New radiocarbon dates from sapropels in three Holocene lakes of the Coorong coastal plain, southeastern Australia

Abstract

The Holocene calcareous mudstone successions of numerous shallow coastal lakes in the Coorong region of southeastern South Australia include prominent organic-rich intervals (0.01 - 1 m thick; total organic carbon 2%), herein termed sapropels. New radiocarbon dates (n = 25) on bulk organic matter in single cores from three such lakes indicate that these sapropels are older in North Stromatolite Lake, part of the Salt Creek lake chain, than 100 km further south in the Robe Range at Old Man Lake and Lake Amy. The onset of sapropel deposition in North Stromatolite Lake at ca 6400 cal y BP marked its isolation from the open Coorong Lagoon, whereas in Old Man Lake, the onset of sapropel formation at ca 4000 cal y BP coincided with the transition from lagoonal to perennial lacustrine conditions as the sea withdrew from the interdunal Robe - Woakwine corridor. In contrast, the depocentre at Lake Amy was never directly connected to the marine system. Here, organic matter from near the base of the lowermost sapropel, dated at ca 4100 cal y BP, provides a minimum age for the onset of lacustrine conditions. Sapropel deposition recorded thus far in these lacustrine sequences ceased earlier in North Stromatolite Lake (ca 5000 cal y BP) than in Old Man Lake (ca 2600 cal y BP) and Lake Amy (ca 1900 cal y BP). This may simply reflect a marked local climatic gradient (drier in the north, wetter in the south). Alternatively, it may indicate that these shallow lacustrine sapropels, unlike their marine and deep-lake counterparts, do not form only during times of high precipitation but may be triggered by other processes such as the enhanced aeolianite influx associated with aridification.