The last forests on Antarctica: Reconstructing flora and temperature from the Neogene Sirius Group, Transantarctic Mountains

Rhian L Rees-Owen,Fiona L Gill,Robert J Newton,Ruza F Ivanović,Jane E Francis,James B Riding,Christopher H Vane,Raquel A Lopes Dos Santos

doi:10.1016/j.orggeochem.2018.01.001

Abstract

Fossil-bearing deposits in the Transantarctic Mountains, Antarctica indicate that, despite the cold nature of the continent’s climate, a tundra ecosystem grew during periods of ice sheet retreat in the mid to late Neogene (17–2.5 Ma), 480 km from the South Pole. To date, palaeotemperature reconstruction has been based only on biological ranges, thereby calling for a geochemical approach to understanding continental climate and environment. There is contradictory evidence in the fossil record as to whether this flora was mixed angiosperm-conifer vegetation, or whether by this point conifers had disappeared from the continent. In order to address these questions, we have analysed, for the first time in sediments of this age, plant and bacterial biomarkers in terrestrial sediments from the Transantarctic Mountains to reconstruct past temperature and vegetation during a period of East Antarctic Ice Sheet retreat. From tetraether lipids (MBT′/CBT palaeothermometer), we conclude that the mean continental summer temperature was ca. 5 °C, in agreement with previous reconstructions. This was warm enough to have allowed woody vegetation to survive and reproduce even during the austral winter. Biomarkers from vascular plants indicate a low diversity and spatially variable flora consisting of higher plants, moss and algal mats growing in microenvironments in a glacial outwash system. Abietane-type compounds were abundant in some samples, indicating that conifers, most likely Podocarpaceae, grew on the Antarctic continent well into the Neogene. This is supported by the palynological record, but not the macrofossil record for the continent, and has implications for the evolution of vegetation on Antarctica.

Highlights

Since the first appearance of angiosperms on Antarctica in the Cretaceous more than 100 million years ago (Ma), Antarctic vegetation has undergone a significant secular change from a diverse fern-conifer dominated ecosystem, to a podocarp-southern beech temperate rainforest during the Late Cretaceous, to a low diversity tundra flora dominated by angiosperms in the Neogene (Dettmann and Thomson, 1987; Francis et al, 2008; Bowman et al, 2014)
The samples were from the Meyer Desert Formation glacigenic deposits, which comprise the upper part of the Sirius Group in the Meyer Desert and Dominion Range region of the Transantarctic Mountains (Fig. 1; Mercer, 1972)
We suggest three possible explanations for the difference between the macrofossil and biomarker record: (i) The presence of relatively degraded diterpenoids alongside better preserved angiosperm-derived triterpenoids is due to the reworking of older organic matter from sediments with a high input of coniferous plant material

Summary

Introduction

Since the first appearance of angiosperms on Antarctica in the Cretaceous more than 100 million years ago (Ma), Antarctic vegetation has undergone a significant secular change from a diverse fern-conifer dominated ecosystem, to a podocarp-southern beech temperate rainforest during the Late Cretaceous, to a low diversity tundra flora dominated by angiosperms in the Neogene (Dettmann and Thomson, 1987; Francis et al, 2008; Bowman et al, 2014). The scarcity of Neogene terrestrial deposits on Antarctica makes reconstructing vegetation difficult It appears that an extensive, low diversity mosaic tundra vegetation existed over a wide geographical range throughout the Oligocene to the mid Miocene (24–14 Ma; Hill, 1989; Raine, 1998; Askin and Raine, 2000; Prebble et al, 2006) and survived multiple episodes of glacial advance and retreat (Ashworth et al, 2007). Questions remain over both the timing of the disappearance of this tundra vegetation and its composition.

Methods

Results

Discussion

Conclusion