Glacial Lowstand Research Articles

Resilient biotic response to long-term climate change in the Adriatic Sea.

Preserving adaptive capacities of coastal ecosystems, which are currently facing the ongoing climate warming and a multitude of other anthropogenic impacts, requires an understanding of long‐term biotic dynamics in the context of major environmental shifts prior to human disturbances. We quantified responses of nearshore mollusk assemblages to long‐term climate and sea‐level changes using 223 samples (~71,300 specimens) retrieved from latest Quaternary sediment cores of the Adriatic coastal systems. These cores provide a rare chance to study coastal systems that existed during glacial lowstands. The fossil mollusk record indicates that nearshore assemblages of the penultimate interglacial (Late Pleistocene) shifted in their faunal composition during the subsequent ice age, and then reassembled again with the return of interglacial climate in the Holocene. These shifts point to a climate‐driven habitat filtering modulated by dispersal processes. The resilient, rather than persistent or stochastic, response of the mollusk assemblages to long‐term environmental changes over at least 125 thousand years highlights the historically unprecedented nature of the ongoing anthropogenic stressors (e.g., pollution, eutrophication, bottom trawling, and invasive species) that are currently shifting coastal regions into novel system states far outside the range of natural variability archived in the fossil record.

Climate and sea‐level controlling internal architecture of a Quaternary carbonate ramp (Northwest Shelf of Australia)

AbstractThe continental shelf of Northwest Australia hosts an extensive tropical carbonate ramp that forms an important template for the interpretation of similar systems in the sedimentary record. Yet, little is known about its development from the middle to late Quaternary, a period of high frequency glacioeustatic changes in sea‐level and climate. This research describes core and seismic‐reflection data from a mid‐ramp to outer ramp transect at the Northwest Shelf. Core material includes the upper 70 m of International Ocean Discovery Program Site U1461 (Expedition 356), which covers the last 500 kyr. During arid glacials, sedimentation is characterized by inorganically precipitated carbonates, including aragonite‐needle mud and ooids. Ooids developed under shallow marine conditions on small‐scale flat‐topped platforms. Seismic and sedimentological evidence indicates that these platforms developed locally on top of the present‐day mid‐ramp and were typically only active during a single glacial period. Aragonite‐needle mud precipitated (inorganically) in shallow‐water areas. Much of these fine sediments were subsequently exported into deeper water where they mixed with pelagic carbonates. Humid interglacials are generally characterized by reduced sedimentation across the Northwest Shelf of Australia, resulting in the amalgamation of glacial lowstand deposits. Yet, substantial amounts of skeletal carbonates were deposited at the studied location during the Holocene and Marine Isotope Stage 11. These sediment accumulations are interpreted as a local feature caused by a decline in current energy. The study presented highlights a ramp system where climate is as important as sea‐level in controlling carbonate deposition.

Integrated Pliocene-Pleistocene magnetostratigraphy and tephrostratigraphy of deep-sea sediments at IODP Site U1424 (Yamato Basin, Japan Sea)

Sediments from the semi-enclosed Japan Sea are sensitive to paleoclimatic perturbations and they offer great opportunities for many regional and global paleoceanographic and paleoclimatic studies. These studies often require a robust chronology. However, due to rare preservation of calcareous microfossils and drastic changes in surface water salinity during glacial lowstands, the construction of a traditional oxygen isotope stratigraphy for Japan Sea sediments is often difficult. Here, we use sediments recovered at Integrated Ocean Drilling Program (IODP) Expedition 346 Site U1424 to build an integrated Pliocene-Pleistocene reference magnetostratigraphy and tephrostratigraphy for sedimentary sequences from the region. Rock magnetic experiments indicate that magnetic remanence of Site U1424 sediments are carried primarily by (titano)magnetite with small contributions from high coercivity minerals (e.g., hematite) and possibly iron sulphides (pyrrhotite and/or greigite). Dark-colored sediments appear to contain less (titano)magnetite probably due to reductive diagenesis under euxinic conditions. Natural remanent magnetization (NRM) of u-channel samples covering a continuous ~ 167.6 m sediment sequence at Site U1424 was repeatedly measured at 1 cm intervals before and after stepwise demagnetization. Despite lower NRM intensity in dark-colored sediments, NRM directional data from both dark- and light-colored sediments are considered suitable for the construction of magnetostratigraphy. Site U1424 sediments clearly recorded the majority of the polarity chrons and subchrons within the last ~ 4.89 Myr, with the Cobb Mountain subchron, the end of Kaena subchron, and the onset of Nunivak subchron less well preserved. Sixteen tephra layers from the site were sampled for chemical composition analyses and the results were correlated to reference tephrostratigraphy of the region. Ages of the identified tephras are consistent with and can be well integrated with the magnetostratigraphy. The resulting age model suggests that sedimentation rates at Site U1424 range between ~ 1.7 and 7.6 cm/kyr with an average of ~ 3.3 cm/kyr. The acquired magnetostratigraphy and tephrostratigraphy at Site U1424 provide a reference chronology that can be correlated with and transferred to other sediment sequences in the region to study paleoceanographic and paleoclimatic changes of the region as well as their links to other regional and global changes.Graphical abstract

