Climate change at the Oligocene-Miocene boundary: new insights from an oyster δ^{18}O record from the Mediterranean margin
Abstract
A major feature of the long-term climate evolution during the Cenozoic is the transition from the "greenhouse" conditions of the late Early Eocene (∼50 million years ago) to our modern "icehouse" conditions with its much lower CO2 levels and significant polar glaciation (Zachos et al., 2008). This transition occurred through a series of steps, amongst which the cooling at the Oligocene-Miocene boundary (known as Mi-1) that is associated to a large-scale expansion of Antarctic ice-sheets. Identification of cooling and polar ice-growth during the Oligocene and Miocene largely relies on δ18O and Mg/Ca benthic foraminifera records from ODP/DSDP sites (e.g. Zachos et al., 2008; Mawbey and Lear, 2014; Beddow et al., 2016), and few records currently exist from shallow environments. In this context the purpose of this study is to provide a new record of the evolution of shallow (<50m) seawater temperatures during the Oligocene and across the Oligocene-Miocene transition, from the oxygen isotope composition of bivalves collected within an open marine setting. Oyster δ18O have been widely used for paleoclimatic studies, with the advantage of being easily screened using cathodoluminescence to avoid altered areas (Kirby 2000; Lartaud et al., 2010). While they have a sparser temporal and spatial distribution, they provide access to seasonal paleotemperature evolution that cannot be approached using foraminifera geochemical data from deep-sea cores. Over 200 oyster shells have been recovered from 26 different levels of the Carry-le-Rouet outcrop on the northern margin of the Liguro-Provençal Basin. This site presents numerous other bivalves in a well-defined chronostratigraphic framework with sediments deposited from the Chattian to the Burdigalian (Oudet et al., 2010; Demory et al., 2011). Bivalves recovered from environments of normal salinity only, based on floro-faunal associations, were carefully selected and screened under cathodoluminescence to select pristine calcite for isotopic analyses. Following a decrease of about 1.5 ‰ for the Chattian to the Aquitanian, our record highlights an increase of about 2 ‰ in oyster δ18O values throughout the Oligocene-Miocene boundary at Carry-le-Rouet, providing a new evidence for the record of cooling and/or increase of ocean δ18O in low-latitude and proximal environments. Interestingly, maximum oyster δ18O values coincide with the occurrence of abundant bryozoans in the sedimentary succession, which supports local cooling of seawater. High-resolution sampling within the biggest bivalve shells, perpendicularly to growth increments, revealed the existence of large seasonal variations of temperature and/or local seawater δ18O, which is presented here for the first time.
- Publication:
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EGU General Assembly Conference Abstracts
- Pub Date:
- April 2018
- Bibcode:
- 2018EGUGA..2015360B