Middle Miocene carbon cycle dynamics: A multi-proxy approach

The development of permanent, stable ice sheets in Antarctica happened during the middle Miocene, about 14 million years ago (Flower and Kennett, 1995; Zachos et al., 2001). The middle Miocene therefore represents one of the distinct phases of rapid change in the transition from the “greenhouse” of the Cretaceous to the “icehouse” of the present day. Accompanying the middle Miocene growth of the Antarctic Ice Sheet are major perturbations in the global carbon system, represented by some of the largest fluctuations in marine carbonate δ13C values in the Cenozoic (Flower and Kennett, 1995; Zachos et al., 2001). A broad positive carbon isotope excursion; the “Monterey Excursion” (Vincent and Berger, 1985) begins in the early Miocene (approximately 16.9 Myr ago) and terminates in the middle Miocene ~13.8Myr ago (Holbourn et al., 2007). Within this broad δ13C excursion, higher frequency fluctuations have been recognised with at least 7 carbon isotope maxima (CM) defined (Woodruff and Savin, 1991). The Ras il-Pellegrin section, Malta spans the 1.1Myrs following the growth of the ice sheet. The simple tectonic history and clay-rich sediments of the Serravallian Blue Clay Formation has led to exceptional preservation of foraminifera and organic biomarkers. Stable isotope analysis of bulk carbonate and planktonic foraminifera Globigerinoides trilobus at Ras il-Pellegrin show the globally recognised positive carbon isotope excursion CM6 and the final stages of the ice sheet expansion (“E3”; Miller et al. 1993). Combined alkenone unsaturation index (UK'37 ) and G. trilobus Mg/Ca ratios were used to reconstruct sea surface temperature. Over CM6 temperatures dropped by ~5°C as the ice sheet expanded, and the temperature contribution to the δ18O record is ~30%, in agreement with other studies (Billups and Schrag, 2002; Lear et al., 2000; Shevenell et al., 2008). Sea surface temperatures estimated from G. trilobus Mg/Ca at this Mediterranean site averaged ~30ºC. Atmospheric pCO2 decreased as the ice sheet expansion progressed, coinciding with CM6. The magnitude of the decrease is on order ~70 ppm, based on alkenone and boron palaeobarometry. The absolute magnitude of atmospheric pCO2 is relatively low (~300 ppm) in general agreement with other Neogene studies (Kürschner et al., 2008; Pagani et al., 1999a; Pearson and Palmer, 2000) in a time of relatively high temperatures. On the basis of ocean carbonate system modelling coupled with the records presented here, the recently suggested “silicate weathering hypothesis” (Shevenell et al., 2008) for CM6 is untenable. The mechanism preferred here is increased organic carbon burial, in agreement with the “Monterey Hypothesis”. Coupled inorganic-organic carbon isotopes spanning the Monterey Excursion (16.9-13.5 Ma) suggest changes in the carbon system consistent with the Monterey Hypothesis. Thermal stratification of the oceans is inferred to have increased during the middle Miocene climate transition