Atmospheric dust flux in northeastern Gondwana during the peak of the late Paleozoic ice age

The silicate mineral fraction of shallow marine carbonates archives dust contributions to the Central Persian Terranes along the northeastern margin of Gondwana (∼30ºS paleolatitude), enabling reconstruction of atmospheric dust loading and circulation for intervals of the late Paleozoic ice age. The Central Persian Terranes hosted cyclic deposition of warm water carbonates from middle Pennsylvanian to earliest Permian time, and our data set includes two ∼28 m sections from the Moscovian and Asselian sampled at 20 cm intervals. Bounding surfaces between successive cycles (high-frequency sequences) are recognized by either abrupt basinward shifts in facies or subtle exposure features; these high-frequency sequences range from 1 m to 5 m thick and are interpreted to record glacioeustatic variations. Time series analysis of the dust fraction through the studied interval supports the hypothesis of orbital forcing for the dust signal. The stratigraphic pattern of the dust flux indicates minimal flux during interglacial highstands (0.19−0.27 g/cm2/kyr) and peak flux during glacial lowstands (3.77−4.57 g/cm2/kyr) after accounting for hiatal time at sequence boundaries. Grain size analysis of the dust for all samples (n = 230) reveals modal sizes (volume-based) of 1−15 µm through the Moscovian interval and 10−75 µm through the Asselian interval. Dust deposition increased during glacial times relative to interglacial times by a factor of 16 to 19. Additionally, the Asselian interval exhibits higher dust flux overall relative to the Moscovian interval, which is interpreted to reflect the more extreme icehouse conditions of the Asselian. Variation in the dust content through the studied sections provides an indicator of temporal changes in atmospheric loading that varied at both glacial−interglacial and higher-frequency (<104 yr) scales. Geochemical data reveal that the Arabian−Nubian Shield and southwestern Pangaea (South America) are the most likely sources of dust deposition in the Central Persian Terranes, with sources shifting during different phases. Increased dust flux during glacials likely reflects multiple factors, including enhanced aridity in the source region, exposure of shelf regions, and potential changes in winds. However, the discrepancy in model reconstructions of the amplitude of glacial−interglacial dust variations indicates that increased production of dust sourced by dynamic glaciation played a large role in enhancing dust flux during glacial phases.

The Origin of Modern Atolls: Challenging Darwin's Deeply Ingrained Theory.

In 1842, Darwin identified three types of reefs: fringing reefs, which are directly attached to volcanic islands; barrier reefs, which are separated from volcanic islands by lagoons; and ring reefs, which enclose only a lagoon and are defined as atolls. Moreover, he linked these reef types through an evolutionary model in which an atoll is the logical end point of a subsiding volcanic edifice, as he was unaware of Quaternary glaciations. As an alternative, starting in the 1930s, several authors proposed the antecedent karst model; in this model, atolls formed as a direct interaction between subsidence and karst dissolution that occurred preferentially in the bank interiors rather than on their margins through exposure during glacial lowstands of sea level. Atolls then developed during deglacial reflooding of the glacial karstic morphologies by preferential stacked coral-reef growth along their margins. Here, a comprehensive new model is proposed, based on the antecedent karst model and well-established sea-level fluctuations during the last 5 million years, by demonstrating that most modern atolls from the Maldives Archipelago and from the tropical Pacific and southwest Indian Oceans are rooted on top of late Pliocene flat-topped banks. The volcanic basement, therefore, has had no influence on the late Quaternary development of these flat-topped banks into modern atolls. During the multiple glacial sea-level lowstands that intensified throughout the Quaternary, the tops of these banks were karstified; then, during each of the five mid-to-late Brunhes deglaciations, coral reoccupied their raised margins and grew vertically, keeping up with sea-level rise and creating the modern atolls.

