Abstract
The Pliocene Epoch (5.2 to 2.58Ma) has often been targeted to investigate the nature of warm climates. However, climate records for the Pliocene exhibit significant variability and show intervals that apparently experienced a cooler than modern climate. Marine Isotope Stage (MIS) M2 (~3.3Ma) is a globally recognisable cooling event that disturbs an otherwise relatively (compared to present-day) warm background climate state. It remains unclear whether this event corresponds to significant ice sheet build-up in the Northern and Southern Hemisphere. Estimates of sea level for this interval vary, and range from modern values to estimates of 65m sea level fall with respect to present day. Here we implement plausible M2 ice sheet configurations into a coupled atmosphere–ocean climate model to test the hypothesis that larger-than-modern ice sheet configurations may have existed at M2. Climate model results are compared with proxy climate data available for M2 to assess the plausibility of each ice sheet configuration. Whilst the outcomes of our data/model comparisons are not in all cases straight forward to interpret, there is little indication that results from model simulations in which significant ice masses have been prescribed in the Northern Hemisphere are incompatible with proxy data from the North Atlantic, Northeast Arctic Russia, North Africa and the Southern Ocean. Therefore, our model results do not preclude the possibility of the existence of larger ice masses during M2 in the Northern or Southern Hemisphere. Specifically they are not able to discount the possibility of significant ice masses in the Northern Hemisphere during the M2 event, consistent with a global sea-level fall of between 40m and 60m. This study highlights the general need for more focused and coordinated data generation in the future to improve the coverage and consistency in proxy records for M2, which will allow these and future M2 sensitivity tests to be interrogated further.
Highlights
The Late Pliocene (3.6–2.58 Ma; Piacenzian Stage; Fig. 1) represents the most recent period in Earth history where average global temperatures were warmer than present-day (Haywood and Valdes, 2004; Dowsett et al, 2010) and peak carbon dioxide (CO2) levels were between ~50 and 125 ppmv higher than pre-industrial (e.g. Bartoli et al, 2011)
Based on a marine pollen record (ODP Site 658), Leroy and Dupont (1994) describe periods of marked aridity in northwestern African climate which occur during positive isotope excursions, including stage 132 (Tiedemann, 1991) which correlates to Stage M2 as proposed by Shackleton et al (1995: S95)
This paper presents the results from the first coupled atmosphere– ocean climate model experiments targeting the simulation of Marine Isotope Stage M2
Summary
The Late Pliocene (3.6–2.58 Ma; Piacenzian Stage; Fig. 1) represents the most recent period in Earth history where average global temperatures were warmer than present-day (Haywood and Valdes, 2004; Dowsett et al, 2010) and peak carbon dioxide (CO2) levels were between ~50 and 125 ppmv higher than pre-industrial (e.g. Bartoli et al, 2011). Local evidence for glaciation prior to the large scale initiation of NHG can be found across the high latitude Northern Hemisphere (McDougall and Wensink, 1966; Geirsdóttir and Eiríksson, 1994; Krissek, 1995; Lagoe and Zellers, 1996; Kleiven et al, 2002; St. John and Krissek, 2002; Gao et al, 2012; Menzies et al, 2013; Knies et al, 2014; see De Schepper et al (2014)), but there is no clear indication of large ice sheets sufficient to explain M2 sea level falls. This study presents the first global climate model simulations targeting the M2 glacial event and uses different potentially analogous Plio-Pliestocene ice sheet configurations that encompass a range of plausible M2 ice sheet volumes
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