Reconstructing the Upper Water Column Thermal Structure in the Atlantic Ocean

The thermal structure of the upper ocean (0–1000 m) is set by surface heat fluxes, shallow wind-driven circulation, and the deeper thermohaline circulation. Its long-term variability can be reconstructed using deep-dwelling planktonic foraminifera that record subsurface conditions. Here we used six species (Neogloboquadrina dutertrei, Globorotalia tumida, Globorotalia inflata, Globorotalia truncatulinoides, Globorotalia hirsuta, and Globorotalia crassaformis) from 66 core tops along a meridional transect spanning the mid-Atlantic (42°N to 25°S) to develop a method for reconstructing past thermocline conditions. We estimated the calcification depths from δ18O measurements and the Mg/Ca-temperature relationships for each species. This systematic strategy over this large latitudinal section reveals distinct populations with different Mg/Ca-temperature relationships for G. inflata, G. truncatulinoides, and G. hirsuta in different areas. The calcification depths do not differ among the different populations, except for G. hirsuta, where the northern population calcifies much shallower than the southern population. N. dutertrei and G. tumida show a remarkably constant calcification depth independent of oceanographic conditions. The deepest dweller, G. crassaformis, apparently calcifies in the oxygen-depleted zone, where it may find refuge from predators and abundant aggregated matter to feed on. We found a good match between its calcification depth and the 3.2 ml/l oxygen level. The results of this multispecies, multiproxy study can now be applied down-core to facilitate the reconstruction of open-ocean thermocline changes in the past

Meridional shifts of the Atlantic intertropical convergence zone since the Last Glacial Maximum

The intertropical convergence zone is a near-equatorial band of intense rainfall and convection. Over the modern Atlantic Ocean, its annual average position is approximately 5° N, and it is associated with low sea surface salinity and high surface temperatures. This average position has varied since the Last Glacial Maximum, in response to changing climate boundary conditions. The nature of this variation is less clear, with suggestions that the intertropical convergence zone migrated north–south away from the colder hemisphere or that it contracted and expanded symmetrically around its present position2. Here we use paired Mg/Ca and δ18O measurements of planktonic foraminifera for a transect of ocean sediment cores to reconstruct past changes in tropical surface ocean temperature and salinity in the Atlantic Ocean over the past 25,000 years. We show that the low-salinity, high-temperature surface waters associated with the intertropical convergence zone migrated southward of their present position during the Last Glacial Maximum, when the Northern Hemisphere cooled, and northward during the warmer early Holocene, by about ±7° of latitude. Our evidence suggests that the intertropical convergence zone moved latitudinally over the ocean, rather than expanding or contracting. We conclude that the marine intertropical convergence zone has migrated significantly away from its present position owing to external climate forcing during the past 25,000 years

The Influence of Salinity on Mg/Ca in Planktic Foraminifers – Evidence from Cultures, Core-top Sediments and Complementary δ18O

The Mg/Ca ratio in foraminiferal calcite is one of the principal proxies used for paleoceanographic temperature reconstructions, but recent core-top sediment observations suggest that salinity may exert a significant secondary control on planktic foraminifers. This study compiles new and published laboratory culture experiment data from the planktic foraminifers Orbulina universa, Globigerinoides sacculifer and Globigerinoides ruber, in which salinity was varied but temperature, pH and light were held constant. Combining new data with results from previous culture studies yields a Mg/Ca-sensitivity to salinity of 4.4 ± 2.3%, 4.7 ± 1.2%, and 3.3 ± 1.7% per salinity unit (95% confidence), respectively, for the three foraminifer species studied here. Comparison of these sensitivities with core-top data suggests that the much larger sensitivity (27 ± 4% per salinity unit) derived from Atlantic core-top sediments in previous studies is not a direct effect of salinity. Rather, we suggest that the dissolution correction often applied to Mg/Ca data can lead to significant overestimation of temperatures. We are able to reconcile culture calibrations with core-top observations by combining evidence for seasonal occurrence and latitude-specific habitat depth preferences with corresponding variations in physico-chemical environmental parameters. Although both Mg/Ca and δ18O yield temperature estimates that fall within the bounds of hydrographic observations, discrepancies between the two proxies highlight unresolved challenges with the use of paired Mg/Ca and δ18O analyses to reconstruct paleo-salinity patterns across ocean basins. The first step towards resolving these challenges requires a better spatially and seasonally resolved δ18Osw archive than is currently available. Nonetheless, site-specific reconstructions of salinity change through time may be valid

