Ba/Ca ratios in benthic foraminifera from recent core tops

The carbon isotope ratio (delta13C) and cadmium content (Cd/Ca) of benthic foraminifera shells have been used to reconstruct deep-water circulation patterns of the glacial oceans. These tracers co-vary with phosphorus in the modern ocean because they are nearly quantitatively regenerated from sinking biological debris in the upper water column. Hence they can be used to reconstruct the distribution of labile nutrients in glacial water masses. Independent constraints on glacial deep-ocean circulation patterns could be provided by a tracer of the distribution of silica and alkalinity, the deeply regenerated constituents of planktonic hard parts. Barium shares key aspects of its behaviour with these refractory nutrients because it is removed from solution in surface waters and incorporated into sinking particles which slowly dissolve deep in the water column and in the sediments. The fractionation of Ba between deep-water masses of the major ocean basins is largely controlled by thermohaline circulation patterns, so Ba conforms to different boundary conditions from Cd and 13C. As Ba substitutes into trigonal carbonates, it is a potential palaeoceano-graphic tracer if the Ba content of foraminifera shells reflects ambient dissolved Ba concentrations. Here we present data from Recent core-top benthic foraminifera which indicate that the Ba content of some recent calcitic benthic foraminifera does co-vary with bottom-water Ba.

Water temperature reconstructions based on paired Uk'37 and Tex86 records, and paired surface and subsurface Mg/Ca records

This dataset contains the collection of available published paired Uk'37 and Tex86 records spanning multi-millennial to multi-million year time scales, as well as a collection of Mg/Ca-derived temperatures measured in parallel on surface and subsurface dwelling foraminifera, both used in the analyses of Ho and Laepple, Nature Geoscience 2016. As the signal-to-noise ratios of proxy-derived Holocene temperatures are relatively low, we selected records that contain at least the last deglaciation (oldest sample >18kyr BP).

Stable oxygen isotope and Mg/Ca ratios on Globorotalia inflata

We present a species-specific Mg/Ca-calcification temperature calibration for Globorotalia inflata from a suite of 38 core top samples from the South Atlantic (from 8° to 49°S). G. inflata is a deep-dwelling planktonic foraminifer commonly occurring in subtropical to subpolar conditions, which qualifies it for reconstructions of the permanent thermocline. Apparent calcification depths and calcification temperatures were determined by comparing measured d18O with equilibrium d18O of calcite based on water column properties. Based on our core top samples, G. inflata apparent calcification depth is constant throughout the South Atlantic mid-latitudes with a depth of 350-400 m within the permanent thermocline. The resulting Mg/Ca-calcification temperature calibration is Mg/Ca = 0.72 +/-0.045/0.042 exp (0.076 +0.006 calcification 2 temperature) (r2 = 0.81) and covers the temperature range 3.1-16.5°C. We applied our Mg/Ca calibration to gravity core PS2495-3 from the Mid-Atlantic Ridge at ca. 41°S to test its validity by reconstructing a low-resolution record covering the last two glacial-interglacial cycles. Our paleotemperature record reveals large changes in temperature for Terminations I and II, when permanent thermocline temperature increased by as much as 8°C. The G. inflata paleotemperature record suggests that oceanic fronts repeatedly migrated over the location of site PS2495-3 during the last 160 kyr. This study shows the potential of G. inflata Mg/Ca to reconstruct paleotemperatures in the permanent thermocline.

Mg/Ca and stable oxygen isotope ratios of Uvigerina spp. from the Southern Ocean

The sensitivity to temperature of Mg/Ca ratios in the shallow-infaunal benthic foraminifera Uvigerina spp. has been assessed. Core-top calibrations over ~1-20 °C show a range in sensitivity of 0.065-0.084 mmol/mol/°C but few data are available spanning the temperature range anticipated in deep-sea records over glacial-interglacial cycles. In contrast to epibenthic foraminiferal species, carbonate ion saturation appears not to affect Mg/Ca significantly. A method based on estimating the ratio of the temperature sensitivity of foraminiferal Mg/Ca to that of d18Ocalcite shows that sensitivity for Mg/Ca at the high end of the observed core-top range (~0.1 mmol/mol/°C) is required for consistency with LGM-Holocene differences in each property as constrained by independent proxy data. This is supported by a Mg/Ca record for Uvigerina spp. generated for the Southern Ocean over the past 440,000 years from Ocean Drilling Program Site 1123 (Chatham Rise, New Zealand). The record shows variability that correlates with climate oscillations. The LGM deep ocean temperature derived from the Mg/Ca record is -1.1 ± 0.3 °C. Transformation to temperature allows estimates to be made of changes in bottom water temperature and seawater d18O and comparison made with literature records. Analysis reveals a ~2.5-kyr lead in the record of temperature over calcite d18O and a longer lead over seawater d18O. This is a reflection of larger phase offsets at eccentricity periods; phase offsets at tilt and precession are within error zero.

