Neogene Benthic Foraminiferal Mg/Ca Records

We use paired measurements of foraminiferal Mg/Ca ratios and oxygen isotope values at several sites spanning the Neogene to investigate the evolution of the oxygen isotopic composition of seawater. Our ultimate goal is to use the oxygen isotopic composition of seawater as a proxy for sea level change, and to compare it to sea-level records established with sequence stratigraphy. Thus far we have generated Mg/Ca records from intermediate water sites 1088 (subantarctic Southern Ocean) and 757 (tropical Indian Ocean) to compare with the published record from intermediate water depth Southern Ocean site 747 (Billups and Schrag, 2002). Comparison of Mg/Ca records reveals that variability among the three sites can be considerable. For example, all sites show a late Miocene maximum (8 Ma), but the amplitude differs among the records. Both Southern Ocean sites (sites 1088 and 747) display a distinct decrease in Mg/Ca ratios during the Pliocene, reaching a minimum during the early Pleistocene (1.5 Ma). In contrast, Mg/Ca ratios at Indian Ocean site 757 remain relatively constant throughout this interval of time. Using the Mg/Ca ratios as an independent temperature proxy, we illustrate that the trends in the calculated oxygen isotopic composition of seawater agree only in that values have increased since the mid Miocene. For example, an increase in the oxygen isotopic composition of seawater, expected from the growth of Northern Hemisphere ice sheets beginning in the Pliocene, is only apparent at Indian Ocean site 757. These observations illustrate that regional water mass hydrography can be variable, which highlights the necessity for generating records from multiple sites in order to obtain a record representative of global, glacioeustatic processes. To this end, we expand this work with records from the North Atlantic and Pacific.