Towards the Calibration of the Globigerinoides ruber (white) paleothermometer

The use of planktonic foraminiferal Mg/Ca ratios to reconstruct past sea surface temperatures (SST) is prevalent in the literature. The perceived simplicity of the underlying chemistry and ease of measurements are alluring. Canonically, temperature is thought to be the primary control on the shell Mg/Ca values. Additionally, an appeal of this proxy is that it can be combined with shell ä18O values to reconstruct changes in the local ä18O of seawater, a proxy for salinity. However, we have identified a salinity effect on the Mg/Ca signal recorded in planktonic foraminifera influencing samples from open ocean locations. This effect causes excess Mg incorporation, higher than predicted by theory, in high salinity regions for the planktonic foraminiferal species, Globigerinoides ruber (white). The shell "excess Mg/Ca" resides within the primary calcite lattice of the shell itself and may be related to the observed cyclic banding of high and low Mg/Ca calcite with the foraminiferal shells. We derive new equations which describe the relationship between shell Mg/Ca, ocean sea surface temperature, salinity, and bottom water ÄCO32-. We also define new equations for ocean temperature and salinity using shell Mg/Ca, ä18O, and bottom water ÄCO32-, which take advantage of the dual sensitivity of shell Mg/Ca and ä18O to temperature and salinity. We apply these results downcore at several locations to assess the influence on paleo-reconstructions. These results are widely applicable to paleoceanographic studies and should allow more accurate reconstructions of both temperature and salinity. Below are brief outlines of the dissertation chapters: 1) A poor correlation between Mg/Ca derived and observed (WOA05) SST was found for 64 coretops in the (sub)tropical Atlantic. Shell-derived SST values from the subtropical gyres were overestimated and the residual "excess Mg/Ca" was well correlated with surface salinity. In this chapter, new calibration equations are developed for the Atlantic Ocean using paired Mg/Ca and ä18O measurements, along with the bottom water ÄCO32-, to predict temperature and salinity. These equations are validated using published coretop data and yield accurate estimates for SST and salinity. 2) The ITCZ is clearly identified in the oceans as the region where temperatures are the highest and salinities are the lowest. These oceanographic fingerprints can be used to track ITCZ variability over the ocean through time. Both canonical equations and the new equations from chapter 1 are used here to reconstruct SST and ä18Oseawater/Salinity gradients since the LGM in the equatorial Atlantic. The marine Atlantic ITCZ migrated in excess of 10° latitude away from its modern position, during both the LGM and early Holocene, supporting climate model results as well as coastal and terrestrial paleohydrological records that document the sensitivity of ITCZ position to both high- and low-latitude forcing. 3) The nature of the excess shell Mg/Ca and the mechanism for incorporation is poorly understood. We investigated excess Mg/Ca using SEM, flow through ICP-MS (FT-ICP-MS) and electron microprobe analyses. SEM and FT-ICP-MS results suggest the excess shell Mg resides within the primary structure of the calcite lattice. Electron microprobe maps of shell Mg/Ca confirm that the excess Mg/Ca lies within the shell itself, likely within the high Mg/Ca calcite bands. These findings suggest the incorporation of shell "excess Mg/Ca" first identified in chapter 1 is not related to post-depositional diagenetic alteration. These results will help elucidate the mechanism responsible for enhanced Mg uptake in high salinity settings. 4) Currently there exist no globally applicable calibration equations relating oceanographic parameters to foraminiferal shell Mg/Ca. In this chapter, we develop new, global calibration equations for G. ruber (white) following the methods of chapter 1. We find that the relationship between shell Mg/Ca and salinity is non-linear, with a threshold value near a salinity of 35, below which there is little influence of salinity on shell Mg/Ca. These equations were validated with published data and appear to be robust. By accounting for the additional influence, alkenone and foraminiferal Mg-Ca derived SST records may be reconciled in for some locations, particularly where there were likely to have been large variations of salinity in the past. These results represent a significant advance for the paleoceanographic community

Stable isotope record of planktonic foraminifera of the Atlantic Ocean

The thermal structure of the upper ocean (0-1000 m) is set by surface heat fluxes, shallow wind-driven circulation, and the deeper thermohaline circulation. Its long-term variability can be reconstructed using deep-dwelling planktonic foraminifera that record subsurface conditions. Here we used six species (Neogloboquadrina dutertrei, Globorotalia tumida, Globorotalia inflata, Globorotalia truncatulinoides, Globorotalia hirsuta, and Globorotalia crassaformis) from 66 core tops along a meridional transect spanning the mid-Atlantic (42°N to 25°S) to develop a method for reconstructing past thermocline conditions. We estimated the calcification depths from d18O measurements and the Mg/Ca-temperature relationships for each species. This systematic strategy over this large latitudinal section reveals distinct populations with different Mg/Ca-temperature relationships for G. inflata, G. truncatulinoides, and G. hirsuta in different areas. The calcification depths do not differ among the different populations, except for G. hirsuta, where the northern population calcifies much shallower than the southern population. N. dutertrei and G. tumida show a remarkably constant calcification depth independent of oceanographic conditions. The deepest dweller, G. crassaformis, apparently calcifies in the oxygen-depleted zone, where it may find refuge from predators and abundant aggregated matter to feed on. We found a good match between its calcification depth and the 3.2 ml/l oxygen level. The results of this multispecies, multiproxy study can now be applied down-core to facilitate the reconstruction of open-ocean thermocline changes in the past

The influence of salinity on Mg/Ca in planktic foraminifers - Evidence from cultures, core-top sediments and complementary δ 18 O

The Mg/Ca ratio in foraminiferal calcite is one of the principal proxies used for paleoceanographic temperature reconstructions, but recent core-top sediment observations suggest that salinity may exert a significant secondary control on planktic foramin