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The diverse lagoons and coastal lakes along the east coast of South Africa occupy incised valleys that were flooded during the rise and subsequent stabilisation of relative sea-level during the Holocene. Sedimentary deposits contained within these waterbodies provide an opportunity to investigate complex hydrological and sedimentological processes, and examine sea-level controls governing system geomorphic evolution. In this paper, we combine diatom and sulfur isotope analyses from two sediment cores extracted from the northern sub-basins of Lake St Lucia, a large shallow estuarine lake that is today largely isolated from direct ocean influence behind a Holocene-Pleistocene barrier complex. Analyses allow the reconstruction of hydrological changes associated with the geomorphic development of the system over the mid-to late Holocene. The sedimentary sequences indicate that St Lucia was a shallow, partially enclosed estuary/embayment dominated by strong tidal flows prior to ∼6200 cal. BP. Infilling was initiated when sea-level rise slowed and stabilised around present day levels, resulting in the accumulation of fine-grained sediment behind an emergent proto-barrier. Diatom assemblages, dominated by marine benthic and epiphytic species, reveal a system structured by marine water influx and characterised by marsh and tidal flat habitats until ∼4550 cal. BP. A shift in the biological community at ∼4550 cal. BP is linked to the development of a back-barrier water body that supported a brackish community. Marine planktonics and enrichments in δ 34 S suggest recurrent, large-scale barrier inundation events during this time, coincident with a mid-Holocene sea-level highstand. Periodic marine incursions associated with episodes of enhanced storminess and overwash remained prevalent until ∼1200 cal. BP, when further barrier construction ultimately isolated the northern basins from the ocean. This study provides the first reconstruction of the palaeohydrological environment at Lake St Lucia and highlights the long-term geomorphic controls that have shaped the recent evolution and natural dynamics of the system. Unlike most coastal lake systems, this system is particularly effective as an archive of geomorphological change. Systems driven by back-barrier modifications, such as Lake St Lucia, highlight how geomorphological changes driven by sediment-supply, climate and sea level can be distributed unevenly over several isolated back-barrier basins. [ABSTRACT FROM AUTHOR]

The Holocene evolution of one of Africa's largest estuaries, Lake St Lucia on the east coast of South Africa, is examined and juxtaposed with previous and contemporary management practices aimed at fixing the estuary state at a single point along the system's overall evolutionary pathway. The estuary has been heavily altered over the course of the last century, mostly through mouth management activities aimed at maintaining a direct connection with the ocean. Based on a high-resolution seismic reflection dataset, LiDAR and bathymetry data of the lake system, together with palaeontological (foraminifera and diatoms) and sulfur isotope geochemical data from three long (14–16 m) piston cores, we investigate the system-wide and coeval evolution of the three different sub-basins (False Bay, North Lake and South Lake) that comprise the current estuarine lake system. The northern and southern sections of the system evolved independently of each other since the Last Glacial Maximum, with each area responding differently to early sea-level forcings. In North Lake, the lagoon evolved from fully estuarine (8300 cal. BP) to a restricted lagoon with occasional marine incursions, steadily decreasing tidal prism and the development of marginal spits by wind-driven lagoonal waves. The tidal inlet closed approximately 6200 cal. BP and fluvial conditions became more dominant as main back-barrier conditions became attuned to fluvial supply. Similar conditions prevailed in the adjacent False Bay basin, albeit with an earlier onset due to sheltering by prominent bedrock peninsulas. In contrast, South Lake evolved from a fully estuarine system (9500 cal. BP) with a persistent ocean connection, to a segmented system with a diminished tidal connection by 5500 cal. BP. The inlet sealed at this point and an isolated back barrier system developed. This prompted the system-wide impoundment of waters causing back-barrier flooding and in situ drowning of bayhead deltas and segmenting spits. By 2000 cal. BP, these areas had flooded sufficiently to connect over a bedrock high that had diverted the initial palaeo-valleys to the north and south. A new, single lagoon, representative of the modern-day system formed, with a new marine connection to the modern day Mfolozi River and contemporary estuary. Periodic desiccation of the lake sub-basins since this point has been apparent. Given the complex management plans implemented to stop excess sedimentation, hypersaline events, and the continuation of "normal" lake levels, this is at odds with the natural system's evolution to a pan or wetland-type state. This geological background should be key to informing management plans that work within the natural evolutionary trajectory of the system, as opposed to attempting to maintain an unattainable status quo. • Examine the Holocene evolution of Lake St Lucia. • Reveal complex back-barrier evolution of two separate systems. • Place evolution in context of sea level and climate forcing. • Juxtapose system evolution with management plans. [ABSTRACT FROM AUTHOR]

