The biogeochemical evolution of the Coorong during the mid- to late Holocene: An elemental, isotopic and biomarker perspective q David M. McKirdy a, * , Carly S. Thorpe a,1 , Deborah E. Haynes a , Kliti Grice b , Evelyn S. Krull c , Galen P. Halverson a , Lynn J. Webster a a Organic Geochemistry in Basin Analysis Group, Centre for Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, University of Adelaide, SA 5005, Australia b Western Australian Organic and Isotope Geochemistry Centre, The Institute for Geoscience Research, Curtin University of Technology, Perth, WA 6163, Australia c CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia article info Article history: Received 26 April 2009 Received in revised form 13 July 2009 Accepted 16 July 2009 Available online 19 July 2009 abstract Since the mid-1990s the Coorong, a 130-km-long coastal wetland of international importance in south- eastern South Australia, has undergone serious decline due largely to prolonged drought and reduced inflows of surface and ground water from its hinterland. Tenuously connected to the ocean at its northern extremity, it is effectively separated into two lagoons by a narrow channel at its mid-point. The southern part is now permanently hypersaline. Here we report the results of an attempt to track the biogeochem- ical status of the estuary over the last 5000 years employing a consortium of elemental, isotopic and bio- marker parameters. The sediment samples analysed came from two cores, each 6–7 m in length, retrieved from sites located halfway along the present-day northern and southern lagoons. The sediments at the southern locality are richer in total organic carbon and total nitrogen than those in the northern core reflecting higher net deposition of organic matter (OM) and nutrients in the southern lagoon. Comparison of the d 13 C org and d 15 N profiles of the two cores reveals that the lower reaches of the Coorong have always been a restricted, O 2 depleted water body that is both more saline and prone to more intensive denitri- fication of NO  3 (aq) than the northern lagoon. Prominent among the aliphatic hydrocarbons preserved in the sediments of the southern lagoon are the C 20 highly branched isoprenoid (HBI), 2,6,10-trimethyl-7- (3-methylbutyl)-dodecane, a putative marker of epiphytic diatoms, and C 15 –C 35 n-alkanes with a marked odd/even predominance in the C 21+ range. These and other biomarkers are consistent with OM derived from a mixed population of algae, cyanobacteria and other eubacteria. 13 C NMR spectra confirm that this OM is largely algal derived, but with a significant contribution from the seagrass Ruppia megacarpa (and possibly other aquatic macrophytes), particularly in pre-European sediments. The C isotopic signature of the HBI alkane is consistently heavier than those of the C 23 ,C 25 ,C 27 and C 29 n-alkanes (presumably from planktonic chlorophytes ± aquatic metaphytes), which in turn are heavier than the coexisting even-car- bon-numbered homologues of likely bacterial origin. With decreasing age these biomarker alkanes dis- play an overall increased depletion in 13 C and enrichment in D, trends that have accelerated over recent decades as the south lagoon has become progressively more hypersaline and ecologically impov- erished. However, coeval negative excursions in the dD profiles of the epiphyte marker and n-alkanes, along with coincident diatom inferred reductions in palaeosalinity, together highlight a series of hitherto unrecognised freshening events attributable to enhanced inflow of surface runoff and shallow groundwater. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The Coorong is an elongate lagoon complex that extends for 130 km southeast from the mouth of the wave-dominated shallow estuary of the River Murray in South Australia (Fig. 1). Separated from the Indian Ocean by Holocene dunes of the Younghusband Peninsula, it is divided by a narrow channel at Parnka Point (viz. Hell’s Gate Passage) into a north lagoon, which is open to the ocean via the Murray Mouth, and a south lagoon which receives sur- face runoff via Salt Creek and groundwater from the southeast 0146-6380/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2009.07.010 q A contribution to the special issue of Organic Geochemistry comprising papers presented at the 15th Australian Organic Geochemistry Conference, Adelaide, September 2008. * Corresponding author. Tel.: +61 8 8303 8146; fax: +61 8 8303 4347. E-mail address: david.mckirdy@adelaide.edu.au (D.M. McKirdy). 1 Present address: Minerals and Metals Group, Golden Grove, PMB 7, Geraldton, WA 6531, Australia. Organic Geochemistry 41 (2010) 96–110 Contents lists available at ScienceDirect Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem