Fine-grained Terrigenous Sediments Research Articles

The orogenic effects on forebulge evolution: A case study from Cenozoic deposits in the Salento Peninsula, Italy

The Salento Peninsula is a part of the larger Apulia Carbonate Platform, one of the carbonate platforms that have developed along the southern margin of the Tethys Ocean since the Triassic, above the Adria microplate. This peninsula represents the culmination of a lithospheric anticline formed due to the subduction of the Adria Plate beneath two mountain belts migrating in opposite directions: the Dinarides-Albanides-Hellenides (DAH) chain, moving from NE to SW, and the southern Apennines (SA) chain, moving from SW to NE. Consequently, the Salento Peninsula represents the foreland sector of these two mountain belts, whose different migration rates, deeply influenced the carbonate sedimentation and paleogeographic evolution of this area over the last 60 Myr. In our study field analysis, a database of 350 wells, and 90 seismic lines calibrated with 3 exploration wells were employed to characterize the physical stratigraphy and depositional setting of the Salento Peninsula and define its paleogeographic evolution during the Paleogene and Neogene. Our results suggest that temporal variations in sedimentation rates and types as well as the reduced thickness (maximum 200 m) of the Cenozoic stratigraphic succession in the internal sectors and both along the Adriatic and Ionian margins of the Salento Peninsula are directly or indirectly linked to the migration rates of the DAH and SA chains. These migration rates played a role in determining changes in subsidence rates, the occurrence of soft-sediment deformation, and the influx of Plio-Pleistocene fine-grained terrigenous sediments, especially along the Ionian margin. Although the investigated succession exhibits a reduced thickness, it comprises several lithostratigraphic units bounded by unconformity surfaces. Within these units, a stack of complete and incomplete simple and composite high- and low-rank depositional sequences has been recognized. Our data suggest that these units are the result of a multifold cyclicity, reflecting the close interaction between eustatic sea-level changes and the tectonic setting of the area.

Controls on Terrigenous Detritus Deposition and Oceanography Changes in the Central Okhotsk Sea Over the Past 1550 ka

The Okhotsk Sea, which connects the high latitude Asian continent and the North Pacific, plays an important role in modern and past climate changes due to seasonal sea ice coverage and as a precursor of the North Pacific Intermediate Water. The long-term glacial-interglacial changes of sea ice coverage and its impacts on terrigenous transport and surface primary productivity in the Okhotsk Sea remain, however, not well constrained. Base on the paleomagnetic, rock magnetic, micropaleontological (diatom), and geochemical studies of the marine sediment core MD01-2414 (53°11.77′N, 149°34.80′E, water depth: 1,123 m) taken in the central Okhotsk Sea, we reconstruct the terrigenous sediment transport and paleoceanographic variations during the past 1550 thousand years (kyr). Seventeen geomagnetic excursions are identified from the paleomagnetic directional record. Close to the bottom of the core, an excursion was observed, which is proposed to be the Gilsa event ∼1550 thousand years ago (ka). During glacial intervals, our records reveal a wide extension of sea ice coverage and low marine productivity. We observed ice-rafted debris from mountain icebergs composed of coarse and high magnetic terrigenous detritus which were derived from the Kamchatka Peninsula to the central Okhotsk basin. Still during glacial intervals, the initiation (i.e., at ∼900 ka) of the Mid-Pleistocene Transition marks the changes to even lower marine productivity, suggesting that sea-ice coverage became larger during the last 900 ka. During interglacial intervals, the central Okhotsk Sea was either devoid of sea-ice or the ice was at best seasonal; resulting in high marine productivity. The weaker formation of Okhotsk Sea Intermediate Water, lower ventilation, and microbial degradation of organic matter depleted the oxygen concentration in the bottom water and created a reduced environment condition in the sea basin. The freshwater supplied by snow or glacier melting from Siberia and Kamchatka delivered fine grain sediments to the Okhotsk Sea. During the stronger interglacial intervals after the Mid-Brunhes Transition (i.e., Marine Isotope Stages 1, 5e, 9, and 11), strong freshwater discharges from Amur River drainage area are in association with intensified East Asian Summer Monsoon. This process may have enhanced the input of fine-grained terrigenous sediments to the central Okhotsk Sea.

