Sea life gives a clue about previous glacier position

Ice_final_2The Article

David K. A. Barnes D.K.A, Sands C.J., Hogg, O.T., Robinson, B.J.O., Downey, R.V., Smith, J.A. (2016). Biodiversity signature of the Last Glacial Maximum at South Georgia, Southern Ocean. Journal of Biogeography. DOI: 10.1111/jbi.12855

Summary

New research published lead by British Antarctic Survey scientists shows that marine life may be very much slower to rebound from the last ice age (Last Glacial Maximum – LGM) at the remote island of South Georgia. Barnes et al (2016) report on a very uneven distribution of marine biodiversity and the implications in the Journal of Biogeography (link). Spectacular South Georgia is best known for Albatrosses, King Penguins and wrecks of historic whaling stations, but most of its biodiversity lives on the seabed (benthos) in one of the world’s largest Marine Protected Areas. Pushed to the edge of the shelf by grounded ice in the LGM, less mobile benthos, such as bryozoans and sponges) have still made little progress back despite having thousands of years to do so. Thus the outer fringe of the shelf of the marine protected area is very much richer in species found nowhere else and we would argue is more important to monitor and manage. This ancient marginal biodiversity is at least hundreds of thousands of years old and scientists from the Antarctic Seabed Carbon Capture Change project (www.asccc.co.uk) estimate that it accumulates carbon six times quicker that the younger shelf closer to shore. When the slowest race in history finally results in seabed life completely recolonizing the shelf it could treble the carbon drawdown in this remotest wilderness – an amazing free ecostystem service. So when anyone asks so what else does biodiversity do for us?…

 

Paper abstract

 

Aim

High-latitude biodiversity distributions can preserve signals of the timing and geography of past glaciations, and as such ground truth ice-sheet models. Discrete polar archipelagos offer the fewest confounding factors for testing historic ice position records in extant biodiversity. At South Georgia, two competing geological hypotheses suggest that either the Last Glacial Maximum (LGM) ice was extensive, nearly covering the continental shelf (H1 Big ice) or restricted to the inner fjords (H2 Little ice). We examined the past configuration of the South Georgia ice cap using seabed biodiversity.

Location

South Georgia, Southern Ocean.

Methods

We used a bespoke camera lander (SUCS) and Agassiz trawl deployments across ‘big ice’ and ‘little ice’ hypothesized positions of LGM grounded ice around the South Georgia continental shelf. We investigated faunal assemblage structure and richness, especially of brooders, and modelled low dispersal taxa, for example, those with limited pelagic larvae (bryozoans and sponges).

Results

We found a striking ‘line’ of major richness discontinuity, with significantly higher richness, especially of brooders and low dispersal model taxa, mainly conforming to the ‘big ice’ hypothesized position. What few bryozoans and sponges occurred inside this line were a subset of those outside.

Main conclusions

Benthic biodiversity is consistent with extensive LGM grounded ice advancing to near the shelf break in most, but not all locations around South Georgia’s shelf, for example, the eastern shelf area. We suggest that most of the shelf is still undergoing recolonization from when grounded ice covered the shelf ~20 kyr ago. Our alternative hypothesis of LGM ice position, H3 ‘Limited-Extensive ice’, best fitted our data and is easily further testable, but if verified, shows that shelf recolonization following glaciation is much slower than previously thought. This contrasts with surprisingly rapid colonization of continental shelves after ice-shelf collapses, but these are not grounded, which may be crucial to polar recolonization rates.