod Sr/Ca. Combined with an oxygen isotope paleotem- perature record from the same samples, the gastropod record suggests that Sr/Casw was slightly higher in the Eocene ($11.4 ` 3 mmol/mol) than today ($8.54 mmol/mol) and remained relatively stable from the mid- to late Cenozoic. We compare our gastropod Cenozoic Sr/Casw record with a published turritellid gas- tropod Sr/Casw record and other published biogenic (benthic foraminifera, fossil fish teeth) and inorganic pre- cipitate (calcite veins) Sr/Casw records. Once the uncertainties with our gastropod-derived Sr/Casw are taken into account the Sr/Casw record agrees reasonably well with biogenic Sr/Casw records. Assuming a seawater [Ca] history derived from marine evaporite inclusions, all biogenic-based Sr/Casw reconstructions imply decreasing seawater [Sr] through the Cenozoic, whereas the calcite vein Sr/Casw reconstruction implies increasing [Sr] through the Cenozoic. We apply a simple geochemical model to examine the implications of divergence among these seawater [Sr] reconstructions and suggest that the interpretation and uncertainties associated with the gastropod and calcite vein proxies need to be revisited. Used in conjunction with records of carbonate depositional fluxes, our favored seawater Sr/Ca scenarios point to a significant increase in the proportion of aragonite versus calcite deposition in shelf sediments from the Middle Miocene, coincident with the proliferation of coral reefs. We propose that this occurred at least 10 million years after the seawater Mg/Ca threshold was passed, and was instead aided by declining levels of atmospheric carbon dioxide