查看更多>>摘要：This study compares the geochemistry of ikaite (CaCO3 center dot 6H(2)O) with that of nearby (< 500 m in distance) recovered methane-derived authigenic carbonates (MDACs) of similar to 10 mol% Mg-calcite. The carbon isotopic composition (delta C-13) of MDACs (lower than -49 parts per thousand Vienna Pee Dee Belemnite or VPDB) suggests a more prominent influence of anaerobic oxidation of methane (AOM) than that of ikaite (approximately -35 parts per thousand VPDB). The higher delta C-13 value of ikaite is attributed to its growth at sediment depths closer to the seafloor, where the proportion of seawater HCO3- relative to methane-derived HCO3- was higher. Assuming that seawater and AOM are the major sources of HCO3- for ikaite growth, the actual C-14 age was estimated to be similar to 14 cal ky before present, close to the age of the sediments observed around the ikaite. For alkalinity increase at shallower sediment depths by the AOM, a higher methane flux is necessary. However, attributing the ikaite growth at shallower sediment depths to higher methane flux alone posed an issue, as no traces of gas hydrate growth were found in the laminated sediments. The oxygen isotopic composition (delta O-18) of the freeze-dried ikaite (ikaite-CO3) was -2-3%. VPDB, which is lower than the corresponding value of similar to 4.0 parts per thousand VPDB for the Mg-calibrated MDACs. It is characteristic of ikaite-CO3 to exhibit such low 6 18 0 values, indicating a unique oxygen isotope fractionation factor for ikaite. The preferential (H2O)-O-18 uptake of ikaite for growth also influenced the delta O-18 value of ikaite-CO3. Hence, owing to (H2O)-O-18 depletion in the ambient interstitial water, the delta O-18 value of the ikaite-CO3 tended to decrease from the interior to the margin. The delta O-18 values of the ikaite-CO3 were archived in yellowish calcite grains that were formed in the pores after patchy dissociation of the ikaite. The delta O-18 values of the grains were close to those of ikaite-CO3. Therefore, the delta O-18-temperature equation for ikaite is necessary for geochemical interpretation of the calcite delta O-18 values.
Casanova-Arenillas, SantiagoRodriguez-Tovar, Francisco J.Martinez-Ruiz, Francisca
15页
查看更多>>摘要：Organic rich layers (ORLs), deposited in the westernmost Mediterranean over the Last Glacial Cycle, have been studied integrating sedimentological and ichnological information from sediment records recovered at Ocean Drilling Program Leg 161 Sites 976 and 977. The conducted study has served to redefine the ORLs initially differentiated in sediment records from Sites 977 and 976, with seven new ORLs recognized, as well as extensions and subtraction of some parts in the ORLs previously defined. In addition, three different ORL types are distinguished according to sedimentological and ichnological features. Type 1 with intermediate thickness is highly bioturbated, mainly consisting of abundant Scolicia and Planolites, frequent Chondrites, and possible Thalassinoides in the base, which are interpreted as supporting dysoxic conditions during deposition. Type 2, corresponding to thicker intervals, shows alternance of highly to moderately bioturbated parts, revealing variable oxygen conditions -from moderate to extreme dysoxic or even suboxic. Type 3 consists of short, laminated intervals characterized by the absence of bioturbation, indicating anoxic or suboxic conditions and determining unfavourable macro benthic habitat. The amended ORLs show a correlation between the Sea Surface Temperatures and established climatic events over the Last Glacial Cycle in the Western Mediterranean.
King, Maxine, VGales, Jenny A.Laberg, Jan SverreMcKay, Robert M....
20页
查看更多>>摘要：The repeated proximity of West Antarctic Ice Sheet (WAIS) ice to the eastern Ross Sea continental shelf break during past ice age cycles has been inferred to directly influence sedimentary processes occurring on the continental slope, such as turbidity current and debris flow activity; thus, the records of these processes can be used to study the past history of the WAIS. Ross Sea slope sediments may additionally provide an archive on the history and interplay of density-driven or geostrophic oceanic bottom currents with ice-sheet-driven depositional mechanisms. We investigate the upper 121 m of Hole U1525A, collected during International Ocean Discovery Program (IODP) Expedition 374 in 2018. Hole U1525A is located on the southwestern external levee of the Hillary Canyon (Ross Sea, Antarctica) and the depositional lobe of the nearby trough-mouth fan. Using core descriptions, grain size analysis, and physical properties datasets, we develop a lithofacies scheme that allows construction of a detailed depositional model and environmental history of past ice sheet-ocean interactions at the eastern Ross Sea continental shelf break/slope since similar to 2.4 Ma. The earliest Pleistocene interval (similar to 2.4- similar to 1.4 Ma) represents a hemipelagic environment dominated by ice-rafting and reworking/deposition by relatively persistent bottom current activity. Finely interlaminated silty muds with ice-rafted debris (IRD) layers are interpreted as contourites. Between similar to 1.4 and similar to 0.8 Ma, geostrophic bottom current activity was weaker and turbiditic processes more common, likely related to the increased proximity of grounded ice at the shelf edge. Silty, normally-graded laminations with sharp bases may be the result of flow-stripped turbidity currents overbanking the canyon levee during periods when ice was grounded at or proximal to the shelf edge. A sandy, IRD- and foraminifera-bearing interval dated to similar to 1.18 Ma potentially reflects warmer oceanographic conditions and a period of stronger Antarctic Slope Current flow. This may have enhanced upwelling of warm Circumpolar Deep Water onto the shelf, leading to large-scale glacial retreat at that time. The thickest interval of turbidite interlamination was deposited after similar to 1 Ma, following the onset of the Mid-Pleistocene Transition, interpreted as a time when most ice sheets grew and glacial periods were longer and more extreme. Sedimentation after similar to 0.8 Ma was dominated by glacigenic debris flow deposition, as the trough mouth fan that dominates the eastern Ross Sea continental slope prograded and expanded over the site. These findings will help to improve estimations of WAIS ice extent in future Ross Sea shelf-based modelling studies, and provide a basis for more detailed analysis of the inception and growth of the WAIS under distinct oceanographic conditions.