Dinoflagellate cyst assemblages, oxygen isotope records of foraminifera, and alkenone and Mg/Ca-based SST estimates for the time interval 3.40-3.18 Ma (Late Pliocene) from five North Atlantic and Caribbean sediment cores

The early Late Pliocene (3.6 to ~3.0 million years ago) is the last extended interval in Earth's history when atmospheric CO2 concentrations were comparable to today's and global climate was warmer. Yet a severe global glaciation during marine isotope stage (MIS) M2 interrupted this phase of global warmth ~3.30 million years ago, and is seen as a premature attempt of the climate system to establish an ice-age world. Our geochemical and palynological records from five marine sediment cores along a Caribbean to eastern North Atlantic transect show that increased Pacific-to-Atlantic flow via the Central American Seaway weakened the North Atlantic Current (NAC) and attendant northward heat transport prior to MIS M2. The consequent cooling of the northern high latitude oceans permitted expansion of the Greenland ice sheet during MIS M2, despite near-modern atmospheric CO2 concentrations. Before and after MIS M2, heat transport via the NAC was crucial in maintaining warm climates comparable to those predicted for the end of this century.

Stable oxygen isotope record and Sr/Ca ratios of corals from IODP Holes M0009D and M0023A

We present monthly resolved records of strontium/calcium (Sr/Ca) and oxygen isotope (d18O) ratios from well-preserved fossil corals drilled during the Integrated Ocean Drilling Program (IODP) Expedition 310 'Tahiti Sea Level' and reconstruct sea surface conditions in the central tropical South Pacific Ocean during two time windows of the last deglaciation. The two Tahiti corals examined here are uranium/thorium (U/Th)-dated at 12.4 and 14.2 ka, which correspond to the Younger Dryas (YD) cold reversal and the Bølling-Allerød (B-A) warming of the Northern Hemisphere, respectively. The coral Sr/Ca records indicate that annual average sea surface temperature (SST) was 2.6-3.1 °C lower at 12.4 ka and 1.0-1.6 °C lower at 14.2 ka relative to the present, with no significant changes in the amplitude of the seasonal SST cycle. These cooler conditions were accompanied by seawater d18O (d18Osw) values higher by ~0.8 per mill and ~0.6 per mill relative to the present at 12.4 and 14.2 ka, respectively, implying more saline conditions in the surface waters. Along with previously published coral Sr/Ca records from the island [Cohen and Hart (2004), Deglacial sea surface temperatures of the western tropical Pacific: A new look at old coral. Paleoceanography 19, PA4031, doi:10.1029/2004PA001084], our new Tahiti coral records suggest that a shift toward lower SST by ~1.5 °C occurred from 13.1 to 12.4 ka, which was probably associated with a shift toward higher d18Osw by ~0.2 per mill. Along with a previously published coral Sr/Ca record from Vanuatu [Corrège et al. (2004), Interdecadal variation in the extent of South Pacific tropical waters during the Younger Dyras event. Nature 428, 927-929], the Tahiti coral records provide new evidence for a pronounced cooling of the western to central tropical South Pacific during the Northern Hemisphere YD event.

Ground truthing the Cd/Ca-carbon isotope relationship in foraminifera of the Greenland-Iceland-Norwegian Seas

