Deposition Of Terrigenous Sediments Research Articles

Age‐Depth Models for Tropical Marine Hemipelagic Deposits Improve Significantly When Proxy‐Based Information on Sediment Composition Is Included

AbstractAccurate age‐depth models for marine sediment cores are crucial for understanding paleo‐oceanographic and ‐climatic changes derived from these archives. To date, information on bulk sediment composition is largely ignored as a potential source of information to improve age‐depth models. In this study, we explore how bulk sediment composition can be qualitatively used to improve age‐depth models. We developed the BomDia algorithm, which produces age‐depth models with realistic sediment accumulation rates that co‐vary in harmony with the bulk sediment composition. We demonstrate that changes in the marine versus terrigenous sediment deposition, based on bulk sediment composition, can be used to significantly improve age‐depth models of hemipelagic marine deposits. Based on two marine records—each containing more than 20 radiocarbon (AMS 14C) dated levels—we show that the mean error of prediction of unused AMS 14C ages significantly improves from 3.9% using simple linear interpolation, to 2.4% (p = 0.003), when bulk sediment composition is included. The BomDia age‐depth modeling approach provides a powerful statistical tool to assess the validity of age control points used and also may assist in the detection of hiatuses. Testing and further development of the BomDia algorithm may be needed for application in depositional settings other than tropical hemipelagic.

The Importance of Ecological Accommodation Space and Sediment Supply for Cold-Water Coral Mound Formation, a Case Study From the Western Mediterranean Sea

The formation of cold-water coral (CWC) mounds is commonly seen as being the result of the sustained growth of framework-forming CWCs and the concurrent supply and deposition of terrigenous sediments under energetic hydrodynamic conditions. Yet only a limited number of studies investigated the complex interplay of the various hydrodynamic, sedimentological and biological processes involved in mound formation, which, however, focused on the environmental conditions promoting coral growth. Therefore, we are still lacking an in-depth understanding of the processes allowing the on-mound deposition of hemipelagic sediments, which contribute to two thirds of coral mound deposits. To investigate these processes over geological time and to evaluate their contribution to coral mound formation, we reconstructed changes in sediment transport and deposition by comparing sedimentological parameters (grain-size distribution, sediment composition, accumulation rates) of two sediment cores collected from a Mediterranean coral mound and the adjacent seafloor (off-mound). Our results showed that under a turbulent hydrodynamic regime promoting coral growth during the Early Holocene, the deposition of fine siliciclastic sediments shifted from the open seafloor to the coral mounds. This led to a high average mound aggradation rate of >130 cm kyr–1, while sedimentation rates in the adjacent off-mound area at the same time did not exceed 10 cm kyr–1. Thereby, the baffling of suspended sediments by the coral framework and their deposition within the ecological accommodation space provided by the corals seem to be key processes for mound formation. Although, it is commonly accepted that these processes play important roles in various sedimentary environments, our study provided for the first time, core-based empirical data proving the efficiency of these processes in coral mound environment. In addition, our approach to compare the grain-size distribution of the siliciclastic sediments deposited concurrently on a coral mound and on the adjacent seafloor allowed us to investigate the integrated influence of coral mound morphology and coral framework on the mound formation process. Based on these results, this study provides the first conceptual model for coral mound formation by applying sequence stratigraphic concepts, which highlights the interplay of the coral-framework baffling capacity, coral-derived ecological accommodation space and sediment supply.