The sedimentary record of Quaternary glacial to interglacial sea‐level change on a subtropical carbonate ramp: Southwest Shelf of Australia

AbstractIn the last decades, the understanding of temperate carbonate systems has improved considerably, but their development over glacial–interglacial timescales is still understudied in comparison to their tropical counterparts. A key question is how do temperate carbonate platforms respond to high‐amplitude, glacial–interglacial sea‐level changes? Integrated Ocean Drilling Program Site U1460 was drilled at the uppermost slope of the Southwest Shelf of Australia at the transition between the subtropical Carnarvon Ramp and the warm–temperate Rottnest Shelf. The origin and composition of the sediments in the upper 25 m below seafloor at Site U1460 were investigated using X‐ray diffraction, scanning electron, and light microscopy. The Middle Pleistocene to Holocene sequence at Integrated Ocean Drilling Program Site U1460 contains a record of sea‐level controlled sedimentary cycles. Carbonate sediments deposited during interglacial sea‐level highstands (Marine Isotope Stages 1, 5, most of 7, 9 and 11) are mainly fine‐grained (<63 µm) and dominated by low‐Mg calcite from pelagic bioclasts such as planktic foraminifera. The glacial lowstand intervals (Marine Isotope Stages 2 to 4, 6, 8, 7d, 10 and 12), instead are coarser‐grained and relatively rich in aragonite and high‐Mg calcite from neritic bioclasts, such as bryozoans. These changes in texture, mineralogy and composition are best explained by the deposition of neritic bioclasts closer to the shelf edge during glacial sea‐level lowstands. During early transgression, reworking of bioclast‐rich coastal dune deposits likely leads to transport and redeposition of neritic clasts on the upper slope. In contrast, dominantly pelagic sediments characterize deposition at the platform edge during interglacial highstands. These results highlight regional differences in the response of temperate carbonate systems to sea‐level change: A previously published model developed for early Pleistocene temperate carbonates from the Great Australian Bight indicates that shelfal material was exported to the upper slope during sea‐level highstands. It is argued that this difference is related to the change in duration and amplitude of glacial–interglacial sea‐level cycles before and after the Mid‐Pleistocene transition.

Submarine landslides on a carbonate platform slope: forward numerical modelling of mechanical stratigraphy and scenarios of failure precondition

Slope failure and landslides are widespread on the submarine slopes of carbonate platforms. They represent a key component of their stratigraphic evolution and a major geohazard. Several Quaternary slope failures and mass transport complexes were identified on the western slope and margin of the Great Bahama Bank (GBB) platform. This study evaluates several hypotheses for the preconditions associated to these events, in relation with the stratigraphic and environmental history of the platform. The forward stratigraphic simulator Dionisos Flow™ allows the slope stratigraphic evolution to be reconstructed on a 2D platform-to-basin section at high temporal resolution according to the eustatic and environmental history of the last 1.7 Myr. The hydromechanical simulator A2 is applied on this high-resolution stratigraphic grid for computing the mechanical stratigraphy of the section, that is the spatiotemporal pore pressure and stress state distribution. The simulated precondition of the platform margin during glacial lowstands results from the combination of two factors: transient overpressure generation by lateral fluid flow from the emerged platform and the steepening stratigraphic trend of the platform. No significant pore fluid overpressure is generated under high sedimentation rates or as a result of shift in hydrostatic gradient during sea-level falls. Precondition in the lower slope is not achieved in this 2D simulation, with the horizontal stress distribution counteracting the effect of overpressure build-up. It is found that cemented levels in the lower slope succession do not represent a significant preconditioning factor.