In order to examine whether the paleoceanographic nutrient proxies, d13C and cadmium/calcium in foraminiferal calcite, are well coupled to nutrients in the region of North Atlantic Deep Water formation, we present da ta from two transects of the Greenland-Iceland-Norwegian Seas. Along Transect A (74.3°N, 18.3°E to 75.0°N, 12.5°W, 15 stations), we measured phosphate and Cd concentrations of modern surface sea water. Along Transect B (64.5°N, 0.7°W to 70.4°N, 18.2°W, 14 stations) we measured Cd/Ca ratios and d13C of the planktonic foraminifera Neogloboquadrina pachyderma sinistral in core top sediments. Our results indicate that Cd and phosphate both vary with surface water mass and are well correlated along Transect A. Our planktonic foraminiferal d13C data indicate similar nutrient variation with water mass along Transect B. Our Cd/Ca data hint at the same type of nutrient variability, but interpretations are hampered by low values close to the detection limit of this technique and therefore relatively large error bars. We also measured Cd and phosphate concentrations in water depth profiles at three sites along Transect A and the d13C of the benthic foraminifera Cibicidoides wuellerstorfi along Transect B. Modern sea water depth profiles along Transect A have nutrient depletions at the surface and then constant values at depths greater than 100 meters. The d13C of planktonic and benthic foraminifera from Transect B plotted versus depth also reflect surface nutrient depletion and deep nutrient enrichment as seen at Transect A, with a small difference between intermediate and deep waters. Overall we see no evidence for decoupling of Cd/Ca ratio and d13C in foraminiferal calcite from water column nutrient concentrations along these transects in a region of North Atlantic Deep Water formation.

Monthly Tahiti coral Sr/Ca and oxygen isotope data from IODP Hole 310-M0024A

The early last glacial termination was characterized by intense North Atlantic cooling and weak overturning circulation. This interval between ~18,000 and 14,600 years ago, known as Heinrich Stadial 1, was accompanied by a disruption of global climate and has been suggested as a key factor for the termination. However, the response of interannual climate variability in the tropical Pacific (El Niño-Southern Oscillation) to Heinrich Stadial 1 is poorly understood. Here we use Sr/Ca in a fossil Tahiti coral to reconstruct tropical South Pacific sea surface temperature around 15,000 years ago at monthly resolution. Unlike today, interannual South Pacific sea surface temperature variability at typical El Niño-Southern Oscillation periods was pronounced at Tahiti. Our results indicate that the El Niño-Southern Oscillation was active during Heinrich Stadial 1, consistent with climate model simulations of enhanced El Niño-Southern Oscillation variability at that time. Furthermore, a greater El Niño-Southern Oscillation influence in the South Pacific during Heinrich Stadial 1 is suggested, resulting from a southward expansion or shift of El Niño-Southern Oscillation sea surface temperature anomalies.

Sr/Ca ratios of coccoliths from the Paleocene-Eocene Thermal Maximum

During the Paleocene-Eocene Thermal Maximum (PETM), rapid release of isotopically light C to the ocean-atmosphere system elevated the greenhouse effect and warmed temperatures by 5-7 °C for 105 yr. The response of the planktic ecosystems and productivity to the dramatic climate changes of the PETM may represent a significant feedback to the carbon cycle changes, but has been difficult to document. We examine Sr/Ca ratios in calcareous nannofossils in sediments spanning the PETM in three open ocean sites as a new approach to examine productivity and ecological shifts in calcifying plankton. The large heterogeneity in Sr/Ca among different nannofossil genera indicates that nannofossil Sr/Ca reflects primary productivity-driven geochemical signals and not diagenetic overprinting. Elevated Sr/Ca ratios in several genera and constant ratios in other genera suggest increased overall productivity in the Atlantic sector of the Southern Ocean during the PETM. Dominant nannofossil genera in tropical Atlantic and Pacific sites show Sr/Ca variations during the PETM which are comparable to background variability prior to the PETM. Despite acidification of the ocean there was not a productivity crisis among calcifying phytoplankton. We use the Pandora ocean box model to explore possible mechanisms for PETM productivity change. If independent proxy evidence for more stratified conditions in the Southern Ocean during the PETM is robust, then maintenance of stable or increased productivity there likely reflects increased nutrient inventories of the ocean. Increased nutrient inventories could have resulted from climatically enhanced weathering and would have important implications for burial rates of organic carbon and stabilization of climate and the carbon cycle.