Late Cretaceous transtensional faulting of the Apulian Platform, Italy

This work focuses on the structural setting of a small area of the Murge Plateau, southern Italy, exposing Turonian bauxites and encompassing Upper Cretaceous carbonates of the Apulian Platform. By combining field geological and structural mapping with mineralogical and petrographic investigations of representative rock samples, we document the control exerted by Late Cretaceous, syn-sedimentary, high-angle faults and dilational fissures striking ca. N105E, N135E, N170E, and N45E. Altogether, these structural elements formed a few km-long, transtensional fault system along which karst-related shafts, dissolution caves, and sinkholes developed. Karst processes first affected the topmost limestone beds of the Valanginian-Cenomanian Bari Fm. Then, during Turonian, in a continental setting, cycles of terrigenous sediment deposition and weathering localized in the karst-related cavities, dilational fissures, and at the top of down-dropped fault blocks. Weathering of the Al–Si rich sediments formed bauxites made up of kaolinite, boehmite, goethite, hematite, and anatase mineral phases. The relative enrichment in kaolinite with respect to boehmite documented for the breccia and conglomerate matrix topping the bauxites is interpreted as due to lower degrees of weathering. Differently, the individual bauxite beds do not show any internal significant variation in terms of their mineral assemblages, but form angular unconformities due to syn-sedimentary faulting. The subsequent marine transgression also occurred under the influence of active tectonics, as displayed by growth folds at the bottom of the Conacian-Early Campanian Altamura Fm. The overall results of this study are in agreement with those previously reported for nearby areas of the Murge Plateau exposing Santonian limestone rocks of the Altamura Fm. Accordingly, we infer that Late Cretaceous transtensional faulting of the Apulian Platform was due to a far field stress state, likely associated to the orogenic processes that involved the northern edge of the Adriatic micro-plate.

Controls Since the mid‐Pleistocene Transition on Sedimentation and Primary Productivity Downslope of Totten Glacier, East Antarctica

AbstractThe rapidly thinning Totten Glacier on the Sabrina Coast, East Antarctica, is the primary drainage outlet for ice within the Aurora Subglacial Basin, which could destabilize under the current atmospheric warming trend. There is growing need for direct geological evidence from the Sabrina Coast to frame late twentieth century Totten melting in the context of past warm climate analogs. Addressing this need, sediment archives were recovered from two sites on the Sabrina Coast slope and rise that record changes in terrigenous sedimentation and primary productivity in the region over glacial cycles since the mid‐Pleistocene transition (MPT). This research presents physical properties, grain size, diatom abundance and assemblages, and geochemical analysis from the two sites to determine how the processes that control sedimentation change between glacial and interglacial phases. The stratigraphic sequences in both cores record cyclic variations in physical properties and diatom abundances, which radiocarbon and biostratigraphic chronologies reveal as 100 Kyr glacial‐interglacial cyclicity. During glacials, terrigenous sediment deposition is enhanced by advanced grounded ice on the shelf, while primary productivity is restricted due to permanent summer sea ice extending past the continental slope. During interglacials, pelagic sedimentation suggests high surface productivity associated with contractions of regional sea ice cover. Comparison with post‐MPT slope records from Wilkes Land and the Amundsen Sea shows that this pattern is consistent in slope sediments around the margin. The higher‐amplitude variations in Antarctic ice volume and sea ice extent post‐MPT ensure that these signals are pervasive around the Antarctic margin.

Caribbean Near-Shore Coral Reef Benthic Community Response to Changes on Sedimentation Dynamics and Environmental Conditions

Coral reefs are facing unprecedented global, regional and local threats that continue to degrade near-shore habitats. Water quality degradation due to unsustainable development practices at coastal watersheds is one of the greatest stressors across multiple spatial scales. The goal of this study was to assess near-shore coral reef benthic community spatio-temporal response to sedimentation patterns, weather, and oceanographic dynamics at Bahia Tamarindo and Punta Soldado in Culebra Island, Puerto Rico. Benthic data were collected across a distance gradient from the shore through high-resolution images at marked belt transects. Environmental data were assessed and contrasted with benthic assemblages using multivariate correlations and multiple linear regression. Coral colony abundance and coral recruit assemblages showed significant variation among seasons, sites and distance zones (PERMANOVA, p<0.01). Species diversity (H’n) increased at both study sites with distance from shore and the most conspicuous coral recruit species were stress-tolerant Porites astreoides, P. porites, and Siderastrea radians. Difference in coral abundance and coral recruits per site had a strong significant negative relationship with sediment characteristics and depth (p<0.05). Near-shore coral reef benthic community structure was significantly different between sites and distance zones from shore, with depth having an important role shaping reef zonation. Changes in benthic community structure were associated with local sediment distribution patterns emerging from human alteration of coastal watersheds and natural events that cause terrigenous sediment deposition and sand resuspension across the reef. Coral cover was significantly lower at zones more exposed to recurrent sedimentation stress (p<0.01). It was also correlated with sediment texture (p=0.006) and terrigenous sediment deposition (p=0.016). Scleractinian coral cover had an inverse relationship with gorgonian and macroalgae cover. In a short-term period, a pattern of increased dominance of encrusting calcareous algae Ramicrusta textilis and invasive sponge Dictyonella funicularis were documented. Changing land use and increased frequency of extreme weather events, as a consequence of global patterns of climate change, may play an important role shaping near-shore coral reefs benthic communities and could threaten the resilience of coastal regions. Therefore, collaborative and trans-disciplinary ecosystem-based management efforts are urgently needed to effectively reduce land-based stressors and foster near-shore coral reef recovery.

