PP41E-01 INVITED
Secondary Ion Mass Spectrometry as a Tool for Measuring Mg/Ca in Small, Rare and Altered Foraminifera
Secondary ion mass spectrometry (SIMS) is useful for measuring the molar ratio of magnesium (Mg) to calcium (Ca) in both primary calcite and diagenetic minerals in foraminifera. The spatial resolution of standard SIMS (10 microns or less) and small amount of material removed (less than 2 ng) makes it possible to avoid targets that include embedding material and encrusting or infilling minerals such as secondary calcite and authigenic clays in diagenetically altered samples. Because analyses are performed on individuals, SIMS is also a viable technique for generating Mg/Ca values from sediment samples in which foraminifera are rare or have low mass. For clean primary calcite samples, Mg/Ca ratios from SIMS compare well to those obtained using inductively coupled plasma mass spectrometry (ICP-MS), while maintaining information regarding the variability of elemental ratios within individual tests. For samples with secondary calcite or stubbornly adhering clays, SIMS enables us to accurately measure primary calcite compositions and to assess and reconcile contamination problems in bulk samples analyzed by the more common solution-based ICP-MS technique. SIMS and NanoSIMS (with resolution of 1 micron) are valuable methods for the identification and avoidance of problems of diagenesis and the analysis of rare or delicate foraminifera. However, because of operator time required to properly target delicate (thin-walled) or contaminated foraminifera, SIMS may not be feasible for Mg/Ca studies where large numbers (hundreds) of samples must be processed and where bulk measurements on multiple individuals will suffice. The presentation will include examples of attempts to use combined Mg/Ca (from SIMS and ICP-MS) and oxygen isotope ratios to quantify upper ocean temperature change in the late Cretaceous.
PP41E-02
The Mg/Ca-Temperature Relationship of Benthic Foraminiferal Calcite: New Core-top Calibrations in the <4°C Temperature Range
Calcite magnesium/ calcium ratios were determined for four species of benthic foraminifera ( Cibicidoides wuellerstorfi, Cibicidoides kullenbergi, Oridorsalis umbonatus and Pyrgo murrhina) from core-top samples collected from the Atlantic, Indian, and Pacific oceans where modern bottom water temperatures are less than 4°C. Mg/Ca of all four species increase with increasing temperature, and the Mg/Ca: temperature relationship does not appear to be necessarily exponential. Mg/Ca of C. kullenbergi is consistently higher than C. wuellerstorfi, implying differences in the uptake of Mg by different species within this genus. The observed Mg/Ca for C. wuellerstorfi at temperatures less than 3°C is lower than those predicted from published Mg/Ca: temperature relationships derived for Cibicidoides spp. at higher temperatures. Previous studies have attributed this change in the Mg/Ca-temperature relationship of C. wuellerstorfi to the effect of decreased carbonate ion saturation (Δ[CO32-]) on Mg/Ca at low temperatures. A relationship between the (Δ[CO32-] and Mg/Ca of 0.0083±0.002 (mmol/mol)/ (μmol/kg) has been established for C. wuellerstorfi from this study. Down-core Mg/Ca and δ18O values for the last 40,000 years were determined for C. wuellerstorfi from eastern equatorial Pacific core TR163-14 in order to carry out a sensitivity study of various Mg/Ca paleotemperature equations. Comparison of temperature estimates derived from three different equations indicate a 1.0 to 2.3°C difference between estimated temperatures, demonstrating the need for caution when choosing a Mg/Ca: temperature equation to estimate past temperatures, as the Mg/Ca-temperature relationship of benthic foraminiferal calcite is considerably different above and below ~3°C.
