Comparing Siliceous Productivity Proxies Along the California Margin During Pliocene Warmth: Smear Slides vs. Biogenic Silica

Abstract:

The early Pliocene (5.3-2.6 Ma) is the most recent time in Earth history when CO₂ levels were similar to today, yet model outputs show average global atmospheric temperatures were 3-4˚C warmer. Given the similar climate boundary conditions in the Pliocene and today (CO₂, continental positions, ocean circulation), the Pliocene is the best analogue for future climate in the paleo-record. Proxy studies reveal Pliocene sea surface temperatures (SST) along the California margin were 3-9˚C warmer than today. The modern California margin is highly productive, driven by strong seasonal upwelling of cold, nutrient-rich water. If the thermocline and nutricline were coupled during the Pliocene as they are today, warmer SSTs would imply lower nutrient concentrations and reduced productivity. Yet previously published oceanic records do not show a clear relationship between SST and productivity.

Alkenone mass accumulation rates (MAR), organic carbon, and biogenic silica show sustained levels of productivity in the Pliocene even as SST was cooling at ODP Site 1022 (40.0˚N, 125.5˚W, depth 1925 m). However, smear slide analysis suggests that diatom MAR decreased during the Pliocene. Because diatoms are the dominant silica-based primary producer, diatom MAR and biogenic silica analyses should display similar variations. The apparent difference between the two proxy records may be due to preservational effects, such as silica dissolution in the sedimentary pore waters and fragmentation of diatom frustules, resulting in reduced diatom MARs, whereas the fragments and non-diatom siliceous producers all contribute to the biogenic silica values.

We present smear slide and biogenic silica analyses as records of diatom productivity at the more southerly ODP Site 1016 (34.0˚N, 122.0˚W, depth 3835 m), and compare it to existing data from ODP Site 1022. In the modern ocean, diatoms dominate California margin phytoplankton under high nutrient upwelling conditions; thus changes in the relative abundance of diatoms should reflect changes in nutrient supply. In order to have confidence in our proxy records we must have a good understanding of the relationship between diatom abundances and biogenic silica values. This combined ODP Sites 1016 and 1022 study will also evaluate other site-specific differences in diatom preservation.