Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series

Paired Mg/Ca and δ18O measurements on planktonic foraminiferal species (G. ruber white, G. ruber pink, G. sacculifer, G. conglobatus, G. aequilateralis, O. universa, N. dutertrei, P. obliquiloculata, G. inflata, G. truncatulinoides, G. hirsuta, and G. crassaformis) from a 6-year sediment trap time series in the Sargasso Sea were used to define the sensitivity of foraminiferal Mg/Ca to calcification temperature. Habitat depths and calcification temperatures were estimated from comparison of δ18O of foraminifera with equilibrium calcite, based on historical temperature and salinity data. When considered together, Mg/Ca (mmol/mol) of all species, except two, show a significant (r = 0.93) relationship with temperature (T °C) of the form Mg/Ca = 0.38 (±0.02) exp 0.090 (±0.003)T, equivalent to a 9.0 ± 0.3% change in Mg/Ca for a 1°C change in temperature. Small differences exist in calibrations between species and between different size fractions of the same species. O. universa and G. aequilateralis have higher Mg/Ca than other species, and in general, data can be best described with the same temperature sensitivity for all species and pre-exponential constants in the sequence O. universa > G. aequilateralis ≈ G. bulloides > G. ruber ≈ G. sacculifer ≈ other species. This approach gives an accuracy of ±1.2°C in the estimation of calcification temperature. The ∼9% sensitivity to temperature is similar to published studies from culture and core top calibrations, but differences exist from some literature values of pre-exponential constants. Different cleaning methodologies and artefacts of core top dissolution are probably implicated, and perhaps environmental factors yet understood. Planktonic foraminiferal Mg/Ca temperature estimates can be used for reconstructing surface temperatures and mixed and thermocline temperatures (using G. ruber pink, G. ruber white, G. sacculifer, N. dutertrei, P. obliquiloculata, etc.). The existence of a single Mg thermometry equation is valuable for extinct species, although use of species-specific equations will, where statistically significant, provide more accurate evaluation of Mg/Ca paleotemperature

Silicate weathering and carbon cycle controls on the Oligocene-Miocene transition glaciation

Changes in both silicate weathering rates and organic carbon burial have been proposed as drivers of the transient “Mi-1” glaciation event at the Oligocene-Miocene transition (OMT; ~23 Ma). However detailed geochemical proxy data are required to test these hypotheses. Here we present records of Li/Ca, Mg/Ca, Cd/Ca, U/Ca, δ18O, δ13C, and shell weight in planktonic foraminifera from marine sediments spanning the OMT in the equatorial Atlantic Ocean. Li/Ca values increase by 1 μmol/mol across this interval. We interpret this to indicate a ~20% increase in silicate weathering rates, which would have lowered atmospheric CO2, potentially forcing the Antarctic glaciation circa 23 Ma. δ13C of thermocline dwelling planktonic foraminifera track the global increase in seawater δ13C across the OMT and during the Mi-1 event, hence supporting a hypothesized global increase in organic carbon burial rates. High δ13C previously measured in epipelagic planktonic foraminifera and high Cd/Ca ratios during Mi-1 are interpreted to represent locally enhanced primary productivity, stimulated by increased nutrients supply to surface waters. The fingerprint of high export production and associated organic carbon burial at this site is found in reduced bottom water oxygenation (inferred from high foraminiferal U/Ca), and enhanced respiratory dissolution of carbonates, characterised by reduced foraminiferal shell weight. Replication of our results elsewhere would strengthen the case that weathering-induced CO2 sequestration preconditioned climate for Antarctic ice sheet growth across the OMT and increased burial of organic carbon acted as a feedback that intensified cooling at this time

Reassessing Mg/Ca temperature calibrations of <em>Neogloboquadrina pachyderma</em> (sinistral) using paired δ^44/40 and Mg/Ca measurements