Chemical evolution of the continental crust from a data-driven inversion of terrigenous sediment compositions

The nature of emerged continents through time is highly debated. Several studies relying on trace element data concluded that the Archaean crust was predominantly mafic, while Ti isotope systematics point to an Archaean crust that was predominantly felsic. Here, we resolve the inconsistency between these two approaches by applying a novel statistical method to a compilation of published elemental concentrations in terrigenous sediments (the OrTeS database). We use a filter based on the Local Outlier Factor to reject sediment samples that have been affected by alteration processes or mineral fractionation during transport. The nature of the emerged continents is calculated using an inverse mixing model based on a Markov Chain Monte Carlo algorithm. A procedure is presented to automatically select elemental ratios that are best suited for constraining the sediment provenance. We find that for all systems that accurately reconstruct the modern-day composition of the continents, a continuous >50% felsic contribution is required to explain the composition of fine-grained terrigenous sediments starting from 3.5 billion years ago. This finding is consistent with an early onset of plate tectonics. We estimate the geothermal gradient in the Archaean upper continental crust by tracking the reconstructed concentrations of the radiogenic heat-producing elements K, U, and Th through time. Radioactive heat production in the bulk continental crust was 50% higher in the Archaean compared to the present, resulting in a continental geothermal gradient that was about 40% higher.

Implications for Provenance since the Last Glaciations in Southeastern Andaman Sea Sediments by Clay Mineralogy

This study presents high-resolution clay mineralogy of Core MASS-III-10, located in the southeastern Andaman Sea. The study aimed to investigate terrigenous sedimentary input from various potential provenances throughout different periods of geological time. The clay mineral assemblages of MASS-III-10 reveal a high amount of smectite at 73% (65-79%), a moderate kaolinite at 12% (10-17%), and a low illite and chlorite content(<15%) over the last 45 ka. Provenance analysis suggests the most fine-grained terrigenous sediments originated from the Irrawaddy Delta Shelf (IDS), with minor sediments deriving from the Andaman Islands, the East Continental Shelf (ECS), and Sumatra provenance. The results show that IDS were predominantly smectite (~44%) while the ECS largely produced kaolinite (47%). The provenance interpretation based on the smectite content revealed that over time there has been no change in the main source of sediment in the Andaman Sea, despite changes in the volume of sedimentary input. Since the last glaciation, the Myanmar provenance (including IDS) has always contributed clay minerals, while other sources made up only minor contributions to the Andaman Sea. The clay minerals from the Myanmar provenance increased when the sea level was at a low stand (MIS 2), potentially due to shoreline retreat which enabled easier transportation of sediment from other sources. During the interglacial period (MIS 1 and 3), the study found a decrease in Myanmar-sourced clay minerals, which may be because the higher sea level made clay mineral deposition more difficult.

The chemistry of fine-grained terrigenous sediments reveals a chemically evolved Paleoarchean emerged crust

Abstract The nature of the rocks exposed to weathering and erosion on continents exerts an important control on weathering feedbacks and the supply of nutrients to the oceans. It also reflects the prevailing tectonic regime responsible for the formation of continents. How the chemical and lithological compositions of the continents evolved through time is, however, still a matter of debate. We use an extensive compilation of terrigenous sediment compositions to better constrain the nature of rocks at the surface of continents at 3.25 Gyr and 250 Myr ago. Specifically, we use geochemical ratios that are sensitive indicators of komatiite, mafic, and felsic rocks in the provenance of the sediments. Our results show that the average Al2O3/TiO2 ratio of fine-grained terrigenous sediments decreased slightly over time from 26.2 ± 1.3 in the Archean to 22.1 ± 1.1 (2SE) in the Phanerozoic. In contrast, in the same time interval, the average Zr/TiO2 ratio stayed nearly constant at ∼245. Considering the distinct behaviors of Al, Ti and Zr during sedimentary processes, we find that hydrodynamic mineral sorting had a minor effect on the chemical composition of Archean fine-grained sediments, but could have been more effective during periods of supercontinents. We show that the compositions of Phanerozoic sediments (Al2O3/TiO2, Zr/TiO2, La/Sc, Th/Sc, Ni/Co, Cr/Sc) are best explained with igneous rocks at the surface of continents consisting of 76 ± 8 wt% felsic, 14 ± 6 wt% Arc-basalts and 10 ± 2 wt% within-plate basalts, most likely in the form of continental flood basalts. Applying the same mass-balance calculations to the Paleoarchean suggests continental landmasses with 65 ± 7 wt% felsic, 25 ± 6 wt% mafic and 11 ± 3 wt% ultramafic rocks (all 2SE), likely in the form of komatiites. The presence of volumetrically abundant felsic rocks at the surface of continents (as evident from the sediment record) as well as at mid-crustal levels (as evident from presently exposed igneous rock record) in Paleoarchean cratons is currently best explained with the onset of subduction magmatism before 3.25 Gyr.

High-resolution clay mineral assemblages in the inner shelf mud wedge of the East China Sea during the Holocene: Implications for the East Asian Monsoon evolution

The inner shelf mud wedge of the East China Sea (ECS) is a high-sedimentation-rate fine-grained sediment unit that has preserved a continuous environmental evolution history since the last deglaciation. We present a high-resolution clay mineralogical study from Core MD06-3040 to semi-quantitatively evaluate terrigenous sediment contributions from various potential provenances throughout the Holocene. The results showed that the clay mineral assemblage is composed of dominant illite (34–49%), moderate smectite (16–41%) and chlorite (15–28%), and minor kaolinite (5–12%). Provenance analysis suggested that most fine-grained terrigenous sediments originated from the Yangtze River, with minor sediments derived from Taiwan island and negligible sediments from nearby Zhejiang and Fujian provinces. Time series variation in the contribution of the Yangtze source fluctuated in the range of 38–80%, whereas that of Taiwan island had a converse variation pattern from ~10% to ~55%, and the contribution of Fujian was relatively stable in the range of 7–11% throughout the Holocene. The fluctuations of clay mineral assemblages and variations of clay mineral contributions from different provenances of Core MD06-3040 were controlled by the variability of precipitation in the Yangtze drainage associated with periodic fluctuations in the East Asian monsoonal circulation.

A 50-Year Sr/Ca Time Series from an Enclosed, Shallow-Water Guam Coral: In situ Monitoring and Extraction of a Temperature Trend, Annual Cycle, and ENSO and PDO Signals

Bell, T.; Lander, M.A.; Jenson, J.W.; Randall, R.H.; Partin, J.W., and Prouty, N.G., 2019. A 50-year Sr/Ca time series from an enclosed, shallow-water Guam coral: In situ monitoring and extraction of a temperature trend, annual cycle, and ENSO and PDO signals. Journal of Coastal Research, 35(2), 269–286. Coconut Creek (Florida), ISSN 0749-0208.Located on the northern edge of the West Pacific Warm Pool and having a developed economy and modern infrastructure, Guam is well positioned and equipped for obtaining natural records of the west Pacific maritime paleoclimate. This study was a proof of concept to explore whether useful climate proxy records might be obtained from coral at readily accessible, even if geochemically nonoptimal, coastal sites. A 50-year Sr/Ca record (1960–2010) was thus obtained from a shallow-water, near-shore Porites lutea colony at a recreational facility inside Guam's Apra Harbor and compared with local and regional meteorological records, including the El Niño–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) indices. The accessibility of the site enabled documentation of relevant environmental variables for 16 months (September 2009–December 2010): seawater δ18O, pH, seawater cations, and nitrate. Time series of seawater δ18O, pH, and cations show evidence of freshwater input from direct rainfall and stream discharge into the harbor. An anomalously higher mean and variable concentrations of Ba suggest the presence of river-borne, fine-grained terrigenous sediment. Nevertheless, the Sr/Ca time series reproduces a long-term warming trend seen in historical records of local air temperature and regional sea-surface temperature (SST) and closely tracks the ENSO and PDO indices over the entire 50-year record. The consistency of the results with Guam's historical instrumental records, previous coral δ18O results from Guam obtained by others, and previous Sr/Ca proxy results for SST in similar environments elsewhere demonstrate that accessible near-shore sites—where environmental conditions can be monitored—can produce useful Sr/Ca records of local and regional climate phenomena.

Geochemical composition of Tanzanian shelf sediments indicates Holocene climatic and sea-level changes

AbstractWe present a high-resolution geochemical and grain-size record from a Holocene sediment core off the Pangani River mouth, Tanzania. Elemental ratios between biogenic elements and Al (i.e., Ca/Al, Mg/Al, and Sr/Al) are mainly influenced by terrigenous dilution on carbonate concentration and/or limitation of carbonate production as a result of variations in the supply of fine-grained terrigenous sediments of the Pangani River. Such elemental ratios increased significantly at the end of the mid-Holocene between 5 and 3.5 ka, demonstrating a gradual transition from the humid early and mid-Holocene to the arid late Holocene in East Africa. Among the elemental ratios between terrigenous elements and Al, Si/Al and K/Al ratios correlate to grain-size variation, indicating a change in sedimentation regime. Fe/Al and Ti/Al ratios show that the sediment source area has shifted from the terrestrial volcanic region of Tanzania (Fe, Ti rich) to the coastal and inner-shelf regions (Fe, Ti poor) around 7.5 ka, in response to arid climate and high sea level. Our geochemical results correspond with a sea-surface temperature record derived from the same sediment core, indicating that the end of the East African Humid Period could have been gradual and related to the cooling water in the western Indian Ocean.

Sedimentary record of recent climate impacts on an insular coastal lagoon in the Gulf of California

Sedimentary records are useful to evaluate environmental changes, either from natural or anthropogenic causes, such as global and climate change. The recent changes in accumulation rates and geochemical characteristics (grain size distribution, elemental composition, organic carbon and carbonate concentrations) recorded in a sediment core from San Jose Island Lagoon (SJIL, Gulf of California) were evaluated to determine its relationship with anthropogenic impacts and climatic variability. The 210Pb-derived chronology was corroborated with 239+240Pu and 137Cs stratigraphic markers. The mass accumulation rate increased up to ∼3 times during the past ∼100 years (0.16 ± 0.03 to 0.51 ± 0.06 g cm−2 yr−1). The contents of terrigenous and marine (salinity) indicator elements, as well as fine-grained sediments, also increased considerably, although no anthropization evidences were observed; indeed, the enrichment factor of trace elements indicated that the ecosystem is still a pristine environment. By using multivariate statistical techniques, we inferred that the larger input of fine-grained terrigenous sediments could be related to the enhancement of soil erosion from the catchment, under the influence of higher rainfall rates, especially during the last 20 years. In addition, the higher concentrations of salinity indicator elements most likely resulted from higher evaporation rates in the lagoon, caused by higher minimum atmospheric temperatures. We concluded that recent climate variability has become the main driver for sedimentary geochemical changes in San Jose Island Lagoon. These observations confirmed the usefulness of 210Pb-dated geochemical sediment records to study the impacts of recent climate variability where long-term environmental data is scarce or non-existent.

