Past Temperature & Precipitation

Summary

The most effective way of constraining ice loss at the ice sheet margins is through measuring change in Surface Mass Balance (SMB). While dynamic change through ocean forcing can trigger rapid mass loss, in periods of rapid prolonged warming like we see today, ice sheet SMB is the dominant driver. ​​​Determining past temperature and precipitation is critical to our ability to calculate change over time in Greenland’s SMB. To improve SMB estimates of change the GRate project will assimilate temperature and precipitation sensitive proxy records from ice cores, and from lake and marine sediment cores along the Greenland margin and surrounding ocean. ​​

Lake sediments contain fossil lipids that are produced by living things and that change their chemical structure with changes in temperature and the water cycle. As part of the GRate project, we are collecting lake sediment cores from sites around Greenland. Back in the lab at the University at Buffalo, we are extracting these lipids and measuring their chemical composition, which we then use to infer past temperature and water cycle changes. Elizabeth Thomas, PI from University @ Buffalo will lead this part of the research. 

Marine sediment samples contain Arctic dinocysts assemblages. They are analyzed as a proxy for sea ice cover along Greenland's coast. Dinocysts are dormant zygotes of dinoflagellates, a small plankton that can be used as paleoclimate markers in the Arctic of past sea ice cover, water temperature and salinity. Through producing and compiling records from dynocyst assemblages in ocean sediment cores in targeted areas along the Greenland coast we will produce and compile ocean temperature and ice data for the area. Anne de Vernal, PI from GEOTOP Montreal will lead the work for the marine and analysis.

Paleoclimate data assimilation involves the combination of information from proxy archives (such as our lake and marine sediment samples) and numerical climate model simulations to produced data-constrained gridded climate histories. Our research team will use data assimilation to combine information from paleoclimate records generated as part of the GRate project and published records of Arctic climate with isotope enabled paleoclimate model simulations. The integrated estimate of temperature and precipitation change in Greenland we produce will provide a comprehensive understanding of past climate evolution in the region, which will be used to provide the boundary conditions necessary to force our ice-sheet model. Greg Hakim and Eric Steig, PIs from University of Washington, and Olivia Truax, postdoctoral scholar from the University of Washington, will lead this part of the research.

Ice Cores

One method of reconstructing past precipitation is with ice core data. Deuterium and Oxygen18 isotopes can be used in reconstructing historic atmospheric temperature and water vapor transport. For the Snow on Ice project data from different length Greenland ice cores were analyzed to examine precipitation records from different time periods. Several deep ice cores that spanned the entire Holocene were used to provide the long-term record needed, with additional shallow cores analyzed to validate a shorter-term record. More recently, the incorporation of independent estimates from gas data (δ15N of N2) to develop a Greenland wide temperature reconstruction, has been explored by others (Buizert et al. (2018). GRate will incorporate such results in our project assimilations. In addtion, we will bring in recent records around the south of Greenland, E Greenland and the NE Greenland Ice Stream, where previously there have been few available records. 

Expanding our use of ice core data will be important to better constrain our findings, yet this alone can not provide the kind of spatial or temporal coverage needed to determine ice sheet margins through time. The  GRate projectlike our previous Snow on Ice project, will expand data coverage by incorporating hydrogen isotopes of leaf waxes found in lake sediments collected around the margins of the Greenland Ice Sheet.

Leaf Waxes

Scrubby plant from Greenland

Image: The Greenland landscape is primarily close cropped scrubby plants. Leaf waxes from these plants, and paleo plant material collected from the lake sediments, can be used as a proxy for precipitation and temperature data.

Summer ice melt is a critical variable in calculating SMB of the ice sheet. We will calculate SMB from a gridded data assimilation product, which incorporates information from climate models and proxy records. In addition to assimilating existing published records, we will generate new records of temperature and water cycle change around Greenland. The Greenland landscape is primarily close cropped scrubby plants. We use leaf waxes produced by plants like these and preserved in ancient lake sediments to reconstruct changes in the water cycle. We also use glycerol dialkyl glycerol tetraethers (GDGTs), lipids produced by bacteria to reconstruct changes in temperature.

Isotope samples

Image Caption: Samples prepared and ready to be run through University @ Buffalo's isotope ratio mass spectrometer to establish a hydrogen isotope ratio. In the lab, solvent is pumped through the mud to release the waxes, next the sample is purified and analyzed for the hydrogen isotopes of the leaf waxes. These are used to reconstruction a picture of paleo-climate temperature and moisture balance in the Holocene. (photo D.Levere University at Buffalo)

Leaf waxes are molecules composed mainly of hydrogen and carbon, produced by terrestrial and aquatic plants as a protective coating on their leaves. Terrestrial plants produce leaf waxes with long carbon chains, whereas aquatic plants produce leaf waxes with shorter carbon chains, allowing us to measure the hydrogen isotopes (δ2H) of aquatic and terrestrial leaf waxes separately. Leaf waxes are blown or abraded off leaves and are deposited and preserved in lake sediment. Leaf wax δ2H reflects plant source water δ2H: i.e., aquatic δ2H reflects mean annual precipitation as recorded in lake water and terrestrial δ2H reflects growing season precipitation as recorded in soil water. The difference between aquatic and terrestrial δ2H tells us about winter snowfall, an important parameter for ice sheet mass balance.

GDGTs are molecules composed mainly of hydrogen and carbon, with some oxygen, and form the cell membrane surrounding bacteria. The chemical structure of the GDGTs changes with temperature. We extract GDGTs from the same samples as leaf waxes, and analyze them to determine past changes in temperature.

To reconstruct temperature and winter snowfall, records will be calculated from five lakes in SW Greenland, where sediment cores were collected, dated, and extracted for lipid analyses as part of the Snow On Ice project. We will add to these cores, collecting at four new locations on NW, N, and E Greenland, filling in gaps in published records.

Paleoceanography

Estelle and sediment core

Image Caption: Sediment cores were collected from along the coast in SW Greenland for the Snow on Ice project, and we will build on this for the GRate project. Plankton collected from the sediments provide important information on temperature data and sea ice cover in the past.

Plankton samples collected from ocean sediments are used to calculate past temperature based on their salinity and temperature preferences. GRate will build on our work from the Snow on Ice project through compiling marine records to establish ocean temperature and salinity focusing at the surface and subsurface areas, as well as sea-ice cover. The depth stratification and environmental preferences of planktic foraminifera are important variables in reconstructing paleoceanographic changes.

Planktic foraminifer assemblages mostly reflect subsurface water because of strong stratification that sorts organisms with very specific salinity needs, while benthic foraminifers from the continental shelf are used to calculate the the same measurements in water masses at 300-500 m water depth. Both measures are important in determining the structure of the water mass, however to date there has been little work done on sub-surface water properties. The work by the GRate team is an important contribution to the field.

NSF logo

 

The GRate project is funded under the National Science Foundation (NSF) Office of Polar Programs