The Mg/Ca temperature calibration of the polar to subpolar planktonic foraminifera Neogloboquadrina pachyderma (sinistral) (sinistral indicates left coiling) was refined by a multiproxy approach combining hydrographic temperature and salinity data with Mg/Ca, delta Ca-44/40, and delta O-18 values from Holocene Nordic seas core top samples. Reliable Mg/Ca-based temperature estimates are limited to foraminiferal tests that calcified in water masses with temperatures above similar to 3 degrees C at habitat depth. In these samples, Mg/Ca and delta Ca-44/40 values are positively correlated (Mg/Ca (mmol/mol) = 0.77 (+/- 0.22) x delta Ca-44/40 (parts per thousand SRM 915a) + 0.52 (+/- 0.12); n = 20, R-2 = 0.76). Both Mg/Ca- and delta Ca-44/40-derived temperatures projected onto their corresponding depth intervals reveal that the &quot;apparent'' calcification depth of N. pachyderma (sinistral) averaging the specimens' whole life cycle is bound to an isopycnal layer defined by water densities (sigma(t)) between 27.7 and 27.8. This implies that N. pachyderma (sinistral) prefers gradually deeper habitats with increasing sea surface temperatures, thus counterbalancing absolute temperature variations. Consequently, the total temperature range recorded in this foraminiferal species is restricted and only partly reflects environmental changes. On the basis of the new Mg/Ca, delta Ca-44/40, and delta O-18 multiproxy data set, we propose a linear Mg/Ca temperature relation for high-latitude N. pachyderma (sinistral): Mg/Ca (mmol/mol) = 0.13 (+/- 0.037) T (degrees C) + 0.35 (+/- 0.17); T &gt; 3 degrees C. In core top samples from polar waters with peak summer temperatures below similar to 3 degrees C, the temperature response in the Mg/Ca and delta Ca-44/40 proxy signal is inversed and poorly correlated. Both Mg/Ca- and delta Ca-44/40-derived temperature estimates pretend significantly higher calcification temperatures than maximum summer sea surface temperatures of these water masses

Temperature and stable isotope variationsin different water masses from the Alboran Sea (western Mediterranean) between 250 150 ka

Mg/Ca, Sr/Ca, and stable isotope measurements have been performed on tests from the planktonic foraminifers Globigerinoides ruber (white), Globigerina bulloides, and Neogloboquadrina pachyderma (right coiling) in samples from Ocean Drilling Program site 977A in the Alboran Sea (Western Mediterranean). The evolution of different water masses between 250 and 150 ka is described. Warm substages were characterized by strong seasonality and thermal stratification of the water column. By contrast, less pronounced seasonality and basin stratification seem to prevail during cold substages. Several periods of stratification due to the low salinity of the upper water mass occurred during the formation of organic-rich layers and also during a possible Heinrich-like event at 220 ka. The three foraminifer species studied show a common and large shell Sr/Ca variability in short timescales, suggesting changes in the global ocean Sr/Ca ratio as one of the main causes of variations in shell composition

Effects of cleaning methods upon preservation of stable isotopes and trace elements in shells of Cyprideis torosa (Crustacea, Ostracoda): implications for palaeoenvironmental reconstruction

The trace element (Sr/Ca and Mg/Ca) and stable isotope (δ¹⁸O and δ¹³C) geochemistry of fossil ostracod valves provide valuable information, particularly in lacustrine settings, on palaeo-water composition and palaeotemperature. The removal of sedimentary and organic contamination prior to geochemical analysis is essential to avoid bias of the results. Previous stable isotope and trace element work on ostracod shells has, however, employed different treatments for the removal of contamination beyond simple ‘manual’ cleaning using a paint brush and methanol under a low-power binocular microscope. For isotopic work pre-treatments include chemical oxidation, vacuum roasting and plasma ashing, and for trace element work sonication, chemical oxidation and reductive cleaning. The impact of different treatments on the geochemical composition of the valve calcite has not been evaluated in full, and a universal protocol has not been established. Here, a systematic investigation of the cleaning methods is undertaken using specimens of the ubiquitous euryhaline species, Cyprideis torosa. Cleaning methods are evaluated by undertaking paired analyses on a single carapace (comprising two valves); in modern ostracods, whose valves are assumed to be unaltered, the two valves should have identical geochemical and isotopic composition. Hence, when one valve is subjected to the chosen treatment and the other to simple manual cleaning any difference in composition can confidently be assigned to the treatment method. We show that certain cleaning methods have the potential to cause alteration to the geochemical signal, particularly Mg/Ca and δ¹⁸O, and hence have implications for palaeoenvironmental reconstructions. For trace element determinations we recommend cleaning by sonication and for stable isotope analysis, oxidation by hydrogen peroxide. These methods remove contamination, yet do not significantly alter the geochemical signal

Paleo-sea surface temperature calculations in the equatorial east Atlantic from Mg/Ca ratios in planktic foraminifera: A comparison to sea surface temperature estimates from UK'37, oxygen isotopes, and foraminiferal transfer function