(Table 1) Calcium isotope composition, Mg/Ca, Sr/Ca and U/Ca ratios of calcareous cysts of Thoracosphaera heimii

We investigated the influences of temperature, salinity and pH on the calcium isotope as well as trace and minor element (uranium, strontium, magnesium) to Ca ratios on calcium carbonate cysts of the calcareous dinoflagellate species Thoracosphaera heimii grown in laboratory cultures. The natural habitat of this species is the photic zone (preferentially at the chlorophyll maximum depth) of temperate to tropical oceans, and it is abundant in deep-sea sediments over the entire Cenozoic. In our experiments, temperatures ranged from 12 to 30 °C, salinity from 36.5 to 38.8 and pH from 7.9 to 8.4. The delta44/40Ca of T. heimii cysts resembles that of other marine calcifiers, including coccolithophores, foraminifers and corals. However, its temperature sensitivity is considerably smaller and statistically insignificant, and T. heimii might serve as a recorder of changes in seawater delta44/40Ca over geologic time. The Sr/Ca ratios of T. heimii cysts show a pronounced temperature sensitivity (0.016 mmol/mol °C**-1) and have the potential to serve as a palaeo-sea surface temperature proxy. No clear temperature- and pH-dependences were observed for Mg/Ca. U/Ca seems to be influenced by temperature and pH, but the correlations change sign at 23 °C and pH 8.2, respectively.

(Table 2) B/Ca, d11B, Mg/Ca and d18O record for ODP Hole 130-806B samples

We present the first continuous records from 0 to 5 Ma (in 0.333 m.y. integrated time steps) of paired boron/calcium (B/Ca) ratios and boron isotopes (d11B) in the planktonic foraminifera Globogerinoides sacculifer (without sacc) from a site in the western equatorial Pacific Ocean (Ocean Drilling Program Site 806). These measurements, the first made in conjunction with calcification temperature (magnesium/calcium ratios) and average shell mass measurements, indicate that pH is not the sole environmental variable controlling B in planktonic foraminiferal calcite. Our data are consistent with calcification temperature exerting a primary control on B concentration and isotopic composition in planktonic foraminifera. If so, calcification temperature must be taken into account if pH for past oceans and atmospheric pCO2 are to be estimated from B isotope measurements in foraminiferal calcite. Doing so will substantially increase the uncertainty of pH estimates. Although this work was designed as a temporal study, its results define new aspects of calibrating the d11B paleo-pH tracer.

Nd isotope data for planktonic foraminifera from within and around sapropels S1 and S5 from ODP Site 160-967

The development of widespread anoxic conditions in the deep oceans is evidenced by the accumulation and preservation of organic-carbon-rich sediments, but its precise cause remains controversial. The two most popular hypotheses involve (1) circulation-induced increased stratification resulting in reduced oxygenation of deep waters or (2) enhanced productivity in the surface ocean, increasing the raining down of organic matter and overwhelming the oxic remineralization potential of the deep ocean. In the periodic development of deep-water anoxia in the Pliocene-Pleistocene Mediterranean Sea, increased riverine runoff has been implicated both as a source for nutrients that fuel enhanced photic-zone productivity and a source of a less dense freshwater cap leading to reduced circulation, basin-wide stagnation, and deep-water oxygen starvation. Monsoon-driven increases in Nile River discharge and increased regional precipitation due to enhanced westerly activity-two mechanisms that represent fundamentally different climatic driving forces-have both been suggested as causes of the altered freshwater balance. Here we present data that confirm a distinctive neodymium (Nd) isotope signature for the Nile River relative to the Eastern Mediterranean-providing a new tracer of enhanced Nile outflow into the Mediterranean in the past. We further present Nd isotope data for planktonic foraminifera that suggest a clear increase in Nile discharge during the central intense period of two recent anoxic events. Our data also suggest, however, that other regional freshwater sources were more important at the beginning and end of the anoxic events. Taken at face value, the data appear to imply a temporal link between peaks in Nile discharge and enhanced westerly activity.

Stable oxygen isotope record and Mg/Ca ratios at the Mid-Pleistocene Climate Transition, ODP Site 181-1123

Earth's climate underwent a fundamental change between 1250 and 700 thousand years ago, the Mid-Pleistocene Transition (MPT), when the dominant periodicity of climate cycles changed from 41,000 to 100,000 years in the absence of significant change in orbital forcing. Over this time, an increase occurred in the amplitude of change of deep ocean foraminiferal oxygen isotopic ratios, traditionally interpreted as defining the main rhythm of ice ages although containing large effects of changes in deep-ocean temperature. We have separated the effects of decreasing temperature and increasing global ice volume on oxygen isotope ratios. Our results suggest that the MPT was initiated by an abrupt increase in Antarctic ice volume at 900 ka. We see no evidence of a pattern of gradual cooling but near-freezing temperatures occur at every glacial maximum.