This study examines both the inlet dynamics, recorded in the back-barrier stratigraphy of Kosi Bay, and the competing effects of waterbody segmentation on along-strike changes in the character and nature of an incised valley fill at Kosi Bay, South Africa. The Kosi Bay system comprises four interconnected lakes (Lake Makhawulani, Lake Mpungwini, Lake Nhlange and Lake Amanzimnyama) subject to varying degrees of marine influence. Formerly, the system's only connection to the ocean was through the Bhanga Nek palaeo-inlet. The location of this system behind a continuous coastal barrier predisposes it to excellent archives of environmental change related to changing sea-level and sediment supply. In this context, two cores were extracted from the system (KB2 and KB4), together with over 150 km of seismic reflection profiling, as well as AMS radiocarbon analyses and legacy data in order to present a new stratigraphic evolution model for this back-barrier system. A total of 5 seismic units were imaged (Units A-E). A major unconformity surface (SB), characterised by broad u-shaped incised valleys, incises the underlying unconsolidated sandy acoustic basement (Unit A). Unit B forms as thickly developed central basin deposits, partially infilling the incised valleys. Two forms of prograding spits are evident -Unit C1 progrades from the palaeo-highs of the valley interfluves into the nearest available accommodation space and Unit C2 progrades from the margins of the system into the basin. These are separated by an erosional surface (T-RS) formed by migrating tidal channels as sea-level rose and marine waters entered the system during the Holocene. Unit C2 spits are in turn associated with slump deposits formed on the steepest part of the margins (Unit D) that were dated to 2900 ± 165 cal. BP. The unusual depth of Surface T-RS is attributed to (1) limited sediment supply during sea-level rise, (2) the presence of easily erodible fine sediments in which this surface erodes and (3) a larger tidal prism when the Bhanga Nek inlet was still open and system segmentation by Units C2 and D had not yet occurred. Unit E caps the stratigraphy and is interpreted as lacustrine fines with a thin layer of acoustically transparent material (gyttja) on top. The deposition of Unit E signifies the closure of the system to the ocean and the overall shift from an estuarine/lagoon environment to a lacustrine one. Recalibrated AMS dates place the system closure to between 3160 and 2900 cal. BP, coeval with the formation of the prograding marginal spits and waterbody segmentation (Unit C2). Along-strike variation in the timing of the basin infilling, attributed to changes to the inlet functioning coupled to the segmentation of the waterbody, is characteristic of this area. At 1450 cal. BP, a new tidal inlet formed to the north of Lake Makhawulani, where impounded waters were diverted via a low point in the back-barrier coastal dune cordon. The formation of this inlet suggests a prolonged period of back-barrier flooding until a low point in the barrier was breached during the Holocene sea-level highstand of 1.5 m. Such inlet development and along-strike back flooding of incised valleys is an unusual attribute of coastal waterbodies of South Africa. • LGM-age incision into unconsolidated Kosi Bay Formation sediments. • Limited sediment supplied to the area during transgressing sea levels. • Post-LGM homogenous central basin fills only partially fill incised valleys. • Development of wind-driven spits signifying early stages of lagoonal segmentation. • Along-strike variation in the timing of the basin infilling. [ABSTRACT FROM AUTHOR]

The morphology of Lake St Lucia, Africa's largest estuary, has been strongly influenced by antecedent topography throughout its Quaternary evolution. The 5 × 20 km southern basin is bounded to landward by a 50 m-high Early/Mid-Pleistocene dune ridge and to seaward by a 100 m-high Late Pleistocene/Holocene dune barrier. Nine seismic units (A-I) are identified in the enclosed basin, along with two regionally developed sequence boundaries (SB1 and SB2) and two tidal ravinement surfaces (tRS1 and tRS2). The lowermost sequence boundary (SB1) is related to regional late-Pliocene hinterland uplift. SB2 is related to the last glacial maximum (LGM) when sea levels fell ∼130 m below present. Lowstand fluvial sediments occupy the base of both SB1 and SB2 incisions. Thick central basin deposits dominate the valley fills, more so in the Pliocene age valleys, the occurrence and thickness of which is attributed to sheltering by a seaward barrier-dune system as well as to the low gradient settings. Prograding tidal bedforms occupy the minor incisions formed within the tidal ravinement surfaces, marking the onset of lagoonal conditions in the area. The system is capped by lagoonal deposits, deposited above tidal flats. A series of prograding spits mark the early stages of lagoonal segmentation in the Holocene. The preservation of only one incision and fill sequence prior to the development of an LGM-aged valley succession is attributed to geological inheritance whereby older valleys were repeatedly re-incised by younger valleys throughout the Pleistocene, causing partial preservation of Pleistocene fills up to the most recent cycle. The older fill remnant relates to a larger magnitude base level fall prior to the Quaternary, which survived erosion during subsequent sea-level falls. Adjacent topography in the form of rocky cliffs and the seaward fronting barrier limit the accommodation space in which infilling material can be hosted outside the major valley network. The competent basement rock into which these valleys have incised protected lowstand fluvial sediments from subsequent tidal ravinement. [ABSTRACT FROM AUTHOR]