Holocene sea-level fluctuations inferred from the evolution of depositional environments of the southern Langebaan Lagoon salt marsh, South Africa

A Holocene sea-level curve is constructed from the facies distribution and radiocarbon ages of sediment recovered from the distal, southern salt marsh of Langebaan Lagoon, on the southwest coast of South Africa. Calibrated ra diocarbon analyses of an oyster-rich bioclastic gravel indicate that the Fl andrian Transgression flooded the lagoon to 0-3 m above present-day levels by 6750 years ago (6.8 ka). Organic matter and shell material dated in dist al lagoonal sediments indicate that sea level returned to present-day level s by 4.9 ka and have since remained within +/-1 m of present-day levels. Bl eached shell and a hiatus in sedimentation suggest an approximate 1 m sea-l evel lowstand between 2.5 and 1.8 ka Changes in the macrobenthos assemblage since 1.5 ka that include the loss of the oyster Ostrea atherstonei, razor clam Solen capensis, brown mussel Arcuatula capensis and periwinkle Oxyste le variegata reflect loss of hard substrate, decreased tidal-flow velocitie s as reworked sands prograded into the southern lagoon and possibly cooler sea-surface temperatures. Calibrated radiocarbon ages of bulk organic matte r from diatom-rich, Zostera muddy quartzose sands indicate a 0.5 m sea-leve l highstand at 1.3 ka followed by a 0.5 m lowstand at 0.7 ka. Dating of fin ing-upward, organic-rich (2 wt % TOC) noncalcareous muds indicates that the present-day salt marsh has grown by aggradation (similar to1 mm y(-1)) and progradation since 0.7 ka.