Paleo-environmental change during the late Holocene in the southeastern Yellow Sea, Korea

Origin of fine-grained terrigenous sediments and integrative interpretation of their detailed variations in the southeastern Yellow Sea (YS) since the Last Glacial Maximum (LGM) have not been well understood. We investigated the provenance and paleo-environmental changes of the southeastern YS by examining clay mineral changes from borehole HMB-102 and HMB-103. Down-core variations in these clay mineral compositions suggest a distinct change in the provenance of fine-grained sediments of the Heuksan Mud Belt (HMB). Before 13.32ka, sediments were mostly derived from the Korean rivers, from the paleo-Huanghe during between 13.32 to 11.08ka, and from the Korean rivers with minor contributions from the paleo-Huanghe during 11.08 to 6.77ka. After 6.77ka, when the sea level was high and the shoreline was near the Korean Peninsula, materials may come from the Korean rivers. The provenance change after LGM occurred due to an increase in water depth caused by climate change and evolution of Kuroshio Current (KC).

Extant coccolithophores from the western equatorial Indian Ocean off Tanzania and coccolith distribution in surface sediments

Living coccolithophores and coccoliths from surface sediments were investigated in the western equatorial Indian Ocean off Tanzania, from the coast and Pemba Channel to the open ocean. In total, 49 plankton samples from 8 stations and 20 surface sediment samples collected in February 2008 were analysed. Coccolithophore standing stock ranged from barren up to almost 23,000 coccospheres/l in the upper water column and the living community comprised 47 different taxa in total. In addition, six more species were only found as detached coccoliths. Coccolithophore numbers were generally low and only Florisphaera profunda, Gephyrocapsa oceanica, and Emiliania huxleyi were numerically important (>5,000 coccospheres/l). Two distinct depth-related assemblages were identified. The upper photic zone (down to about 50m) in the Pemba Channel was dominated by G. oceanica and can be related to slightly increased nutrient conditions. In contrast, the upper photic zone community in the open ocean (down to about 70m) is characterised by high numbers of Umbellosphaera irregularis, typically indicating oligotrophic stratified waters, high sea-surface temperatures and strong water stratification. The lower photic-zone flora in the whole study area is generally dominated by F. profunda and accompanying species. A total of 40 species were identified in the surface sediments and total coccolith numbers vary drastically from <1000 to 18,200 x 106 coccoliths/g of sediment. The absolute numbers are much lower near-shore and strikingly enhanced further offshore mainly due to heavy dilution by fine-grained terrigenous sediments. Nevertheless, the occurrence of species in the plankton communities compares well to those of the surface sediment assemblages although plankton samples are from just one season. This points to an excellent preservation potential in this area, which can be explained by high supply of clay-rich sediments and a therefore rapid burial of the coccoliths in shelf and upper slope sediments.

Holocene shelf sedimentation patterns off equatorial East Africa constrained by climatic and sea-level changes

Equatorial East Africa experienced significant variations in paleoclimatic and paleoceanographic conditions during the Holocene. These environmental changes influenced sedimentation patterns on the continental shelf. To date, however, little is known about the sediment source, its transport to, and deposition on, the Tanzanian shelf. This paper presents a new high-resolution Holocene sedimentary record off northeast Tanzania (equatorial East Africa) and provides insights into how sedimentation patterns responded to climatic and oceanographic changes during the Holocene. Based on grain-size distribution patterns and mineral assemblages, three types of shelf sediments were identified: Type I (fine-grained terrigenous sediment) is dominated by clay minerals that originated from continental weathering; Type II (coarse-grained terrigenous sediment) is mainly composed of feldspar and quartz, derived from reworking of pre-existing deposits; and Type III (biogenic marine sediment), with low- and high-magnesium calcite, was produced by marine carbonate-secreting organisms. The high input of Type I sediment during the early Holocene (10–8cal kyr BP) was caused by river mouth bypassing. This supply-dominated regime was controlled by intense river discharge and subsequent resuspension of mud in shelf settings, responding to the humid climate in the hinterland and sea-level rise with low rate off Tanzania. The first occurrence of Type II sediments was around 8cal kyr BP and dominated when sedimentation rates lowered. This accommodation-dominated regime was caused by shoreface bypassing due to an arid climate and sea-level highstand. Type III sediments increased significantly from the early to late Holocene, resulting from the weakening dilution effect of the terrigenous component. The sedimentation pattern on the Tanzanian shelf shifted from allochthonous to autochthonous sedimentation constrained by climatic changes and relative sea-level fluctuations at the end of the early Holocene. Our results also suggest that, with respect to sea-level rise, high sedimentation rate on the continental shelf could be caused by high terrigenous sediment input as a result of a dynamic interplay between the shelf topography and humid climatic conditions inland.