Reef-flat and back-reef development in the Great Barrier Reef caused by rapid sea-level fall during the Last Glacial Maximum (30–17 ka)

Abstract Reef growth patterns and the development of associated environments have been extensively studied from reef deposits from Holocene and previous interglacial highstands. However, reefs that grew during glacial lowstands are comparatively poorly understood. Here we show the formation of reef-flat and back-reef environments following rapid sea-level fall (15–20 mm yr−1 and 20–40 m in magnitude) during the Last Glacial Maximum (LGM) on the present shelf edge of the Great Barrier Reef. Sedimentological and foraminiferal analyses of unconsolidated reef sediments recovered in cores 111–140 m below sea level at Hydrographers Passage during Integrated Ocean Drilling Project (IODP) Expedition 325 reveal the occurrence of a benthic foraminiferal assemblage dominated by the genera Calcarina and Baculogypsina, which is common in modern reef-flat and back-reef environments in the Great Barrier Reef and elsewhere. This assemblage is associated with higher foraminiferal proportions in reef sediments and higher proportions of well-preserved Baculogypsina tests in the same intervals, which also characterize reef-flat environments. Radiocarbon (14C–accelerator mass spectrometry) ages of reef-flat dwelling foraminifers (n = 22), which indicate the time when these foraminifers were alive, are consistent with the timing of the two-step sea-level fall into the LGM as defined by the previously published well-dated coralgal record. This foraminiferal evidence suggests the development of geomorphically mature fringing reefs with shallow back-reef lagoons during the LGM. Our results also imply that back-reef sediment accumulation rates during the LGM lowstand were comparable to those during the Holocene highstand.

Seismic stratigraphy of the inner to mid Agulhas bank, South Africa

The inner to mid continental shelf of the Agulhas Bank, which forms part of the Palaeo-Agulhas Plain, is scattered with Pleistocene deposits. Their wide lateral extension is the expression of a flat underlying substrate, availability of accommodation space, and depositional processes in response to glacio-eustatic sea-level change. We present seismic sub-bottom profiles up to 30 m deep, sediment cores up to 5 m in length and Pleistocene deposits that date back to Marine Isotope Stage (MIS) 7 from the inner to mid shelf between the Breede River in the West and Plettenberg Bay in the East. Radiocarbon and optically stimulated luminescence dates are integrated with microfossil analysis into a seismic stratigraphic model comprised of twenty Quaternary facies units within two depositional sequences bounded by shelf-wide unconformities. Sequence Boundary 1 (SB1) corresponds to the erosional unconformity with bedrock and SB2 to the MIS 2 glacial lowstand. Incised palaeo-river channels are associated with both sequence boundaries and cored deposits also mapped seismically from estuarine, lacustrine and fluvial systems are grouped to represent the lower floodplain. The most pervasive stratigraphic pattern in these shelf deposits is made up of the depositional sequence remnant of the Falling Stage Systems Tract (FSST) forced regression from MIS 5e–2. The other dominant stratigraphic group is the Transgressive Systems Tract (TST) associated with the Post-glacial Marine Transgression. The TST makes up an almost equal proportion of deposits in both sequences in the sedimentological record as the FSST, despite the shorter temporal span of the TST. A Wave Ravinement Surface marks the rise in sea level from the Last Glacial Maximum in a landward direction.

Carbonate drifts as marine archives of aeolian dust (Santaren Channel, Bahamas)

AbstractSediment data from the Bahamian Santaren carbonate drift reveal the variability of trans‐Atlantic Saharan dust transport back to about 100 ka bp (Marine Isotope Stage 5·3) and demonstrate that carbonate drifts are a valuable pelagic archive of aeolian dust flux. Carbonate drift bodies are common around tropical carbonate platforms; they represent large‐scale accumulations of ocean‐current transported material, which originates from the adjacent shallow‐water carbonate factory as well as from pelagic production, i.e. periplatform ooze. Subordinately, there is a clay‐size to silt‐size non‐carbonate fraction, which typically amounts to less than 10% of the sediment volume and originates from aeolian and fluvial input. Sedimentation rates in the 5·42 m long core GeoHH‐M95‐524 recovered 25 km west of Great Bahama Bank in the Santaren Channel ranges from 1·5 to 24·5 cm ka−1 with lowest values during the last glacial lowstand and highest values following platform re‐flooding around 8 ka bp. These sedimentation rates imply that carbonate drifts have not only the potential to resolve long‐term environmental changes on orbital timescales, but also millennial to centennial fluctuations during interglacials. The sediment core has been investigated with the aim of characterizing the lithogenic dust fraction. Laboratory analyses included X‐ray fluorescence core scanning, determination of carbonate content and grain‐size analyses (of bulk and terrigenous fraction), as well as visual inspections of the lithogenic residue; the age model is based on oxygen isotopes and radiocarbon ages. Data show that the input of aeolian dust in the periplatform ooze as indicated by Ti/Al and Fe/Al element ratios abruptly increases at 57 ka bp, stays elevated during glacial times, and reaches a Holocene minimum around 6·5 ka bp, contemporary to the African Humid Period. Subsequently, there is a gradual increase in dust flux which almost reaches glacial levels during the last centuries. Grain‐size data show that the majority of dust particles fall into the fine silt range (below 10 μm); however, there is a pronounced coarse dust fraction in the size range up to 63 μm and individual ‘giant’ dust particles are up to 515 μm in size. Total dust flux and the relative amounts of fine and coarse dust are decoupled. The time‐variable composition of the grain‐size spectrum is interpreted to reflect different dust transport mechanisms: fine dust particles are delivered by the trade winds and the geostrophic winds of the Saharan Air Layer, whereas coarse dust particles travel with convective storm systems. This mode of transport ensures continuous re‐suspension of large particles and results in a prolonged transport. In this context, grain‐size data from the terrigenous fraction of carbonate drifts provide a measure for past coarse dust transport, and consequently for the frequency of convective storm systems over the dust source areas and the tropical Atlantic.

Paleoshorelines and lowstand sedimentation on subtropical shelves: a case study from the Fraser Shelf, Australia

Evidence of submerged paleoshorelines on tectonically stable continental shelves is represented by relict coastal depositional features that formed during periods of lower sea level. This study investigates two paleoshoreline features that extend 70 km from the Barwon Bank along the southeast Fraser Shelf, off eastern Australia. They are defined by well-lithified, mixed carbonate-siliciclastic deposits now at water depths of 60 m and between 80 and 100 m, located approximately 40 km seaward of the modern shoreline. High-resolution multibeam bathymetry data show geomorphic features that we interpret as paleodunes in the case of the 60 m feature, and paleobarriers in the case of the 80–100 m feature. Sub-bottom profiles also show evidence of prograded beach foresets and paleochannels as components of the Barwon Bank, which is a low and elongate bank that rises about 30 m above the surrounding shelf to a depth of 60 m. Carbonate cements in the rocks forming the paleoshoreline features reveal diagenetic influence of meteoric waters, suggesting that subaerial processes contributed to the formation and preservation of both features. Radiocarbon dating of bioclastic grains (coralline algae, benthic forams) yields an age range between 22.8 and 20.4 ka for the paleobarrier, and an age of 17.3 ka for the paleodune. The morphology of the submerged paleodunes is analogous to modern parabolic dunes on nearby Fraser and Moreton islands. The position and morphology of the paleodunes provide proxy information about the climate history of Australia during the late Quaternary, as these dunes are a likely consequence of the general continent-wide aridity during the Last Glacial Maximum. The preservation of the eolian dunes during marine transgression, despite their direct exposure to coastal hydrodynamic processes, suggests that the dunes were armoured, stabilised and lithified during the glacial lowstand, prior to drowning.

Sedimentary proxies for Pacific water inflow through the Herald Canyon, western Arctic Ocean

Pacific water inflow to the Arctic Ocean occurs through the shallow Bering Strait. With a present sill depth of only 53 m, this gateway has been frequently closed during glacial sea-level low stands of the Pleistocene. Here, we investigate the sedimentological and mineralogical response to sea-level rise and the opening of the Bering Strait during the last deglaciation in a 6.1 m-long marine sediment core (SWERUS-L2-4-PC1) from the Herald Canyon. Grain size data indicate an abrupt erosional contact at 412 cm down core that likely formed when Pacific waters first started to flow into the Arctic Ocean around 11 cal ka BP, and was topographically steered into the Herald Canyon. A transitional unit between 412 and 390 cm appears to be a condensed interval with minimal local sedimentation. The underlying sediments, deposited in a shallow, river-proximal setting, exhibit a rather uniform bulk and clay mineral composition similar to mineral assemblages from surface sediment samples of the Chukchi Sea. Enhanced contributions from Pacific waters above 390 cm (< 8.5 cal ka BP) are reflected by elevated chlorite/illite and (chlorite + kaolinite)/illite ratios, and are anti-correlated with intervals of higher illite/smectite ratios, interpreted as periods of enhanced advection of shelf transformed waters originating from the East Siberian Sea. Clay mineral changes in the Holocene drift sediments are best explained by the interplay between two origins for bottom waters in the Herald Canyon and are consistent with modern oceanographic observations.