A New Seismic Stratigraphy in the Indian‐Atlantic Ocean Gateway Resembles Major Paleo‐Oceanographic Changes of the Last 7 Ma

AbstractThe exchange of water masses between the Indian Ocean and the Atlantic constitutes an integral interocean link in the global thermohaline circulation. Long‐term changes in deep water flow have been studied using seismic reflection profiles but the seismic stratigraphy was poorly constrained and not resolved for the time period from the late Miocene onward. Here we present results from International Ocean Discovery Program Site U1475 (Agulhas Plateau) located over a sediment drift proximal to the entrance of North Atlantic Deep Water into the Southern Ocean and South Indian Ocean. Site U1475 comprises a complete carbonate‐rich stratigraphic section of the last ~7 Ma that provides an archive of climate‐induced variations in ocean circulation. Six marker reflectors occurring in the upper 300 m of the drift are identified here for the first time. The formation of these reflectors is mainly due to density changes that are mostly caused by changes in biogenic versus terrigenous sediment deposition. Synthetic seismograms allow age assignments for the horizons based on biostratigraphy and magnetostratigraphy. Prominent reflectors are related to late Pleistocene glacial/interglacial variability, the middle and early Pleistocene transitions, and the onset of the northern hemisphere glaciation. A peculiar early Pliocene interval (~5.3–4.0 Ma) bounded by two reflectors is characterized by fourfold elevated sedimentation rates (&gt;10 cm/kyr) and the occurrence of sediment waves. We argue that this enhanced sediment transport to the Agulhas Plateau was caused by a reorganization of the bottom current circulation pattern due to maximized inflow of North Atlantic Deep Water.

Tolerance of tropical seagrasses Zostera muelleri and Halophila ovalis to burial: Toward an understanding of threshold effects

Seagrass meadows are critically important coastal habitats, but they are undergoing rapid global declines linked to degradation of coastal catchments and the expansion of development in coastal zones, which can lead to increased deposition of terrigenous sediments onto seagrass meadows. In the Asia-Pacific region, which includes the Great Barrier Reef, direct sedimentation effects are recognised as the major threat to seagrass growth and survival. However, tolerance to burial varies among species. Understanding thresholds for burial tolerance at the species-level is therefore critical to effective management of human impacts on seagrass meadows. This research examined the effects of sedimentation on two tropical seagrass species, Zostera muelleri and Halophila ovalis in an experimental mesocosm and field setting. The research suggests that shoot density in these species declines significantly at burial depths of 5–7.5 mm in a mesocosm setting, with very low levels of growth observed above 10 mm burial depth. Fine sediments (including fine sands, silts and clays) appear to have a greater negative effect on shoot density in both species, compared to coarser sandy sediments. In situ, the effect of burial is less pronounced and is likely to be mitigated by a range of factors, including clonal integration. These species also demonstrate different adaptations to burial disturbance, including by resisting burial effects (Z. muelleri) or by regenerating from the buried rhizome after disturbance (H. ovalis).

Holocene environmental change along the southern Cape coast of South Africa – Insights from the Eilandvlei sediment record spanning the last 8.9 kyr