Historic impact of watershed change and sedimentation to reefs along west-central Guam

Using coral growth parameters (extension, density, calcification rates, and luminescence) and geochemical measurements (barium to calcium rations; Ba/Ca) from coral cores collected in west-central Guam, we provide a historic perspective on sediment input to coral reefs adjacent to the Piti-Asan watershed. The months of August through December are dominated by increased coral Ba/Ca values, corresponding to the rainy season. With river water enriched in barium related to nearshore seawater, coral Ba/Ca ratios are presented as a proxy for input of fine-grained terrigenous sediment to the nearshore environment. The century-long Ba/Ca coral record indicates that the Asan fore reef is within the zone of impact from discharged sediments transported from the Piti-Asan watershed and has experienced increased terrestrial sedimentation since the 1940s. This abrupt shift in sedimentation occurred at the same time as both the sudden denudation of the landscape by military ordinance and the immediate subsequent development of the Asan area through the end of the war, from 1944 through 1945. In response to rapid input of sediment, as determined from coral Ba/Ca values, coral growth rates were reduced for almost two decades, while calcification rates recovered much more quickly. Furthermore, coral luminescence is decoupled from the Ba/Ca record, which is consistent with degradation of soil organic matter through disturbance by forest fires, suggesting a potential index of fire history and degradation of soil organic matter. These patterns were not seen in the cores from nearby reefs associated with watersheds that have not undergone the same degree of landscape denudation. Taken together, these records provide a valuable tool for understanding the compounding effects of land-use change on coral reef health.

Crystal-chemical microheterogeneity of Precambrian globular dioctahedral mica minerals

The paper presents a novel method for determining the crystal-chemical heterogeneity of finely dispersed dioctahedral 2: 1 mica mineral phases based on the modeling of their powder diffraction patterns. We used three samples, which were taken from Lower-Middle Riphean rock sections of the Anabar and Olenek uplifts in northern Siberia, and one sample from the Upper Riphean Inzer Formation in the southern Urals. Choice of globular samples was determined by contrast features of their chemical composition and different lithological types of the host terrigenous rocks that are commonly transformed at the level of deep catagenesis.Based on structural formulas, unit cell parameters, and coordinates of atoms occupying the cells, we modeled powder diffractograms that made it possible to determine the sizes of coherent scattering domains and probability parameters, which characterize the type, content, and distribution of stacking faults in each of the studied samples. The modeling results demonstrated that each sample represents a physical mixture of individual micaceous phases of different compositions. The paper discusses scales and modes of the crystalchemical heterogeneity of micaceous varieties, which make up globules in the coarse- and fine-grained terrigenous sediments, as well as their various specific crystal-chemical characteristics. The probable physicochemical settings, which were responsible for specific features of the structural and crystal-chemical heterogeneity of micaceous varieties in each of the studied four samples, are also discussed.

Are Coral Clasts from a Turbid Near-Shore Reef Environment a Suitable Material for Radiocarbon Analysis?