How sea level change mediates genetic divergence in coastal species across regions with varying tectonic and sediment processes.

Platetectonics and sediment processes control regional continental shelf topography. We examine the genetic consequences of how glacial-associated sea level change interacted with variable nearshore topography since the last glaciation. We reconstructed the size and distribution of areas suitable for tidal estuary formation from the last glacial maximum, ~20 thousand years ago, to present from San Francisco, California, USA (~38°N) to Reforma, Sinaloa, Mexico (~25°N). We assessed range-wide genetic structure and diversity of three codistributed tidal estuarine fishes (California Killifish, Shadow Goby, Longjaw Mudsucker) along ~4,600km using mitochondrial control region and cytB sequence, and 16-20 microsatellite loci from a total of 524 individuals. Results show that glacial-associated sea level change limited estuarine habitat to few, widely separated refugia at glacial lowstand, and present-day genetic clades were sourced from specific refugia. Habitat increased during postglacial sea level rise and refugial populations admixed in newly formed habitats. Continental shelves with active tectonics and/or low sediment supply were steep and hosted fewer, smaller refugia with more genetically differentiated populations than on broader shelves. Approximate Bayesian computation favoured the refuge-recolonization scenarios from habitat models over isolation by distance and seaway alternatives, indicating isolation at lowstand is a major diversification mechanism among these estuarine (and perhaps other) coastal species. Because sea level change is a global phenomenon, we suggest this top-down physical control of extirpation-isolation-recolonization may be an important driver of genetic diversification in coastal taxa inhabiting other topographically complex coasts globally during the Mid- to Late Pleistocene and deeper timescales.

Glacial Cycles Influence Marine Methane Hydrate Formation

AbstractMethane hydrates in fine‐grained continental slope sediments often occupy isolated depth intervals surrounded by hydrate‐free sediments. As they are not connected to deep gas sources, these hydrate deposits have been interpreted as sourced by in situ microbial methane. We investigate here the hypothesis that these isolated hydrate accumulations form preferentially in sediments deposited during Pleistocene glacial lowstands that contain relatively large amounts of labile particulate organic carbon, leading to enhanced microbial methanogenesis. To test this hypothesis, we apply an advection‐diffusion‐reaction model with a time‐dependent organic carbon deposition controlled by glacioeustatic sea level variations. In the model, hydrate forms in sediments with greater organic carbon content deposited during the penultimate glacial cycle (~120–240 ka). The model predictions match hydrate‐bearing intervals detected in three sites drilled on the northern Gulf of Mexico continental slope, supporting the hypothesis of hydrate formation driven by enhanced organic carbon burial during glacial lowstands.

Late Pleistocene Bryant Canyon turbidite system: Implications for Gulf of Mexico minibasin petroleum systems

The western ancestral Mississippi shelf-margin delta fed the Bryant Canyon turbidite system in the intraslope basin province of the northwestern Gulf of Mexico (GOM) during the penultimate glacial lowstand of sea level (130–160 year BP). The Bryant Canyon links a chain of 15 fill-and-spill minibasins on the continental slope. On the upper and lower continental slopes, minibasins are narrow (1–3 km), elongate (3–6 km), and follow salt ridges. On the middle slope, minibasins are larger (8–15 km) semicircular basins. Three main depositional facies are recognized based on seismic-facies interpretation: (1) ponded turbidites (T), (2) mass-transport deposits (MTDs), and (3) bypass channelized turbidites (C). Thick, intrabasinal, muddy MTD wedges sourced from high-relief internal walls of the minibasins alternate with and frequently cap externally derived deposits. Tabular extrabasinal MTD deposits originated from shelf-margin delta or canyon-wall failures upslope. The T and MTD facies deposits each make up approximately 40% of basin fill, and the C facies deposits comprise approximately 20%. The T facies deposits form perched lobes at canyon inlets into basins and ponded units on distal sides of basins. Channels in the C facies are similar in width (500–2000 m) and relief (20–100 ms) to channels in productive GOM subsurface minibasins. Syntectonic activity of salt diapirs typically began midway through filling of Bryant Canyon minibasins and then preferentially uplifted northern portions of basin deposits. Local salt-tectonic activity caused greater basin relief and thicker capping MTDs than in subsurface minibasins to the west (e.g., Brazos-Trinity Basin IV) or east (e.g., Mississippi Canyon). Bryant Canyon minibasins provide excellent modern analogs for subsurface Miocene to Pleistocene GOM chains of minibasins because of similar scales and depositional facies. The youngest Bryant T facies deposits and their overlying incised, thick, channel deposits contain the most sand-prone facies and suggest the best potential for petroleum reservoirs in subsurface minibasins.