PP41E-03 INVITED
Controls on Mg/Ca variation in planktonic foraminifera: insights from microanalysis of laboratory cultured Orbulina universa
The bulk shell Mg/Ca composition of foraminifera (including Orbulina universa) increase exponentially with seawater temperature at ~10% per °C, however, individual O. universa exhibit large-amplitude internal Mg/Ca growth banding that suggests factors in addition to temperature may influence foraminiferal calcite Mg/Ca composition. To shed light on the origin of Mg/Ca banding in O. universa we have performed high resolution compositional profiling of single shells grown in laboratory-cultures under a 12 hour high-light (day)-12 hour dark (night) cycle, synchronized with transfer of living foraminifers between trace element or isotopically spiked and natural seawater. These experiments confirm that the Mg/Ca banding reflects a diel cycle and specifically that high Mg/Ca bands form at night and low-Mg/Ca bands form during the day. Our results, together with existing models for and measured daily changes in [CO32-] and calcite saturation state within the foraminiferal microenvironment, point to the important interplay between day-time photosynthesis (by algal symbionts) and night-time respiration in determining the Mg/Ca composition of foraminiferal calcite. They are also consistent with previously reported increases in bulk Mg/Ca composition with decreasing seawater [CO32-] and pH. The results of other experiments undertaken to elucidate the effects of seawater properties on the nature of both compositional banding and the development of gametogenic calcite within O. universa will also be discussed.
PP41E-04
Controls on the uptake of magnesium, other trace metals and stable isotopes in foraminiferal calcite from a seasonal sea-ice environment
The stable isotope and trace metal chemistry of planktonic foraminiferal calcite has become a key geochemical tool for the understanding of past sea surface temperature, salinity and nutrient conditions. However, the use of proxies such as Mg/Ca at high latitudes, in particular the Southern Ocean, is hampered by the lack of preservation, poor sensitivity at low temperatures and other complicating factors such as seasonality in sea-ice cover, salinity, temperature and carbon fluxes. Here, we present a year long monthly time-series study of the foraminifera, Neogloboquadrina pachyderma (sinistral), from sediment traps deployed off the West Antarctic Peninsula in a seasonal sea-ice environment. This species is found in sediment cores from polar regions and is consequently a promising means for understanding past high latitude environmental change. Our data show a clear seasonal signal in the uptake of metals heavier than calcite (e.g. Cd, Ba, Sr). However, the uptake of stable isotopes and lighter metals, including Mg, is more complex and is evidently sensitive to environmental parameters, most likely a combination of ambient temperature, salinity and carbonate ion concentration. We discuss the implications of seasonality, low temperature and sea-ice for the use of pachyderma Mg/Ca, other trace metals and stable isotopes as palaeoproxies.
PP41E-05 INVITED
High Quality Proxy Data From Foraminiferal Calcite Using Flow-Through Time Resolved Analysis
Flow through time resolved analysis (FT-TRA) offers increased potential over standard batch methods for extracting pristine El/Ca ratios from biogenic calcite, even in samples that are severely overgrown. The potential of the technique is rooted in its ability to resolve differences in solubility by making thousands of determinations (typically 8300) during the leaching and dissolution process. These time-resolved ratios can be used to isolate shell material from other phases and provide a real measure of uncertainties in the proxy. FT-TRA is based on the premise that biogenic calcite is one of the most soluble phases in a sample of foraminiferal tests and early methods were successful at dissolving carbonate phases while leaving more resistant phases like clays and low-Sr barite behind. More recent FT methods use deionized water and dilute nitric acid (3mM) along with highly controlled concentration gradients and flow-rate protocols to achieve improved resolution. Previous work has shown that FT-TRA dissolves different shell parts in the same sequence that occurs naturally on the sea floor. The natural formulation of FT-TRA yields insight into the formation, preservation and dissolution of biogenic calcite and associated authigenic minerals in addition to producing high quality proxy data. Examples of FT-TRA separations and their use in paleoceanography will be given in this presentation.
