John S. Compton

Rare earth element behavior in soils and salt pan sediments of a semi-arid granitic terrain in the Western Cape, South Africa

The rare earth element (REE) contents of soils and pan sediments of the Darling Hills granitic terrain of southwestern South Africa were determined to study the behavior of REE during weathering in a semi-arid, Mediterranean climate. The P-rich, Stype bedrock granite has a middle REE (MREE) enrichment relative to post-Archean Australian sedimentary rocks (PAAS) that primarily reflects the REE content of feldspar and accessory apatite. Initial chemical weathering of the granite results in the loss of REE, particularly Ce, Eu and the heavy REE (HREE). The enrichment of REE in soils relative to bedrock granite corresponds to clay mineral and amorphous Fe oxide content in the case of the light REE (LREE) and precipitation of pedogenic calcite in the case of the HREE. Quartz sand dilutes the REE content, and concentration of K-feldspar produces positive Eu anomalies. Cerium is rapidly precipitated during weathering and retained in clayey, upper soil horizons as insoluble CeO2. Most pan sediments have a bowl-shaped REE pattern that is derived from the accumulation of eroded catchment soils modified by precipitation of evaporative carbonates. REE enrichment, particularly LREE, and the large negative Ce anomaly of oxidized basal pan sediment relative to the bedrock granite suggest that REE entering the pan by groundwater flow are sorbed onto amorphous Fe oxides. In contrast, goethite-rich nodules from the pan have a low REE content and show HREE enrichment relative to the granite. Precipitation of pedogenic calcite and evaporative pan carbonates greatly increases the retention of the HREE in these semi-arid soils. Diagenetic pan dolomite has a REE pattern similar to pedogenic and evaporative calcite, but with a significantly lower REE content. The REE patterns indicate that, although most abundant in the bedrock granite, the MREE are least retained by soils during weathering. The LREE are generally retained by clayey and amorphous Fe-oxide-rich soils and sediments, whereas HREE are retained by precipitation of soil and pan carbonates. D 2003 Elsevier B.V. All rights reserved.

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 radiocarbon analyses of an oyster-rich bioclastic gravel indicate that the Flandrian 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 distal lagoonal sediments indicate that sea level returned to present-day levels by 4.9 ka and have since remained within 61 m of present-day levels. Bleached shell and a hiatus in sedimentation suggest an approximate 1 m sea-level lowstand between 2.5 and 1.8 ka. Changes in the macrobenthos assemblage since 1.5 ka that include the loss of the oysterOstrea atherstonei, razor clamSolen cap ensis, brown musselArcuatula capensis and periwinkleOxystele variegata reflect loss of hard substrate, decreased tidal-flow velocities as reworked sands prograded into the southern lagoon and possibly cooler seasurface temperatures. Calibrated radiocarbon ages of bulk organic matter from diatom-rich,Zostera muddy quartzose sands indicate a 0.5 m sea-level highstand at 1.3 ka followed by a 0.5 m lowstand at 0.7 ka. Dating of fining-upward, organic-rich (2 wt% TOC) noncalcareous muds indicates that the present-day salt marsh has grown by aggradation (1mmy- 1) and progradation since 0.7 ka.

A tenfold increase in the Orange River mean Holocene mud flux: implications for soil erosion in South Africa.

Soil erosion poses a major threat to sustainable agriculture in southern Africa but is difficult to quantify. One measure of soil erosion is the sediment flux of rivers. The Orange River is the principal source of sediment to the western margin of South Africa with an estimated mean mud flux over the last 11 500 years (the Holocene epoch) of 5.1 (3.2—7.4) million metric tons/year (Mt/yr). A total of 43 gigatons (Gt; 1015 g) representing 72% of the Holocene mud flux has accumulated on the shelf in the Orange River prodelta and mudbelt, a clayey fine-silt deposit focused on the inner to middle shelf. Only 8% (5 Gt) of the mud flux occurs in Holocene calcareous ooze on the slope. Comparison of the clay to mud ratio of offshore deposits with Orange River suspended sediment and catchment soils indicates that 20% (11 Gt) of the Holocene mud flux has been lost as clay beyond the margin. The Orange River mud flux prior to the building of large dams (1930—1969) is ten times greater than the mean Holocene mud flux and is reconciled with estimates of soil erosion within the catchment. A tenfold increase in the Orange River mud flux implies up to a hundredfold increase in total soil erosion depending on the extent of mud storage over periods of decades to centuries within the catchment. Erosion has shifted from areas of high relief and rainfall of the Drakensberg escarpment during the Holocene to intensely cultivated lands of low relief having moderate to high rainfall in the eastern catchment and to a lesser extent, grazing areas of the southern Orange River catchment.