Bathyal records of enhanced silicate erosion and weathering on the exposed Luzon shelf during glacial lowstands and their significance for atmospheric CO2 sink

We present a new high-resolution multiproxy data set of mass accumulation rates, Sr-Nd isotopes, as well as major and trace elements for the siliciclastic sediment fraction from International Marine Global Change Study Core MD06-3052, located on the continental slope of the western Philippine Sea. We integrate our new data with published grain sizes and sea surface temperatures from the same core, as well as with Equatorial Pacific sea level, and East Asian summer monsoon precipitation, in order to constrain at high-resolution changes in physical erosion and chemical weathering intensities on Luzon, and sediment source-to-sink processes. We assess the potential significance of chemical weathering of arc silicates in regulating global atmospheric CO2 since 156kyr BP. Sr-Nd isotopes show that the siliciclastic sediments were dominantly sourced from volcanic rocks exposed on Luzon (~68–100%), with a lesser contribution from Asian dust (~0–32%). Different indices indicate that stronger physical erosion and chemical weathering occurred during Marine Isotope Stage (MIS) 6 (130–156kyr BP), as well as in the latter stage of MIS 3 and MIS 2 (14–40kyr BP). The large sea-level lowstands and associated significant exposure of continental shelf in the western Philippine Sea during these two cold periods should favor physical erosion and chemical weathering of unconsolidated sediments on the exposed shelf. Furthermore, we notice the relatively good coherence between chemical weathering intensities on Luzon and global atmospheric CO2 concentrations over these cold intervals. We suggest that strengthening of chemical weathering of silicates on Luzon and other tropical arcs (within 20° of the Equator) during the Quaternary glacial sea-level lowstands may significantly contribute to the lowering of atmospheric CO2 concentrations during ice ages. We estimate that a significant fraction, up to ~16% (i.e., ~8ppmv), of all atmospheric CO2 ultimately sequestered by silicate weathering may be processed through an area corresponding to only ~1% of the exorheic drainage area worldwide.

Pleistocene reefs of the Egyptian Red Sea: environmental change and community persistence.

The fossil record of Red Sea fringing reefs provides an opportunity to study the history of coral-reef survival and recovery in the context of extreme environmental change. The Middle Pleistocene, the Late Pleistocene, and modern reefs represent three periods of reef growth separated by glacial low stands during which conditions became difficult for symbiotic reef fauna. Coral diversity and paleoenvironments of eight Middle and Late Pleistocene fossil terraces are described and characterized here. Pleistocene reef zones closely resemble reef zones of the modern Red Sea. All but one species identified from Middle and Late Pleistocene outcrops are also found on modern Red Sea reefs despite the possible extinction of most coral over two-thirds of the Red Sea basin during glacial low stands. Refugia in the Gulf of Aqaba and southern Red Sea may have allowed for the persistence of coral communities across glaciation events. Stability of coral communities across these extreme climate events indicates that even small populations of survivors can repopulate large areas given appropriate water conditions and time.

