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doi:10.2204/iodp.proc.318.203.2015

Introduction

Integrated Ocean Drilling Program (IODP) Expedition 318 to the Wilkes Land margin of Antarctica recovered a sedimentary succession ranging in age from the early Eocene to the Holocene. Site U1356 is at the transition between the continental rise and the abyssal plain at 4003 meters below sea level (mbsl) and was cored to 1006.4 meters below seafloor (mbsf) (see Figure F16 in the “Site U1356” chapter [Expedition 318 Scientists, 2011b]). The sediments are divided into 11 lithostratigraphic units, and age control is based on biostratigraphic datums from siliceous microfossils (diatoms and radiolarians), calcareous nannofossils, and organic- walled dinoflagellates (dinocysts) integrated with a magnetostratigraphy that is correlated, where feasible, to the geomagnetic polarity timescale (GPTS) (Tauxe et al., 2012). This data report is concerned with samples from the lowermost 110 m of Site U1356 (lithostratigraphic Units X and XI), dated to the early to middle Eocene (53.6–46 Ma) with a hiatus from 51.9 to 49.3 Ma (Tauxe et al., 2012). Unit X (Cores 318-U1356A-96R through 100R; 895.5–948.8 mbsf) consists of interbedded stratified and massive sandstones, diamictites, silty claystones, and siltstones. Unit XI (Cores 101R to 106R; 948.8–1000.08 mbsf) consists predominantly of dark green bioturbated claystone and subordinate laminated siltstone and sandstone interbeds (see the “Site U1356” chapter [Expedition 318 Scientists, 2011b]). During the Eocene, Site U1356 was at a latitude of ~65°S, and the dinocyst assemblages suggest a midshelf depositional environment much shallower than today (Pross et al., 2012).

Shore-based analyses of biotic climate proxies (pollen and spores) and indexes based on branched tetraether lipids reveals that the climate in lowland settings along the Wilkes Land coast in the early Eocene (Unit XI) supported the growth of highly diverse, near-tropical forests characterized by mesothermal to megathermal floral elements with extremely mild terrestrial winters (>10°C) (Pross et al., 2012), while sea-surface temperatures were extremely warm (Bijl et al., 2013). The middle Eocene (Unit X) is characterized by a marked sea-surface temperature and continental cooling (2°–4°C) (Bijl et al., 2013), loss of megathermal flora, and dominance of a temperate rain forest assemblage (Pross et al., 2012). Furthermore, ongoing work on biomarkers from Site U1356 confirms the presence of the bacterial-derived, C₃₁ (17α, 21β) homohopane within a suite of immature hopanes. The timing of the concentration fluctuations and the molecular structure of hopanes and their compound-specific carbon isotope values (δ¹³C) reveal the extent to which wetlands fluctuated in response to external, orbital forcing (Toney et al., 2012).

The initial report of alkenones in the Eocene sediments of Site U1356 was significant because alkenones can be utilized both for paleotemperature estimation (e.g., Conte et al., 2006; Prahl and Wakeham, 1987) and as a proxy for the estimation of past pCO₂ levels (e.g., Zhang et al., 2013). Thus, we undertook shore-based analysis of 50 samples from Site U1356 to confirm the initial report of the presence of alkenones and to assess whether sufficient concentrations were present for compound-specific δ¹³C measurements.

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