Abstract This study investigates Holocene sediments from Eilandvlei, a coastal lake located within the Wilderness embayment at the southern Cape coast of South Africa. The evolution of the present estuarine/coastal lake system is reconstructed based on seismic data as well as a multi-proxy approach on a 30.5 m sediment core spanning the last ~8.9 kyr. Geochemical (Ca, TOC/S, Br/TOC) and micropalaeontological data (diatoms, foraminifera) reflect changes in the degree of marine influence at the core site. The embayment likely developed via distinct phases of connectivity to the Indian Ocean caused by sea level changes and dune progradation. Marine conditions prevailed at the core site from ~8900 to 4700 cal BP. The rapid sea level rise during the early Holocene caused the inundation of a palaeovalley that most likely had formed at lower sea levels during the Pleistocene. Towards the mid-Holocene the sea level exceeded its present height around ~7500 cal BP creating a marine embayment. At ~4700 cal BP, the embayment became distinctly more disconnected from the ocean turning into a lagoon system that persisted until ~1200 cal BP. Subsequently, the marine influence further decreased and the present estuarine/coastal lake system was established. Grain size and geochemical data (Fe, Si/Al, chemical index of alteration (CIA)) further reflect changes in the deposition of terrigenous sediments at the core site. While the sedimentation of fine-grained (

A 30 Ma history of the Amazon River inferred from terrigenous sediments and organic matter on the Ceará Rise

The history of the Amazon River is a much-discussed subject, and the timing of the development of a transcontinental system in particular is a matter of some controversy, with estimations varying between the Early Miocene and the Pliocene or even the Pleistocene. To shed further light on this, we studied the sediment provenance of an Oligocene to Late Pleistocene marine sedimentary section from the Ceará Rise (ODP Site 925), a topographic high in the central Atlantic Ocean, using major element concentrations and Nd isotopic composition in 85 samples. In addition, the carbon isotopic composition of bulk organic matter and changes in the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) were used to identify periods of increased river outflow. On the basis of these results, we suggest that the history of the development of the Amazon River is characterized by specific steps. During the late Oligocene/Early Miocene (30–18.3 Ma), the terrigenous mass accumulation rates (TARs) were high, and sediment and GDGT compositions suggest that a large river system existed, which at times received weathering products from a younger and probably Andean sediment source. A shift to a younger Andean sediment provenance after 8.7 Ma indicates that the Amazon River became permanently connected with the Andes. Between 18.3 and 4.5 Ma, TARs were generally low, and GDGTs were derived for the most part from in situ production in marine waters. Around 4.5 Ma, the river expanded, probably due to ongoing tectonic activity, and uplift in the Andes increased Andean rock erosion. This led to a strong increase in terrigenous sediment deposition and enhanced organic matter preservation on the Ceará Rise, and the delivery of terrestrial (both soil and riverine) branched GDGTs to the Ceará Rise.

The Sarmatia megablock as a fragment of the Vaalbara supercontinent: Correlation of geological events at the Archean‒Paleoproterozoic transition

The results of correlation between geological events in the period of 2.8‒2.0 Ga provide grounds to assume that the Sarmatia lithospheric megablock definable in the southern part of the East European Craton belonged to the ancient Vaalbara supercontinent consisting of the Pilbara and Kaapvaal cratons. In the period of 2.8‒2.6 Ga, all of them represented fragments of the continental crust consolidated at approximately 2.8 Ga and subjected to continental rifting, which was accompanied by intense basite volcanism. In the period of 2.50‒2.45 Ga, these three cratons were characterized by similar tectonic settings and accumulation of banded iron formations. Precisely these banded iron formations of the largest Transvaal, Hamersley, Kursk, and Kremenchug‒Krivoi Rog iron ore basins accumulated in the period of 2.50‒2.45 Ga in a single oceanic basin serve as a basis for adequate paleotectonic reconstructions of the Vaalbara supercontinent. In the period of 2.45‒2.20 Ga, all three cratons were subjected to a long-lasting break in sedimentation followed by activation of continental rifting with terrigenous sediment deposition, which terminated with basite volcanism ca. 2.2 Ga. These events gave start to the Vaalbara breakup, which represented a multistage process with alternating divergence and convergence phases of supercontinent fragments until the Kaapvaal and Zimbabwe, Pilbara and Yilgarn, and Sarmatia and Volgo-Uralia cratons, respectively, became eventually united.