Enhanced silicate weathering of tropical shelf sediments exposed during glacial lowstands: A sink for atmospheric CO2

Atmospheric CO2 and global climate are closely coupled. Since 800ka CO2 concentrations have been up to 50% higher during interglacial compared to glacial periods. Because of its dependence on temperature, humidity, and erosion rates, chemical weathering of exposed silicate minerals was suggested to have dampened these cyclic variations of atmospheric composition. Cooler and drier conditions and lower non-glacial erosion rates suppressed in situ chemical weathering rates during glacial periods. However, using systematic variations in major element geochemistry, Sr–Nd isotopes and clay mineral records from Ocean Drilling Program Sites 1143 and 1144 in the South China Sea spanning the last 1.1Ma, we show that sediment deposited during glacial periods was more weathered than sediment delivered during interglacials. We attribute this to subaerial exposure and weathering of unconsolidated shelf sediments during glacial sealevel lowstands. Our estimates suggest that enhanced silicate weathering of tropical shelf sediments exposed during glacial lowstands can account for ∼9% of the carbon dioxide removed from the atmosphere during the glacial and thus represent a significant part of the observed glacial–interglacial variation of ∼80ppmv. As a result, if similar magnitudes can be identified in other tropical shelf-slope systems, the effects of increased sediment exposure and subsequent silicate weathering during lowstands could have potentially enhanced the drawdown of atmospheric CO2 during cold stages of the Quaternary. This in turn would have caused an intensification of glacial cycles.

Lowstand wedges in carbonate platform slopes (Quaternary, Maldives, Indian Ocean)

AbstractSeismic, hydroacoustic and sedimentological data were used to analyse the response of atoll‐slope sedimentation in the Maldives to the late Quaternary sea‐level change. The slope deposits, as imaged in multichannel seismic profiles, are arranged into stacked aggrading to backstepping basinward thinning wedges. In a piston core recovered at the lower slope of one of the atolls, the sediment texture ranges from packstone to rudstone. Major components are blackened bioclasts, the large benthic foraminifers Operculina and Amphistegina, together with Halimeda debris and red algae. Radiocarbon dating at a core depth of 66 cm indicates that the wedge sedimentation stopped or was largely reduced after 16 ka BP. Therefore, the atoll‐slope deposits largely consist of sediment formed in situ and deposited during the last glacial lowstand in sea‐level. This is in apparent contradiction to the concept of highstand shedding of tropical carbonate platforms, which requires slope sedimentation during sea‐level highstands, when the platform is flooded. Rather than intrinsic factors, such as sediment bypass along the steep slope, the extrinsic process of current winnowing of the slope appears to be a major controlling factor in the production of this feature. This process may be relevant for other case studies of carbonate platforms, as currents may be accelerated around such edifices, leading to slope winnowing and sediment deposition in more current‐protected zones. The study results also have consequences for the interpretation of outcrop and seismic subsurface data of carbonate platform slope series, because such slope sediment wedges are not necessarily formed during sea‐level highstands, but can consist of lowstand wedges only.

Temporal and spatial patterns of sediment deposition in the northern South China Sea over the last 50,000 years

Sediment depositional patterns along the upper continental slope of the northern South China Sea (SCS) have been studied using two sediment cores (GeoB16601-6, 20°09.07′N, 116°14.38′E, 1012m water depth and GeoB16602-4, 18°57.12′N, 113°42.64′E, 951m water depth) recovered during RV SONNE Cruise SO-221 “INVERS”. Sediment cores were analyzed for bulk sediment element composition by X-Ray Fluorescence (XRF) core scanning and clay mineral assemblage by X-Ray Diffraction (XRD). The bulk sediment element and the clay mineral compositions of the two cores suggest similar depositional patterns between 50kyrBP and ~29kyrBP, and ~14kyrBP to present, but a clear difference in depositional patterns during the last glacial and deglacial (~29–14kyrBP) when sea level was below −90m. Between ~29–14kyrBP, a higher kaolinite percentage in core GeoB16602-4 in comparison to core GeoB16601-6 is interpreted to reflect a higher contribution of clay supplied by the Pearl River to core site GeoB16602-4. In contrast, core GeoB16601-6 received less Pearl River supplied clayey material during the same period. When the sea level was below −90m during the last glacial lowstand, the detrital fine-grained materials supplied by the Pearl River were mostly transported by coastal currents to the southwest, resulting in a higher contribution of Pearl River discharged material at site GeoB16602-4 than at site GeoB16601-6. We suggest that sea-level induced modifications of the land-ocean distribution together with changes in the palaeo-physiographic conditions, such as the proximity of the palaeo-rivers to the individual core sites might be responsible for the different sediment depositional patterns in the study area. Thus, the overriding control of sea-level induced changes on the sediment depositional environment might mask climate-related changes in sediment depositional pattern in the northern SCS.