Exploring Northern Hemisphere Ice Sheet Variability in the Pliocene using Ice Rafted Debris Records and Iceberg Trajectory Modelling

Abstract:

The Greenland Ice Sheet (GRIS) contains approximately 7.36m of sea level equivalent that could melt over the next 1000 years. Therefore understanding the response of the GRIS during past warm periods is fundamentally important. The Late Pliocene (LP) has similarities to the projected 21^st Century climate, and there is enough available data to constrain ice sheet and iceberg modelling studies, thus it make the LP a useful palaeoclimate modelling target.

Within the LP, the mid Piacenzian Warm Period (mPWP) is particularly well documented. The USGS PRISM (Pliocene Research, Interpretation & Synoptic Mapping) project focuses on the mPWP (3.26 –3.025Ma) and was chosen for the similarities to modern in paleogeography, CO₂ levels and because fossil assemblages of LP foraminifera are largely composed of extant species. Data sets of PRISM are used as boundary conditions in climate models which simulate ice sheet and iceberg scenarios in warm conditions of the mid/high latitudes. In the mPWP, temperatures were 2–3°C warmer than present and CO₂ level was about 405ppmv. Multi-climate and multi-ice sheet modelling studies show retreat in the GRIS to higher elevation.

However, immediately prior to the mPWP, Marine Isotope Stage M2 (3.3Ma) is a cold period in the warmer LP background. Localized evidence of ice during the M2 exists but if a larger northern hemisphere (NH) glaciation occurred, evidence has been erased. Evidence shows a drop in sea level up to 60m and CO₂ at 220ppmv. Climate models show a medium/large NH ice cover is plausible at the M2. The exact extent during both warm and cold periods in the LP remains unclear.

Evidence of this extent can be seen in marine sediment cores as ice rafted debris (IRD) which helps decipher the state of the ice sheet. The distribution of mPWP North Atlantic IRD in space and time tells us about the location of iceberg-producing glaciers of the NH. Using the M2 and mPWP climate scenarios, we have modelled iceberg trajectories to see where IRD would be found in the North Atlantic under warm and cold LP conditions. These are compared with the location of IRD evidence in the North Atlantic and presented here.