We present two ∼270 kyr paleo-sea surface temperature (SST) records from the Equatorial Divergence and the South Equatorial Current derived from Mg/Ca ratios in the planktic foraminifer Globigerinoides sacculifer. The present study suggests that the magnesium signature of G. sacculifer provides a seasonal SST estimate from the upper ∼50 m of the water column generated during upwelling in austral low-latitude fall/winter. Common to both down-core records is a glacial-interglacial amplitude of ∼3°–3.5°C for the last climatic changes and lower Holocene and glacial oxygen isotope stage 2 temperatures compared with interglacial stage 5.5 and glacial stage 6 temperatures, respectively. The comparison to published SST estimates from alkenones, oxygen isotopes, and foraminiferal transfer function from the same core material pinpoints discrepancies and conformities between methods

Contrasting multiproxy reconstructions of surface ocean hydrography in the Agulhas Corridor and implications for the Agulhas Leakage during the last 345,000 years

Planktonic δ18O and Mg/Ca-derived sea surface temperature (SST) records from the Agulhas Corridor off South Africa display a progressive increase of SST during glacial periods of the last three climatic cycles. The SST increases of up to 4°C coincide with increased abundance of subtropical planktonic foraminiferal marker species which indicates a progressive warming due to an increased influence of subtropical waters at the core sites. Mg/Ca-derived SST maximizes during glacial maxima and glacial Terminations to values about 2.5°C above full-interglacial SST. The paired planktonic δ18O and Mg/Ca-derived SST records yield glacial seawater δ18O anomalies of up to 0.8‰, indicating measurably higher surface salinities during these periods. The SST pattern along our record is markedly different from a U37K'-derived SST record at a nearby core location in the Agulhas Corridor that displays SST maxima only during glacial Terminations. Possible explanations are lateral alkenone advection by the vigorous regional ocean currents or the development of SST contrasts during glacials in association with seasonal changes of Agulhas water transports and lateral shifts of the Agulhas retroflection. The different SST reconstructions derived from U37K' and Mg/Ca pose a significant challenge to the interpretation of the proxy records and demonstrate that the reconstruction of the Agulhas Current and interocean salt leakage is not as straightforward as previously suggested

Glacial-interglacial modulation of the marine nitrogen cycle by high-latitude O2 supply to the global thermocline

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 19 (2004): PA4007, doi:10.1029/2003PA001000.An analysis of sedimentary nitrogen isotope records compiled from widely distributed marine environments emphasizes the global synchrony of denitrification changes and provides evidence for a strong temporal coupling of these variations to changes in nitrogen fixation as previously inferred. We explain the global coherence of these records by a simple physical control on the flux of dissolved oxygen to suboxic zones and the coupling to fixation via the supply of phosphorus to diazotrophs in suitable environments. According to our hypothesis, lower glacial-stage sea surface temperature increased oxygen solubility, while stronger winds in high-latitude regions enhanced the rate of thermocline ventilation. The resultant colder, rapidly flushed thermocline lessened the spatial extent of denitrification and, consequently, N fixation. During warm periods, sluggish circulation of warmer, less oxygen rich thermocline waters caused expansion of denitrification zones and a concomitant increase in N fixation. Local fluctuations in export productivity would have modulated this global signal.Financial support for this work was provided by the Natural Sciences and Engineering Research Council of Canada and by a WHOI postdoctoral fellowship to MK

Uncertainties in seawater thermometry deriving from intratest and intertest Mg/Ca variability in <em>Globigerinoides ruber</em>

Laser ablation inductively coupled plasma-mass spectrometry microanalysis of fossil and live Globigerinoides ruber from the eastern Indian Ocean reveals large variations of Mg/Ca composition both within and between individual tests from core top or plankton pump samples. Although the extent of intertest and intratest compositional variability exceeds that attributable to calcification temperature, the pooled mean Mg/Ca molar values obtained for core top samples between the equator and &gt;30°S form a strong exponential correlation with mean annual sea surface temperature (Mg/Ca mmol/mol = 0.52 exp0.076SST°C, r2 = 0.99). The intertest Mg/Ca variability within these deep-sea core top samples is a source of significant uncertainty in Mg/Ca seawater temperature estimates and is notable for being site specific. Our results indicate that widely assumed uncertainties in Mg/Ca thermometry may be underestimated. We show that statistical power analysis can be used to evaluate the number of tests needed to achieve a target level of uncertainty on a sample by sample case. A varying bias also arises from the presence and varying mix of two morphotypes (G. ruber ruber and G. ruber pyramidalis), which have different mean Mg/Ca values. Estimated calcification temperature differences between these morphotypes range up to 5°C and are notable for correlating with the seasonal range in seawater temperature at different sites. Copyright 2008 by the American Geophysical Union