Provenance analysis for Middle Eocene sediments in the West Kamchatka sedimentary basin (Tigil area)

The paper presents the results of reconstruction of Middle Eocene provenances for the West Kamchatka sedimentary basin (WKSB) corresponding to the Tigil area. It has been established that the early (Eocene) evolution stage of WKSB was marked by the deposition of terrigenous sediments in intermontane depressions followed by the accumulation of shallow-marine sediments after transgression. In terms of the composition, sandstones of the Middle Eocene Snatol Formation correspond to graywackes. With respect to the geochemistry of sandstones, their provenances were confined to an active continental margin and island arc. The mineral composition of the heavy fraction suggests an alternating dominance of felsic and mafic rocks in the provenances. Dating of the clastic zircon from sandstones of the Snatol Formation by the LA-ICP-MS method revealed a wide variation range of their age. The most significant peak is close to the age of calc-alkaline magmatism in the Okhotsk–Chukotka volcanic belt. This fact provides insight into the Eocene paleogeography: the major rock provenances were located in the Okhotsk–Chukotka volcanic belt and the eastern Achaivayam–Valagin island arc. Local sources of clastic material were represented by the Utkholok and Kinkil volcanic belts.

Linkages between Quaternary climate change and sedimentary processes in Hala Lake, northern Tibetan Plateau, China

Profundal lake sediment cores are often interpreted in line with diverse and detailed sedimentological processes to infer paleoenvironmental conditions. The effects of frozen lake surfaces on terrigenous sediment deposition and how climate changes on the Tibetan Plateau are reflected in these lakes, however, is seldom discussed. A lake sediment core from Hala Lake (590km2), northeastern Tibetan Plateau spanning the time interval from the Last Glacial Maximum to the present was investigated using high-resolution grain-size composition of lacustrine deposits. Seismic analysis along a north–south profile across the lake was used to infer the sedimentary setting within the lake basin. Periods of freezing and melting processes on the lake surface were identified by MODIS (MOD10A1) satellite data. End-member modeling of the grain size distribution allowed the discrimination between lacustrine, eolian and fluvial sediments. The dominant clay sedimentation (slack water type) during the global Last Glacial Maximum (LGM) reflects ice interceptions in long cold periods, in contrast to abundant eolian input during abrupt cold events. Therefore, fluvial and slack water sedimentation processes can indicate changes in the local paleoclimate during periods of the lake being frozen, when eolian input was minor. Inferred warm (i.e., ∼22.7 and 19.5cal.ka BP) and cold (i.e., ∼11–9 and 3–1.5cal.ka BP) spells have significant environmental impacts, not only in the regional realm, but they are also coherent with global-scale climate events. The eolian input generally follows the trend of the mid-latitude westerly wind dynamics in winter, contributing medium-sized sand to the lake center, deposited within the ice cover during icing and melting phases. Enhanced input was dominant during the Younger Dryas, Heinrich Event 1 and at around 8.2ka, equivalent to the well-known events of the North Atlantic realm.

Geochemistry and genesis of metabasites from the Kolpakova Group of the Sredinnyi Range in Kamchatka

The data on the Sm, Nd, Rb, Sr, and Pb isotope compositions obtained for sedimentary and igneous rocks from the Sea of Okhotsk [1] and recent informa� tion on zircons from metaterrigenous rocks and gran� itoids of the Sredinnyi Range crystalline massif in Kamchatka [2–5] imply the existence of an epiconti� nental crust beneath this basin and influx of terrige� nous material constituting sedimentary sequences of the Sredinnyi Range from the eroded mature conti� nental crust. The new data on volcanogenic rocks from the ter� rigenous sequences of the Sredinnyi Range indicating that deposition of terrigenous sediments was accom� panied by intense volcanism synchronous with the ini� tiation of the formation of the Okhotsk–Chukotka Volcanic Belt supplement to our knowledge of this region [6]. The wide distribution of mafic and ultra� mafic volcanites in terrigenous sections suggests the existence of a magmatic chamber beneath the sedi� mentary basin, which served presumably as a source of heat and fluids responsible for granitization and meta� morphism of rocks constituting the Sredinnyi Range of Kamchatka. In this communication, we present new data on the geochemistry and mineralogy of metabasites from the Kolpakova Group of the Sredinnyi Range massif in Kamchatka with discussion of a possible mechanism of their formation among terrigenous sequences of the Cretaceous epicontinental marginal basin. The Kolpakova Group of apoterrigenous meta� morphic rocks over 2500 m thick crops out at the base of the observable section of the Sredinnyi Range mas� sif, where it consists of the lower, middle, and upper lithological units. The lower unit, which is most wide� spread in the massif limits, is composed of migmatized kyanite–garnet–biotite ± staurolite ± muscovite and garnet–biotite–muscovite plagiogneisses and migma� tites. The middle unit is largely represented by biotite and garnet–biotite plagiogneisses with migmatization slightly less intense as compared with that in the lower unit. The upper unit crops out in the northern part of the massif on the right side of the Zolotaya River head and left side of the Srednyaya Andrianovka River. It is composed of alternating amphibolites, garnet amphibolites, clinopyroxene–amphibole (frequently with garnet) crystalline schists, garnet–amphibole, garnet–biotite–amphibole, garnet–biotite, and kyan�

High‐resolution migration history of the Subtropical High/Trade Wind system of the northeastern Pacific during the last ~55 years: Implications for glacial atmospheric reorganization

AbstractGuaymas Basin, Gulf of California, is a restricted basin located under the present‐day wet/dry subtropical divide (~27°N) and is ideally circumstanced for detecting variations in the North Pacific Subtropical High (NPSH)/Trade Wind system. Controlled by climate cell boundary displacement, NPSH midwinter location was the primary influence on timing and intensity of upwelling seasons in Guaymas Basin. Analysis of high‐resolution X‐ray fluoresence data and sediment fabric log from 75% laminated Core MD02‐2517/2515, western Guaymas Basin, reveals systematic changes in NPSH behavior over the last ~55 kyr BP. Southward displacement of the wet/dry subtropical divide controlled upwelling‐related diatom productivity, while sea level and regional rainfall controlled terrigenous supply. The basin was oxic during the glacial, and preservation was ensured by high burial rate due to the increased deposition of terrigenous sediment. Sediment fabric style (number and/or thickness of laminae, plus color banding and homogeneous intervals) changes systematically through the core and gives insights into the number of seasons occurring in Guaymas Basin, and the occurrence and intensity of the upwelling season. Five millennial‐scale low flux events with close timing to Heinrich events and ten decadal/centennial‐scale low biogenic silica events occurring in the interval ~33–16.5000 years Before Present (kyr BP) are interpreted as times of extreme aridity. At ~16.5 kyr BP, a regime shift from terrigenous‐dominated oxic to evenly balanced biogenic‐terrigenous dysoxic conditions occurred. Although there was a further extreme arid event at ~11.5 kyr BP, ~16.5 kyr BP was essentially the beginning of the interglacial.

Maerl-bed mapping and carbonate quantification on submerged terraces offshore the Cilento peninsula (Tyrrhenian Sea, Italy)

On the continental shelf off the Cilento peninsula (eastern Tyrrhenian Sea) the occurrence of more than 13 km2 of maerl beds was documented through acoustic surveys. Swath bathymetric data along with a dense grid of chirp-sonar profiles were acquired over more than 180 km2. The maerl facies was characterized on the basis of the components analysis of 32 grab samples collected at selected sites. Mapped maerl-beds are predominant on submerged terraces located at variable water depth (wd) between 42 and 52 m. This preferred distribution on submerged terraces is probably associated with relatively vigorous bottom currents generated by local circulation that hinders the deposition of terrigenous sediments. Calcareous red algae result to be the most important producers of carbonates from 40 down to 60 m wd. We calculated the coralline carbonate accumulation from the percentage cover of coralline algae (thin section mapping) × 1 cm-thick layer of sediment × measured coralline density. The total coralline cover (living plus dead) in the Cilento area is 13.96 km2, with a total 316800 tons of algal carbonate in the surface 1 cm layer, that correspond to 20430 g m-2. Living maerl is recorded at a depth of 47 m, with a live coralline cover of about 40% over a minimum area of about 1.2 km2. This live maerl has a thickness of about 1 cm and is composed mainly of unattached branches of Lithothamnion corallioides (P.L.Crouan & H.M.Crouan) P.L.Crouan & H.M.Crouan, 1867. The molluscan association of the maerl bed is dominated by characteristic species of the Coastal Detritic Biocoenosis. The production of carbonate by living coralline algae has been calculated as weight of live corallines in 1 cm-thick layer × 100 y-1 × total area-1 and corresponds to 90.8 g m-2 y-1.