Use of coral skeletons to determine growth histories of reefs situated in warm, clear tropical waters is well established. Recently, however, there has been increasing awareness of the significance of reefs occurring in environments that are considered as marginal for coral growth, such as turbid inshore settings characterized by episodes of elevated turbidity, low light penetration, and periodic sediment burial. While these conditions are generally considered as limiting for coral growth, coral reefs in these settings can exhibit high live coral cover and species diversity, and thus can be both ecologically and geologically significant. Turbid-zone reefs are also commonly concentrated along eroding shorelines with many analogues to erosional shorelines developed during the Holocene transgression. A growing number of studies of these previously undocumented reefs reveal that the reef deposits are detrital in nature, comprising a framework dominated by reef rubble and coral clasts and set within a fine-grained terrigenous sediment matrix. In addition to the recognized effects of diagenesis or algal encrustations on the radiocarbon signature of coral samples, episodic high-energy events may rework sediments and can result in age reversals in the same stratigraphic unit. As in other reef settings, the possibility of such reworking can complicate the reconstruction of turbid-zone reef growth chronologies. In order to test the accuracy of dating coral clasts for developing growth histories of these reef deposits, 5 replicate samples from 5 separate coral clasts were taken from 2 sedimentary units in a core collected from Paluma Shoals, an inshore turbid-zone reef located in Halifax Bay, central Great Barrier Reef, Australia. Results show that where care is taken to screen the clasts for skeletal preservation, primary mineralogical structures, and δ13C values indicative of marine carbonate, then reliable 14C dates can be recovered from individual turbid reef coral samples. In addition, the results show that these individual clasts were deposited coevally.

Are Coral Clasts from a Turbid Near-Shore Reef Environment a Suitable Material for Radiocarbon Analysis?

Use of coral skeletons to determine growth histories of reefs situated in warm, clear tropical waters is well established. Recently, however, there has been increasing awareness of the significance of reefs occurring in environments that are considered as marginal for coral growth, such as turbid inshore settings characterized by episodes of elevated turbidity, low light penetration, and periodic sediment burial. While these conditions are generally considered as limiting for coral growth, coral reefs in these settings can exhibit high live coral cover and species diversity, and thus can be both ecologically and geologically significant. Turbid-zone reefs are also commonly concentrated along eroding shorelines with many analogues to erosional shorelines developed during the Holocene transgression. A growing number of studies of these previously undocumented reefs reveal that the reef deposits are detrital in nature, comprising a framework dominated by reef rubble and coral clasts and set within a fine-grained terrigenous sediment matrix. In addition to the recognized effects of diagenesis or algal encrustations on the radiocarbon signature of coral samples, episodic high-energy events may rework sediments and can result in age reversals in the same stratigraphic unit. As in other reef settings, the possibility of such reworking can complicate the reconstruction of turbid-zone reef growth chronologies. In order to test the accuracy of dating coral clasts for developing growth histories of these reef deposits, 5 replicate samples from 5 separate coral clasts were taken from 2 sedimentary units in a core collected from Paluma Shoals, an inshore turbid-zone reef located in Halifax Bay, central Great Barrier Reef, Australia. Results show that where care is taken to screen the clasts for skeletal preservation, primary mineralogical structures, and d13C values indicative of marine carbonate, then reliable 14C dates can be recovered from individual turbid reef coral samples. In addition, the results show that these individual clasts were deposited coevally. DOI: 10.2458/azu_js_rc.55.16349

Inverse Relationship of Marine Aerosol and Dust in Antarctic Ice with Fine-Grained Sediment in the South Atlantic Ocean: Implications for Sea-Ice Coverage and Wind Strength

This research seeks to test the hypothesis that natural gamma radiation (NGR) from Ocean Drilling Program Site 1094, which displays variability over the last glacial-interglacial cycle similar to dust in the Vostok ice core, reflects fine-grained terrigenous sediment delivered by eolian processes. Grain size was measured on 400 samples spanning 0–20 m in a composite core. Accumulation of the <63μ size fraction at Site 1094 and dust in Vostok exhibit a negative correlation, suggesting the fine sediments are not dominantly eolian. However the technique used for grain size measurements cannot distinguish between terrigenous and biogenous materials; therefore it is possible much fine-grained material is diatoms. An inverse correlation between fine sediments and NGR supports this interpretation, and implies terrigenous materials were at times diluted by microfossils from high biological productivity. Fine marine sediments correlate positively with temperature and negatively with marine aerosol Na+ in Vostok. One plausible explanation is extensive sea-ice of cold intervals steepened ocean-continent temperature gradients, intensified winds, and led to increased transport of dust and marine aerosol to Antarctica yet also reduced biological productivity at Site 1094. Such a reduction despite increases in NGR, potentially representing Fe-rich dust influx, would require light limitation or stratification associated with sea-ice.

