SUBMARINE HYDROTHERMAL ACTIVITY AND SECULAR CHANGES IN MID-CRETACEOUS SEAWATER CHEMISTRY

The primary climatic forcing mechanism of the mid-Cretaceous (Aptian-Albian, 122-98 Ma) was global tectonic activity associated with the progressive breakup of Pangea, and the emplacement of several Large Igneous Provinces. The global carbon cycle was impacted through changes in productivity, organic carbon burial, and shifts in carbonate deposition/dissolution. Reconstructions of ocean crust production and strontium isotope geochemistry, paired with microfossil biostratigraphy, suggest that tectonism intensified during the latest Aptian as shown by minimum 87Sr/86Sr ratios in deep-sea carbonates. According to Stanley and Hardie (1998), a significant decrease in seawater Mg/Ca should accompany periods of increased tectonism and hydrothermal flux through spreading centers. The models of Wilkinson and Algeo (1989) and Stanley and Hardie (1998) predict that the mid-Cretaceous has the lowest seawater Mg/Ca ratios of the Mesozoic and Cenozoic oceans. Additionally, major evolutionary innovations in the marine biosphere, including a 92% turnover in planktic foraminifers, occur across the Aptian-Albian boundary, corresponding to large-scale changes in seawater chemistry (Leckie et al., 2002).

Mg/Ca in foraminiferal calcite has been used not only as an independent proxy of seawater temperature but also to differentiate the ice volume and paleotemperature signal from the Cenozoic deep-sea d¹⁸O record. If the Mg/Ca ratios of foraminiferal calcite are to be interpreted as a paleotemperature proxy across the Aptian-Albian transition, then decreases in Mg/Ca, as measured at DSDP sites 511 (Falkland Plateau) and 392A (Blake Nose), suggest substantial cooling. However, d¹⁸O paleotemperature estimates from these sites depict warming and increased thermal stratification over this interval. Our data support that a secular change in seawater Mg/Ca dominates the signal from the latest Aptian through the Albian. Coincident with this decrease in seawater Mg/Ca is an increase in foraminiferal Li/Ca and Sr/Ca. Foraminiferal lithium isotopic analyses are in progress to elucidate the source of lithium enrichment which may provide an independent tracer of increased hydrothermal activity across the Aptian-Albian boundary.