Terrigenous sediment export from the western margin of South Africa on glacial to interglacial cycles

Similar to many shelves globally, Pleistocene terrigenous sediment deposition on the western shelf of South Africa is strongly influenced by high-amplitude sea-level fluctuations over glacial to interglacial climate cycles. Here, variations in the glauconitic quartzose sand and terrigenous mud content of gravity cores from the western slope of South Africa are used to evaluate the dynamics of shelf sediment export over climate cycles of the last 450 kyr. Calcareous ooze on the slope contains a mean of 0.2 to 8 wt.% glauconitic quartzose sand derived from erosion of Neogene outer shelf sediments and 13 to 40 wt.% terrigenous mud sourced from the Orange River. Quartz sand content on the slope increases during glacial periods and is sharply reduced across glacial terminations and throughout interglacial highstands. Relative sea-level changes on the margin estimated from the quartz sand content on the slope are consistent with the timing, but not amplitude, of proposed eustatic sea-level records. A possible mechanism of sand erosion on the outer shelf is the vertical mixing forced by internal tides which may intensify over the shelf break as sea level is lowered during glacials. Turbidites are rare and bottom currents are generally capable of suspending mud but not sand except locally along the margin. Estimated variations in the bulk sedimentation rate and terrigenous mud mass accumulation rate on the slope suggest that export of terrigenous mud off the shelf during glacial lowstands is transported beyond the slope at intermediate and deeper water depths to the Southern Ocean.

Assessing far-field effects of terrigenous sediment loading in the coastal marine environment

Major rain events erode coastal catchments, and muddy plumes of terrigenous sediment can extend far offshore. Surface waters gradually clear as terrigenous sediments sink, although near- bed turbidity may remain high due to resuspension by waves and tides. This may adversely affect large suspension-feeding benthic epifauna, structurally and functionally important organisms in coastal marine systems, by clogging their filtration structures and decreasing their feeding efficiency. While terrigenous sediment concentrations likely decrease with distance from the coast, sensitivities of suspension feeders to this stressor may increase. We tested this hypothesis using controlled addi- tions of terrigenous sediment at estuarine and coastal sites in northern New Zealand. None of the large, solitary suspension feeders (pinnid bivalves Atrina zelandica, sponges Aaptos spp., and ascid- ians Styela plicata) were completely buried or killed by experimental deposition of terrigenous sediment. However, after living in the deposits for 3 wk, the condition of all 3 taxa declined relative to controls, and clearance rates of A. zelandica and Aaptos spp. were reduced (averaging about 40% less). A. zelandica outside the estuary (Site MI, coarse ambient sediment) were more sensitive to terrigenous material than A. zelandica inside the harbour (Site TK, fine ambient sediment), which was probably related to the greater background suspended sediment concentrations at TK to which the A. zelandica were accustomed. Impacts to large, structure-forming species such as A. zelandica, Aaptos spp., and S. plicata may eventually affect ecosystem structure and function, particularly if the frequency or magnitude of terrigenous sediment loading and resuspension increases.

Deposition of terrigenous sediment on subtidal marine macrobenthos: response of two contrasting community types

Changes in patterns of land-use associated with human population growth throughout the world have altered the regime of terrigenous material export from catchments to rivers and, sub- sequently, to estuarine and marine communities. Although terrigenous sediment is now widely recognised as a disturbance agent in estuarine and marine communities, experimental studies on its impacts in subtidal habitats are rare. Terrigenous deposits in the intertidal zone de-water at low tide and are subjected to erosion by waves during emersion, so experimental results in spatially wide- spread subtidal habitats where these processes are muted may differ substantially. We deposited ter- rigenous sediment into replicate experimental plots at 6 m depth at 2 subtidal sites, creating 3 treat- ment levels (magnitudes of terrigenous material addition) inside and outside a small harbour in northern New Zealand. We tracked the persistence of the terrigenous deposits (3 and 7 mm thick- ness) over time and sampled macrobenthic communities at both sites on Days 7, 14 and 30, to com- pare and contrast their responses relative to controls. The diverse coarse sand community outside the harbour (Site MI) was more sensitive to terrigenous materials than that which lived in muddier sedi- ments inside the harbour (Site TK), as indicated by multivariate and univariate analyses: both the 3 and 7 mm treatments caused significant change at Site MI, whereas only the more severe 7 mm treat- ment caused significant change at Site TK. The terrigenous sediments we added matched the grain size of Site TK sediments better, and macrobenthic animals living in turbid tidal estuaries are proba- bly better conditioned to cope with high suspended sediment concentrations and sediment deposition rates. However, beyond a critical threshold, terrigenous sediment had a negative influence on com- munities at both sites, and they had not recovered by the time the experiment was terminated 30 d later.

