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Constraining Greenland Ice Sheet Mass Balance Using Salt Marsh Sediment Sequences
A research project of the Department of Geography.
The Greenland Ice Sheet (GIS) occupies centre stage in current debates about the nature and impacts of global warming, with a combination of model simulations, satellite observations and airborn altimeter data all suggesting that rising northern hemisphere temperatures are causing accelerated melting, with slight inland thickening offset by significant thinning and accelerated ice flow at the margins. A consequence of this is that the GIS is now making an increasingly positive contribution to global sea-level rise. Models predict that there is a critical temperature threshold - or “tipping point” - beyond which the GIS will disappear entirely, and that this threshold will be passed this century.
However, many of these observations are short-term and span only the last decade or so and, although we now appreciate that the ice sheet is more dynamic than previously thought, it is not clear whether the current dynamics, especially those associated with the major ice streams, are typical of the past. For these reasons, it is important that we determine the century-scale mass balance trajectory of the GIS, so that these recent observations can be placed in an appropriate longer-term context.
Since the mid 1990s, we have developed new millennial-scale RSL records using the “islolation basin” approach. The method has enabled us to define a new glacial-isostatic rebound model using our observations from west and east Greenland. However, as awareness of the dynamic response of the GIS to climate heightened, so we have shifted our focus to developing RSL records from the last few hundred years.
Recently, we have pioneered the extraction of century and decadal-scale RSL records from salt marshes that fringe the GIS. Our approach has several advantages over the “isolation basin” method:
- The salt marshes can be sampled at close sampling intervals (typically 10 cm vertical separation between samples) from laterally continuous stratigraphic sequences that can be easily exposed in the field. In contrast, drowned isolation basins are few in number and each basin yields only one index point for the late Holocene rise in RSL
- The salt marshes contain plant macrofossils for radiocarbon dating and are also suitable for other geochemical analyses e.g. lead and cesium
- They also contain diatoms that can be compared with present-day assemblages to develop quantitative sea-level reconstructions via transfer functions
- Salt marsh deposits typically extend from at least 1m below the high tide mark up to the current highest level of tidal flooding - covering the last 1000 years or so, right up to the present day
The intention of this project is to establish a network of field sites that can provide a bridge between the longer-term record of RSL change and the recent observations of the geodetic era, ideally by coupling salt marsh sites with those supporting a permanent GPS receiver under the GNET programme. The work is important to a variety of researchers interested in establishing the changing mass balance of the Greenland Ice Sheet. Corrections for bedrock motions associated with GIA are required by those interpreting a range of geodetic data, including measurements of vertical land motion (GPS), regional gravity changes (GRACE - Gravity Recovery and Climate Experiment), as well as changes in ice height (satellite and airborne altimetry).
Modifying techniques first developed in temperate and low latitude settings, we have established well-defined RSL records from two field sites in Disko Bugt, from near Sisimiut and from a third site at Nanortalik, on the south tip of Greenland. We identify a pronounced regional slow-down in the rate of RSL after c. AD 1600 which we think may record a hitherto unrecognised change in ice sheet mass balance. We are presently exploring the possible cause of this change, and the subsequent trends in RSL leading up to the present day, using a variety of ice sheet modelling experiments. In 2014 we collected new salt marsh records from SE Greenland with the hope that we will be able to reconstruct RSL changes in the last 1000 years from the entire southern sector of the GIS in the near future.