Stages of Late Proterozoic magmatism and periods of Au mineralization in the Yenisei Ridge

Geological, isotopic, and geochemical data permitted distinguishing the Mesoproterozoic (1.6–1.05 Ga), Early Neoproterozoic (1.05–0.8 Ga), and Late Neoproterozoic (0.8–0.6 Ga) stages of magmatism and crustal evolution in the Yenisei Ridge. Each of them contributed to the regional Au metallogeny. In the Early Mesoproterozoic, crustal destruction and stretching in the southwestern Siberian craton (Yenisei Ridge) led to the initiation of a pericratonic trough, the formation of rift mafic associations (Rybnaya–Panimba volcanic belt), and the accumulation of fine-grained terrigenous sediments (Sukhoi Pit Group). Black carbonaceous shales and the picrite basalt–basalt association were enriched in Au. In the early Neoproterozoic, the terrigenous strata of the Sukhoi Pit Group were deformed, metamorphosed, and granitized as a result of the Grenville orogeny. Granite-gneiss domes formed in the earlier, syncollisional, period (1.05–0.95 Ga) of this stage, and K–Na granitoid plutons formed in the late collisional one (0.88–0.86 Ga). Premineral metasomatites formed in the zone where these plutons influenced enclosing black shale–terrigenous strata. They host Au deposits, which formed later. The formation of quartz-reef zones correlates with that of thrust nappes (0.85–0.82 Ga). The latter is genetically related to the final stage of the evolution of a collisional orogen. In the Late Neoproterozoic, rift and intraplate magmatism was most intense and frequent (780, 750, 700, 670–650 Ma) in the Tatarka–Ishimba fault system. Manifestations of Mesoproterozoic volcanism and all the Au deposits of the Central metallogenic belt in the Yenisei Ridge are concentrated here. The three periods of gold–arsenopyrite–quartz, gold–sulfide, and Au–Sb mineralization correlate well with the initiation and evolution of rift structures and the manifestations of intraplate magmatism at 800–770, 720–700, and 670–650 Ma. The tectonomagmatic processes which took place in these periods might have been crucial in Au ore accumulation.

1200 year paleoecological record of coral community development from the terrigenous inner shelf of the Great Barrier Reef

Increased terrestrial sediment and nutrient yields are regarded as significant threats to coral reef health. Within the central Great Barrier Reef lagoon, where water quality has reportedly declined since European settlement (since ca. A.D. 1850), inner-shelf reef conditions have purportedly deteriorated. However, the link between reef decline and water-quality change remains controversial, primarily because of a lack of pre-European period ecological baseline data against which to assess contemporary ecological states. Here we present a high-resolution record of reef accretion and coral community composition from a turbid-zone, nearshore reef on the inner shelf of the Great Barrier Reef; the record is based on six radiocarbon date–constrained cores, and extends back to ca. 1200 calibrated yr B.P. Results demonstrate not only the potential for coral communities to initiate and persist in settings dominated by fine-grained terrigenous sediment accumulation, but also that a temporally persistent (but low diversity) suite of corals has dominated the reef-building community at this site for at least the past millennium. Furthermore, the coral assemblages exhibit no evidence of community shifts attributable to post-European water-quality changes. While extrapolation of these findings to other turbid-zone reefs must remain tentative, the study raises important questions about the resilience of inner-shelf reefs that are under terrestrial sediment influence and subject to elevated turbidity conditions, and demonstrates the potential to develop detailed, millennial time scale, coral community records from Holocene reef systems.

28th DeBeers Alex. Du Toit Memorial Lecture, 2004. On Cryogenian (Neoproterozoic) ice-sheet dynamics and the limitations of the glacial sedimentary record

28th DeBeers Alex. Du Toit Memorial Lecture, 2004. On Cryogenian (Neoproterozoic) ice-sheet dynamics and the limitations of the glacial sedimentary record