What: The overarching goal of this study is to improve our understanding of ice dynamics, glacial history and paleoproductivity of the Pliocene Weddell Sea using sediment cores from ODP Sites 693 and 695.
Who: Suzanne O’Connell, 4 Students
When: early/mid- June to early/mid-July
Where: Students will spend the first week to ten days at the International Ocean Discovery Program (IODP) core facility at Texas A&M University in College Station, Texas. There they will measure magnetic susceptibility, XRF and image the archive half of split cores. They will also sample cores for future work. They will return to Wesleyan for the second two weeks to work on sample and data preparation
Project Description: Today the Southern Ocean plays multiple critical roles in regulating earth’s climate and biogeochemical cycles. 1) Along the Polar Front Zone, Antarctic Intermediate Water (AIW) sinks, carrying CO2 and O2 to mid ocean depths. 2) South of the Polar Front, CO2 from the deep ocean upwells and directly exchanges2 with the atmosphere and provides nutrients to support a rich biological community. 3) Further south, in the Weddell Sea, the densest water in the entire ocean, Antarctic Bottom Water (AABW) sinks carrying CO2 and O2 to the deepest ocean. Thus the Southern Ocean, and in particular the Weddell Sea is a major source and sink for CO2 and therefore a potential regulator for Earth’s climate.
We don’t know how similar the modern ocean is to the Pliocene ocean, but using sediments from ODP Expedition 113, we have an opportunity to discover more about Antarctica and the Weddell Sea during this critical climatic time. During the Pliocene, Earth’s climate began the transition from an obliquity to an eccentricity dominated climate system as shown in the most recently compiled benthic foraminifera record (Lisiecki and Raymo, 2004). In addition, paleoclimate estimates suggest that atmospheric CO2 levels were at 400 ppm, levels Earth’s atmosphere has just recently achieved.
Deciphering Earth’s climatic history from geologic samples is fraught with problems. These include diagenisis, low sedimentation rates, sediment disturbance, missing intervals, etc. Nevertheless, it is worth persevering, as that is all that is available. Ocean sediment cores provide an excellent opportunity to identify paleoceanographic changes using sediment composition (size and mineralogy), microfossil assemblages, bioturbation and magnetic properties.
In this project we will examine Pliocene sediments from two ODP sites on different sides of the Weddell Sea, Site 693 on the East Antarctic Continental Slope in the southeastern Weddell Sea and Site 695 roughly 400 km from the tip of the Antarctic Peninsula in the northwestern Weddell Sea (Fig. 1, Table 1). These two sites are moderately to well recovered (Fig.2); provide a thick Pliocene sedimentary sequence and high sedimentation rates. Sediments are dominantly silty and clayey muds with varying amounts of biosilica, primarily diatoms. Dropstones are common at both sites and Site 695 has more volcaniclastic sediments.
Extensive research on these cores has been held back because of the problem of age determination. With the results of ANDRILL (Konfirst et al., 2011) and ODP Leg 318 (Tauxe et al., 2012), better chronostratigraphy is possible for these carbonate-free sediments. We have begun to use the new diatom biostratigraphy to narrow the possible sediment ages (Table 2) and will continue this effort. The diatom stratigraphy indicates very high sedimentation rates, >10cm/1000 years and puts the sediments that we focused on during the summer 2014 Keck project older than the Pliocene warm period (3.3 and 2.9 million years ago) that we anticipated. (Chandler et al., 1994; Fedorov et al., 2013).
Diatoms ages at Site 693, between about 50 and 110 mbsf, an almost continuously recovered portion place it in the lower Pliocene with a very high sedimentation rate of > 10 cm/1000 years. This provides an opportunity to observe sedimentary changes on an obliquity scale. An obliquity signal has been identified from the ANDRILL 1B site in the Ross Sea (Konfirst et al., 2011), with higher productivity and more diatoms during warmer periods and more hemipelagic muds during cooler intervals.
Another approach to determining orbital forcing and sedimentation rates is wavelet analysis (Darby et al., 2012; Sen et al., 2009). The orbital cycles identified at Site 693, using wavelet analysis on a variety of core measurements such as XRF, spectra and color (Hall et al., 2014) also show a strong obliquity-driven forcing. We are just beginning an analysis to identify the sedimentary changes that result from the forcing. One component appears to be a glauconite signal. Glauconite is found in the underlying Cretaceous sediments (OConnell, 1990). Cretaceous sediments were also recovered in dredge hauls of the canyon wall (Futterer et al., 1990). Futterer et. al (1990) speculate that the canyons were carved by glacial activity. Relative abundances of glauconite in combination with other sediment measurements (XRF, diatoms, grain size, etc.) will be used during the current academic year to attempt to define glacial interglacial cycles from Core 693A-8R. The results of this study will inform the specific projects proposed for 2015-2016 research.
Possible student research projects for either Site 693 or Site 695.
Changes in productivity. Higher productivity is associated with higher wt% biosilica, higher Ba/Al ratios (XRF) and higher diatom valve concentrations (Cook et al., 2013). If there was someone at a home institution to identify specific diatom species more subtle changes in biological communities might also be identified (Cortese and Gersonde, 2008). Most geochemistry labs can measure wt% biosilica (Mortlock and Froelich, 1989). Possibly C/N and N isotopes might also enhance understanding productivity signals and changes. The XRF data will be collected during the summer at IODP.
Infaunal communities. Bioturbation varies throughout Site 693. High-resolution core images make it possible to identify different marine-floor sedimentary conditions using specific trace fossils or ichnofabric. These in turn might show glacial/interglacial or high and low productivity cyclicity. This would be done at the home institution using high-resolution core images.
Paleomagnetic stratigraphy and provenance tracer. The magnetic reversal stratigraphy is poorly defined for Site 693 (e.g. Fig. 33, p 373 (Barker and Kennett, 1988)). Yet, based on the IODP 318 Expedition data, the current age calibrations of several diatom events in the Pliocene are “too young” to correlate with the Global Paleomagnetic Time Scale (Fig. 16 in (Tauxe et al., 2012)). The high sedimentation rate at this interval could help to improve the diatom chronostratigraphy. Rock magnetic properties themselves can also be used to trace provenance (Brachfeld et al., 2013; Tauxe et al., 2012). This project would require access to paleomagnetic facilities.
Wavelet Analysis: can be done on the principal components of the XRF data, spectral data on the sediment, and possibly other measurements. The wavelets are used to identify frequency spectra within the analyzed material. By assigning an age to the frequency, a sedimentation rate is estimated. This can be used to look for orbitally forced and non-orbitally forced changes in the depositional environment.
Provenance/mineralogy: can be addressed in multiple ways. Dropstones are common and predominantly ice rafted. Thin section and geochemical analyses of dropstones might allow a source region to be identified. This in turn can suggest iceberg travel paths (Williams et al., 2010). Icebergs are thought to flow for greater distances during colder climate intervals. Sand grains are common throughout the cores. The mineralogy of grains in grain mounts can be identified petrographycally and chemically. With access to an XRD, a student could determine the mineralogy of different sediment size fractions.
Grain-size analysis: Grain size analysis can help to identify potential transport mechanisms of the sediment. In particular, we would like to see if there is a very fine fraction that might be wind blown. Prevailing westerly winds suggest that windblown sediment from South America is unlikely to reach Antarctica today or even during the last glacial (McCave et al., 2014). In particular at Site 695, is there a In samples with a fine fraction, that component could be separated and analyzed for chemistry and mineralogy to see if the source was similar to the sand-size fraction.
References
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