PP41E-06 INVITED
Ostracode Mg/Ca Paleothermometry: Applications and Complications
Ostracode (bivalved Crustacea) shell Mg/Ca paleothermometry has wide applicability in Cenozoic paleoclimatology over 101 to 107 year timescales because they are commonly fossilized, live in freshwater, shallow- and deep-marine habitats, and grow by molting, which minimizes Mg/Ca variability due to ontogenetic variability. Two empirically derived Mg/Ca-temperature calibrations based on core top and culturing include one for the shallow marine, estuarine genus Loxoconcha (5 to 30°C) and another for deep-sea genus Krithe (<1 to 14°C). The former produced a temperature history for Chesapeake Bay for the last millennium, which has been intensively analyzed in the context of the hockey stick temperature curve. The latter produced evidence for decreased deep-sea temperature during glacial intervals and the first Atlantic-wide reconstruction of deep-sea temperature during the warm mid-Pliocene. In addition to temperature, however, factors such as host-water magnesium concentrations, salinity, intra-shell, intra-population, and interspecific variabilility, seasonality, biological factors (shell secretion rate), and post-mortem dissolution can contribute to scatter in calibration datasets and uncertainty in paleotemperature estimates. We will review these processes, present a new 2000 year Chesapeake temperature record, and discuss its relation to twentieth century climate change.
PP41E-07
Deglacial sea surface and deep chlorophyll maximum temperature variations in the eastern equatorial Pacific: implications for regional palaeoceanography
The eastern equatorial Pacific (EEP) is a major upwelling region in which multiple current systems interact and surface ocean temperatures and productivity strongly vary. We examined two high sedimentation rate cores from the EEP, ME 24J and ME 27J, in order to assess changes in upwelling dynamics there since the Last Glacial Maximum (LGM). We will present temperature estimates from both cores for the sea surface (SST) using Uk37 and the deep chlorophyll maximum (DCM) using Mg/Ca from N. dutertrei which are well known to be DCM dwellers. Because Mg/Ca in planktonic foraminifers is significantly influenced by post-depositional calcite dissolution, we used the Globorotalia menardii fragmentation index and equations developed by Mekik et al. (2007) to correct the Mg/Ca temperature estimates for dissolution. Our results show a strong increase in DCM temperatures with a concurrent drop in SST during the deglacial, while the uncorrected temperature estimates also show a warming during the deglacial but less so than the dissolution corrected estimates. Subsequently, SST rises while dissolution corrected DCM temperatures steadily drop until they reach their present day values (15-18 degrees C). Our new records will be discussed in terms of changes in water column structure, water mass distributions and upwelling dynamics during the last 21 kyrs. We will also revisit various approaches used to correct foraminiferal Mg/Ca records for dissolution. Mekik, F., R. Francois and M. Soon, 2007. A novel approach to dissolution correction of Mg/Ca-based paleothermometry in the tropical Pacific. Paleoceanography, in press.
PP41E-08
Fidelity of δ18Oseawater estimates using foraminiferal shell Mg/Ca and δ18O
Paired foraminiferal shell Mg/Ca and δ18O analyses are widely used to estimate surface ocean δ18Oseawater, a proxy for surface salinity. We present a systematic assessment of the fidelity of shell- based δ18Oseawater estimates using an Atlantic meridional transect (43N-25S) of 58 coretop measurements of G. ruber (white) Mg/Ca and δ18O. Core locations were selected near the flanks of the mid-Atlantic ridge, at or above the modern lysocline depth, from trigger weight samples only, and of late Holocene age. G. ruber (white) shells for Mg/Ca analyses were cleaned using the full reductive-oxidative cleaning protocol. These results document a large, systematic offset between shell-derived and observed δ18Oseawater values. With increasing salinity, shell Mg/Ca compositions are significantly elevated above their expected values determined from isotopic calcification temperatures. This "excess Mg" residual is highly correlated with surface salinity or alkalinity (r2=0.75). This "excess Mg/Ca" effect is apparent in published Atlantic G. ruber (white) Mg/Ca and δ18O data. Temperature and salinity signals are evidently embedded in both shell δ18O and Mg/Ca and this actually enhances detection of salinity gradients. We develop multivariate regression equations that accurately estimate mean annual SST (r2=0.8) and salinity (r2=0.8) values along the transect using shell Mg/Ca, δ18O, and bottom water Δ CO32- as predictors.