Quantitative links between fluvial sediment discharge, trapped terrigenous flux and sediment accumulation, and implications for temporal and spatial distributions of sediment fluxes

The southwestern-most Okinawa Trough (SOT), characterized by high sedimentation rates (>0.1 cm/yr), has the potential for recording high-resolution episodic events, such as storm floods and seismic activities, at least on a regional scale. To retrieve data on past climate change from nearby sediment cores and quantitatively reconstruct it, particularly with respect to precipitation (or typhoon-induced flood events), a linkage between fluvial sediment discharge and terrigenous sediment flux is warranted. Apparent sediment fluxes, observed with four arrays of sediment traps deployed in the SOT, were found to vary with fluvial sediment discharges. Empirical equations for individual arrays of sediment traps are site-dependent and related to the scenario of initial supply, transport and final deposition of terrigenous sediments (i.e. land–sea interaction). Using these equations and hydrological data from 1950 to 2000, the long-term temporal and spatial variations of settling sediment fluxes were simulated. Simulation results agree well with sediment mass accumulation rates derived from literature data on 210Pb and 137Cs chronology. The simulated spatial patterns of sediment fluxes along a slope–trough section illustrate that sediment plumes can disperse concurrently in two manners, namely near-bottom and mid-depth plumes, and the flood-driven plumes can travel very long distances, approaching 125°E or beyond. The sediment burial budget in the SOT was estimated to be approximately 5.2 Mt/yr, representing about 80% of riverine exports from the Lanyang Hsi, Taiwan. This is the first study dealing qualitatively and quantitatively with two parameters, namely terrigenous sediment flux and fluvial sediment discharge.

Skeletal-Carbonate Neptunian Dikes of the Capitan Reef: Permian, Guadalupe Mountains, Texas, U.S.A.

Sedimentary dikes in the Permian (Guadalupian) Capitan Reef of West Texas and New Mexico are composed of siliceous sand, skeletal carbonate, or cement. Skeletal-carbonate neptunian dikes appear to be a relatively uncommon type; only four are known by the authors, but they are important in providing information about platform growth, sediment sources, and dispersal on the platform, and the biota that were present on the platform. The dikes occur within massive poorly sorted skeletal grainstones typical of the reefal part of the Capitan Formation (the Capitan-massive) that is correlative with upper Guadalupian Yates B strata on the platform. They formed during later Yates B, Yates C, and Tansill deposition on the platform, following closely a major period of progradation of the platform margin. The dikes are approximately vertical, and parallel to and within about 100 m of the platform margin. Fissure formation was a probable consequence of flexing and fracturing of lithified shallow-water Capitan-massive over compacting unconsolidated slope and basinal sediment. At least three stages of opening and filling are recorded in the largest and most complex dike, with each sequential fissure opening forming on the basinward side of the previous cemented filling. In two of the dikes, centimeter-thick deposits of vertically laminated peloidal grainstone, which formed in situ within microbial films on the fissure wall, preceded filling by skeletal sediment. Skeletal fossils within two of the dikes are well preserved and represent organisms that either were living within the fissures or were living on the adjacent sea floor and were washed into the fissures soon after death. Other skeletal grains in these dikes, and all of those in the other two dikes, are abraded, strongly micritized, and encrusted with microbial micrite, indicative of having lain on the sea floor for a considerable time before being deposited in the fissures. Fossils in the dikes are characteristic of the reefal and adjacent outer-shelf facies tracts, but they are a biased record of the biota of that depositional area in that they consist largely of vagile or only weakly attached members of the dweller and baffler guilds living on the reef surface and of organisms living within the fissures; members of the reefal framework communities are largely absent. The absence in the dikes of fossils diagnostic of more distant facies tracts indicates that there was little across-platform movement of carbonate sediment on the platform. Deposition of terrigenous sediment is correlated with a sea-level low-stand, indicating that land-derived sediment moved across the platform otherwise infrequently.