The strontium isotope systematics of the Orange River, Southern Africa

The 87Sr/86Sr ratio of Orange River water increases from 0.7081 at its headwaters in the basalt of the Drakensberg mountains, to a value of 0.7126 before its confluence with the Vaal River tributary. The concomitant increase in Sr concentration is from 0.08 to 1.28 μM. The Vaal River, which drains Precambrian and Permian/Triassic rocks, has higher Sr concentrations, from 0.95 to 6.69 μM, and more radiogenic 87Sr/86Sr values, from 0.713 to 0.731. The geochemistry of the dissolved load of the Orange and Vaal rivers corresponds with that of the dominant catchment lithologies: Jurassic basalt, Permian/Triassic Karoo sediments and Proterozoic/Archaean para- and ortho-gneisses/granitoids. Chemical weathering rates in the Orange River system are at least a factor of 3 lower than the global average value of 36 ton/km2/yr. The highest chemical and physical weathering rates occur in the Upper Orange River, associated with high runoff and relief. In the low-relief Vaal River, chemical weathering predominates over physical weathering.

Origin and age of phosphorite from the Last Glacial Maximum to Holocene transgressive succession off the Orange River, South Africa

A major erosional unconformity between upper Cretaceous sediments and the Last Glacial Maximum (LGM) transgressive succession occurs on the inner continental shelf off the Orange River. Vibracores from 122 m water-depth consist of upper Pleistocene shelly, phosphatic sands deposited in a high-energy shoreface environment during the LGM lowstand between 22 and 19 ka. The gravel-rich shoreface sand grades into condensed shell-rich delta-front sands as sea level rose between 19 and 14.3 ka. An abrupt facies transition to prodelta muddy sands corresponds to the rapid rise in sea level associated with Melt Water Pulse (MWP) 1A at 14.3 ka and the facies transition to clayey silt corresponds to MWP 1B at around 11 ka. A large decrease in macro- and microfossil diversity indicates low-oxygen conditions as the sea level rose. Phosphorite recovered from throughout the succession includes large interior mollusc moulds, sand-size peloids and friable nodules. Sr-derived ages of the phosphorite range from early Miocene to Quaternary. X-ray amorphous friable Ca-phosphate nodules occur in organic-rich Holocene mud deposits. Quaternary sediment on the shelf represents a complex condensed section, in which reworked phosphorite pebbles show multiple episodes of phosphogenesis often separated by millions of years within individual pebbles. The phosphorite indicates that organic-rich mud deposition on the shelf was prevalent during the early to middle Miocene marine transgressions and sea-level highstands that were associated with increased upwelling as indicated by the extensive early to middle Miocene eolianites of the Tsondab Sandstone Formation. Most friable phosphorite that formed during Quaternary marine transgressions and highstands was probably fragmented and transported off shelf during succeeding marine regressions.

Holocene Evolution of the Sixteen Mile Beach Complex, Western Cape, South Africa

Abstract The Sixteen Mile Beach Complex is one of numerous active dune accumulations along the West Coast of southern Africa. The beach complex is composed of a sandy beach, coast-parallel dunes, and a dune cordon. Grain size analyses, calcium carbonate content, sand texture and composition, and radiocarbon ages were determined to understand the Holocene evolution of the Sixteen Mile Beach Complex. Changes in sand grain size and dune morphology allow the complex to be divided into three parts. The beach and the dune cordon at the southern end are composed of fine sand. The central beach has a rapid decrease in the fine sand fraction that coincides with the transition from the dune cordon to a single large coast-parallel dune ridge. The northern beach is composed of medium sand and consists of a series of prograded, vegetated coast-parallel dune ridges. The formation of these distinct regimes is a reflection of the different amounts of wave energy received by the complex from the predominant southwest swell. The radiocarbon analyses of bulk sand samples show a progressive younger age of the carbonate beach sand toward the north, which reflects an increase in grain size and recently broken shell. The mean age of the beach is 2.4 ka based on an accelerator mass spectrometry (AMS) date of picked pink-colored carbonate grains. The fresh bulk carbonate beach sand has a mean age of 7.4 ka and reflects the presence of reworked Pleistocene beach and dune sand. Corrected bulk sand radiocarbon ages indicate that the dune cordon has been active since 4.5 ka with a mean dune migration rate of 5.3 m/y. However, variations in sand supply, sea level, and climate indicate a complex and erratic evolution of the dune cordon.

