Holocene sea level and environmental change on the west coast of South Africa: evidence from plant biomarkers, stable isotopes and pollen

Michael E Meadows,Brian M Chase,Arnoud Boom,Hannah L Grimes,Andrew S Carr

doi:10.1007/s10933-015-9833-7

Abstract

We present an 8000-year biomarker and stable carbon isotope record from the Verlorenvlei Estuary, South Africa. We assessed how leaf wax lipids, insoluble macromolecular organic matter, bulk C/N data and compound-specific stable carbon iso- topes were linked to the site's palynological record and to evidence for regional sea level and environ- mental change. Down-core trends in bulk d 13 C are closely coupled to trends in pollen types from saline- tolerant taxa. These trends are mirrored by variations in the incorporation of reduced sulphur into macro- molecular organic matter. This process, quantified with the thiophene ratio, is closely associated with periods of higher sea level 8,000-4,300 cal yr BP. We propose the thiophene ratio is a proxy for relative marine influence within (peri) estuarine sediments. All measured variables indicate differences between ear- ly-middle Holocene (8,000-4,300 cal BP) and late Holocene conditions at Verlorenvlei. The former period was more saline and preserves more labile macromolecular organic matter. Marine influence declined after 4,300 cal yr BP, and although the abundance of short-chain-length n-alkanes suggests continued presence of wetland flora until 2,500 cal yr BP, organic matter preservation became poorer and a drying trend was inferred, most notably for the interval 2,500-900 cal BP. Increasing freshwater inundation is apparent during the last 700 cal yr, consistent with several records from this region. Leaf wax n-alkane distributions are largely uncorrelated with bulk or- ganic matter variables, with the exception of the abundance of C31 and C33 n-alkanes, which are negatively correlated with d 13 CTOC. Furthermore, C31-C33 n-alkane d 13 C values are uncorrelated with

Highlights

Data are suggestive of a dominantly terrestrial or riparian plant organic matter source throughout the Holocene (Meyers 1997), sediments deposited after 4,000 cal yr BP tend to be associated with higher total organic carbon (TOC)/total nitrogen (TN) and lower δ13CTOC compared to those deposited prior to 4,000 cal yr BP (ESM Fig. 7)
The data are indicative of a vegetation response to varying salinity around Klaarfontein, which was strongly affected by marine inundation
We propose that the timing and relative influence of marine incursions can be further constrained by the thiophene ratio, which provides indirect evidence for marine inundation via evidence for the incorporation of reduced sulphur into macromolecular organic matter

Summary

Introduction

Down-core trends in bulk δ13C are closely coupled to trends in pollen types from saline-tolerant taxa These trends are mirrored by variations in the incorporation of reduced sulphur into macromolecular organic matter. This process, quantified with the thiophene ratio, is closely associated with periods of higher sea level 8,000-4,300 cal yr BP. They are higher than our newly measured terrestrial (C3) vegetation C29 and C31 endmember values of -35 ± 2 and -34 ± 1‰, respectively These patterns are best explained by a dominant contribution of local riparian vegetation to the C23-C29 n-alkanes, but time-varying contributions of non-local leaf waxes to the C31-C33 signals. The mid-latitude westerly systems and the South Atlantic Anticyclone (Stager et al.2012), and possesses an important archaeological record (Parkington et al 1988; Parkington 2012)

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Discussion

Conclusion