Abstract:
High resolution samples from ODP Hole 1050C (Blake Nose) provide a natural experiment of foraminiferal paleobiology and short term paleoceanographic variability in a tropical/subtropical greenhouse ocean. During the Maastrichtian the Blake Nose was located at a paleolatitude of approximately 30 degrees N, near the western edge of the Tethys; paleodepth is estimated at 1-2 km. Shallow burial increases the probability that depositional geochemical signals are preserved. The hemipelagic interval studied is composed dominantly of nannofossil chalk (80 to 95% CaCO3) with a significant amount of terrigenous material and abundant well-preserved foraminifera. Shipboard data shows approximately 50 cm thick cyclic alternations in color and magnetic susceptibility that are thought to reflect the approximately 20 kyr precessional cycle. Because the time scale is short, the location does not change, and sedimentological features repeat; paleoceanographic factors can be isolated. Causally linked changes should be correlated from cycle to cycle. Across several cycles, we found variations in stables isotopes (oxygen up to 0.8 and carbon up to 0.6), the relative abundance of planktic foraminiferal species, the mineralogy of acid-insoluble residues, and elemental abundances that all match lithologic cycles. High planktic oxygen values are correlated with high planktic but low benthic carbon values indicating cooler and/or more saline surface waters were associated with higher productivity and vice versa. High productivity intervals are also characterized by high Ca concentrations, enrichment in feldspar, and increases in the relative abundance of Heterohelix spp., Globigerinelloides spp., and Laeviheterohelix glabrans. Conversely, low productivity intervals have high Fe concentrations, enrichment in quartz and clay minerals, and relative increases in Globotruncana spp. and Pseudoguembelina spp. These correlations suggest linkage among climate, continental weathering, ocean productivity, and foraminiferal communities on geologically short time scales. Determining what processes are driving productivity cycles (e.g., continental fluxes, upwelling) is not simple, but foraminiferal populations seem to respond in a systematic fashion which will allow better interpretation of foraminiferal assemblages elsewhere.