Petra Dekens, PhD, San Francisco State University
Petroleum resources found in coastal waters are the result of high productivity in the overlying water column and preservation in the sediments. This study aims to characterize productivity and preservation along the California margin during the early Pliocene warm period (3-5 Ma). Today, coastal upwelling, driven by equatorward winds along the California coast, brings cold nutrient-rich water from below the thermocline to the surface. This seasonal upwelling leads to low sea surface temperatures (SST) and high primary productivity. During the early Pliocene California margin SSTs were 2 to 9˚C warmer compared to today. These SST records lead to questions about how productivity differed during this time of global warmth. Initial indicators of coccolithophorid productivity do not show a correlation between SST and productivity, indicating that the nutricline and thermocline were decoupled through the last 5 Ma. Alkenone mass accumulation rates (MAR), which is a proxy of coccolithophorid productivity, was similar during the early Pliocene (3-5 Ma) compared to the Pleistocene (with an increased MAR from 1.5-3 Ma). This project is testing the hypothesis that warm nutrient-rich water was upwelled during the early Pliocene, leading to primary productivity similar to today, even while SST were significantly warmer.
Different phytoplankton types thrive under different hydrographic conditions. For example, diatoms tend to dominate during times of vigorous upwelling, while coccolithophorids are able to outcompete diatoms for nutrients when the water is well stratified and nutrient concentrations are lower. Therefore, we must determine the relative abundance of the dominant types of phytoplankton present in the surface ocean in order to characterize upwelling conditions during the early Pliocene. This project is applying two productivity indicators: lithostratigraphy using grain size analysis, and organic biomarkers. The use of these two proxies will not only characterize changes in productivity and water column stratification, it will also allow us to differentiate between changes in primary productivity and preservation within the sedimentary environment.
The organic biomarker and grain size analyses will be done at two sites along the California margin: ODP site 1014 is (32.8˚N, 120.0˚W) and ODP site 1022 (40.0˚N, 125.5˚W). An undergraduate student, Jenny Miles, did a BS thesis project in my lab where she used both grain size analysis and smear slides to characterize the sediments at ODP site 1022. We had hoped that the sediment could be characterized using a laser particle size analyzer, as that analysis is quite fast. Jenny worked in collaboration with Ivano Aiello to do the laser particle size analysis and then used a petrographic microscope to verify the grain types and calibrate that to the laser particle size analyzer data. Unfortunately, the two data sets were not well correlated. Jenny’s smear slide characterizations of ODP site 1022 did lead to some interesting results however. Her data show that at approximately 3.5 Ma ODP site 1022 shifted from a system with a higher abundance of diatoms to one with a much higher abundance of coccolithophorids. At this stage it is unclear if this signal is due to changes in productivity or changes in preservation.
My current graduate student, Valerie Schwartz, is working on this project on several fronts. She is increasing the resolution of the smear slide data at ODP site 1022 in addition to doing smear slide analysis at ODP site 1014. I hope to recruit an additional undergraduate student to do the laser particle size analysis at ODP site 1014. In addition, Valerie has begun the complex task of identifying additional biomarkers in the sediments. Biomarkers have been used to reconstruct SSTs and the relative abundances of different phytoplankton on many time scales. Biomarkers make it possible to reconstruct the relative abundances of diatoms, coccolithophorids, and dinoflagelates. We have worked with colleagues at UC Santa Cruz to learn the laboratory techniques required to extract the organic fraction from the sediment and separate the extract into its different components using silica column separation. The next step in this process is to work with colleagues at UC Santa Cruz and the Virginia Institute of Marine Science to identify the organics present in the sediment. This step involves developing the GC-MS methods (column choice, oven program, etc) as well as incorporating a GC-MS library to identify the molecules.