Foraminifera of Langebaan Lagoon salt marsh and their application to the interpretation of late Pleistocene depositional environments at Monwabisi, False Bay coast, South Africa

The relationship between marine benthic foraminifera assemblages and the environment has been examined from samples collected across the southern edge of Langebaan Lagoon. The assemblages show a vertical zonation that can be related primarily to altitude above mean sea level and secondarily to type and abundance of plant cover. The foraminiferal analysis distinguishes three zones: a high marsh zone dominated by a monospecific Trochammina inflata assemblage, a middle marsh zone consisting of variable abundances of T. inflata and Jadammina macrescens ; and a low-marsh and tidal zone dominated by calcareous species such as Ammonia japonica, Ammonia parkinsoniana , Elphidium sp. A. Elphidium cf. articulatum and Quinqueloculina sp. The altitudinal range of these foraminiferal zones is used to reconstruct relative changes in late Pleistocene sea level from a succession exposed at Monwabisi on the False Bay coast.

Variation in palaeo-shorelines explains contemporary population genetic patterns of rocky shore species

Processes driving and maintaining disjunct genetic populations in marine systems are poorly understood, owing to a lack of evidence of hard barriers that could have shaped patterns of extant population structure. Here, we map two genetically divergent lineages of an obligate rocky shore fish, Clinus cottoides, and model sea-level change during the last 110 000 years to provide the first evidence of a vicariant event along the southern coastline of Africa. Results reveal that lowered sea levels during glacial periods drastically reduced rocky intertidal habitat, which may have isolated populations in two refugia for at least 40 000 years. Contemporary coastal dynamics and oceanography explain secondary contact between lineages. This scenario provides an explanation for the origin of population genetic breaks despite a lack of obvious present-day geographical barriers and highlights the need for including palaeo-oceanography in unravelling extant population patterns.

Submerged shorelines and landscape features offshore of Mossel Bay, South Africa

Abstract Coastal geomorphic systems have been studied widely to understand the responses of shorelines to fluctuating sea levels. Submerged shorelines, remnant of Pleistocene sea-level lowstands, are well preserved on the South African continental shelf. This paper describes work undertaken to better understand offshore coastal environments now submerged by high sea levels off the South African south coast near Mossel Bay, offshore of the Pinnacle Point archaeological locality. Multibeam bathymetry and side-scan sonar reveal evidence of past sea-level fluctuations and submerged coastal landscape features on the seabed. These results form the basis of an ongoing palaeoenvironmental reconstruction for this part of the shelf. We describe seven significant geomorphic features that show a submerged environment that differs significantly to the immediate adjacent coastal plain. However, these features are comparable to other stretches of the present South African shoreline that serve as modern analogues. We propose that features on the continental shelf primarily reflect geological substrate, gradients and Pleistocene sea-level fluctuations. Early modern humans were likely to have had a different set of resources to use in this Pleistocene landscape compared to those available along the presently exposed coast.

Pleistocene Dolomite from the Namibian Shelf: High 87Sr/86Sr and δ18O Values Indicate an Evaporative, Mixed-Water Origin

ABSTRACT Dolomites from Upper Pleistocene sandstone and diatomaceous mud recovered from a coast-parallel, 4-km-wide trough on the Namibian shelf between Hottentot Bay and Spencer Bay have 87Sr/86Sr ratios between 0.70946 and 0.71309. Marine mollusc shells associated with the dolomites have a modern seawater Sr isotope ratio of 0.70915. The greater-than-seawater Sr ratios indicate unequivocally that groundwaters were involved in dolomite formation. Groundwaters, containing radiogenic Sr derived from weathering of Precambrian basement rocks, were focused into local troughs, mixed with seawater, and evaporated in the arid climate. The 13CPDB values of -4.0 to -2.4o/oo indicate uptake of bicarbonate derived in part from microbial degradation of organic matter, and 18OPDB values of 1.4 to 6.4o/oo suggest that the dolomite cement precipitated from a variably evaporated groundwater-seawater mixture. Dolomite cementation of the basal bioclastic sand and gravel occurred during emergence when sea level was below the trough sill depth of 65 m, most likely between 500 and 130 ka. Overlying calcareous and diatomaceous Eemian mud and sand are dolomitic only in the deepest, central trough area and contain 26 to 45 wt % disseminated dolomite. The 13CPDB values of -0.3 to -0.5o/oo and 18OPDB values of 5.7 to 6.4o/oo suggest that the disseminated dolomite formed from an evaporated groundwater-seawater mixture largely by replacement of biogenic carbonate when the shelf was subaerially exposed between 75 and 11 ka. Holocene transgressive shelly gravels and muddy sands contain only reworked Pleistocene dolomite, but Holocene dolomite may occur in coastal salt pans onshore.