查看更多>>摘要：The deep-water sedimentary processes and morphological features offshore Madeira Island, located in the Central-NE Atlantic have been scantly studied. The analysis of new multibeam bathymetry, echo-sounder profiles and few multichannel seismic reflection profiles allowed us to identify the main geomorphologies, geomorphic processes and their interplay. Several types of features were identified below 3800 m water depth, shaped mainly by i) the interplay between northward-flowing Antarctic Bottom Water (AABW) and turbidity currents and ii) interaction of the AABW with oceanic reliefs and the Madeira lower slope. Subordinate and localized geomorphic processes consist of tectono-magmatic, slope instability, turbidity currents and fluid migration. The distribution of the morphological features defines three regional geomorphological sectors. Sector 1 represents a deep-seafloor with its abyssal hills, basement highs and seamounts inherited from Early Cretaceous seafloor spreading. Sector 2 is exclusively shaped by turbidity current flows that formed channels and associated levees. Sector 3 presents a more complex morphology dominated by widespread depositional and erosional features formed by AABW circulation, and localized mixed contourite system developed by the interplay between the AABW circulation and WNW-ESE-flowing turbidite currents. The interaction of the AABW with abyssal hills, seamounts and basement ridges leads to the formation of several types of contourites: patch drifts, double-crest mounded bodies, and elongated, mounded and separated drifts. The patch drifts formed downstream of abyssal hills defining an previously unknown field of relatively small contourites. We suggest they may be a result of localized vortexes that formed when the AABW's flow impinges these oceanic reliefs producingthe erosional scours that bound these features. The bottom currents in the area are known to be too weak (1-2 cm s(-1)) to produce the patch drifts and scours. Therefore, we suggest that these features could be relics at present, having developed when the AABW was stronger than today, as during glacial/end of glacial stages.
查看更多>>摘要：As an incipient back-are basin, the mineralization characteristic in the Okinawa Trough (OT) is quite unique. Many studies have been conducted in OT hydrothermal deposits, however the mineralization condition and metal source differences between the middle (MOT) and southern Okinawa Trough (SOT) are still unclear. The rare earth elements (REEs) and Sr, S isotope compositions of the OT hydrothermal deposits were determined in this paper in order to solve these problems. The Eu anomalies of the hydrothermal sulfide samples show large variation, with delta Eu values range from 0.62 to 21.38, which reflects the differences in the redox condition of their parental hydrothermal fluids. The Sr-87/Sr-86 ratios of the hydrothermal deposit samples are abnormally high, indicating the fluid/sediment interaction during the fluid circulation. The delta S-34 values of sulfide samples from the MOT fall between those of basement volcanic rocks and seawater, which can be attributed to the mixing of volcanic and seawater-derived sulfur. While samples from the SOT have abnormally low delta S-34 values, remobilization of biogenic sulfur in sediments is the major cause of the sulfur isotope anomaly. Both of S and Pb isotopes indicate that the proportions of sediment component in the SOT hydrothermal deposits are higher than those of the MOT. Whereas the Sr isotope compositions of the hydrothermal deposit samples show no systematic difference between the MOT and SOT. We propose that the decoupling of Sr and S, Pb isotopes may be caused by the fluid mobility differences between these three elements during the fluid/sediment interaction.
查看更多>>摘要：The Holocene hydro-ecological and environmental change of the Nile Delta remains a knowledge gap. The study aimed to approach this objective by using multi-proxies of lipid biomarkers and bulk organic carbon (OC) in a well-dated MZ-1 core in Manzala lagoon off the Nile Delta. The data defined five distinctive stages at decadal-centennial scale. Stage I (ca. 8.2-7.7 ka) was characterized by remarkedly high reconstructed pH and low mean annual air temperature (the 8.2-ka cold event). The obviously low TOC and C/N, but higher short-chain n-alkanes indicated more marine OC deposition. This was followed by highly fluctuated organic geochemical proxies at Stage II (ca. 7.7-5.8 ka) indicating episodic but significant terrestrial OC input delivered by Nile floods during the African Humid Period (AHP, 11.0-5.4 BP). Subsequent increase in short- and medium-chain n-alkanes suggested macmphytes prevailing in the lagoon during Stage III (ca. 5.8-5.4 ka) with cooling-drying climate transitioned from the AHP. Stage IV (ca. 5.4-1.2 ka) witnessed decreased short-chain n-alkanes of marine origin and increased long-chain n-alkanes of terrestrial origin when the Nile climate entered into the mega-trend of aridification. Abnormal pulses of terrestrial OC with high TOC likely reflected the modified landscape due to early human occupation around the area. The highly fluctuated charcoal fluxes since ca. 6 ka implied such clearing-firing-aided land exploitation activity. The lagoon eco-setting was further affected by intensified anthropogenic-OC input after the latest millennium. Our study highlighted the land-ocean interactions on modulating the paleo-lagoonal ecology of the Nile Delta, which should be incorporated into healthy management in the Nile Delta.
Ferreira, Andressa L.Vital, HeleniceStattegger, KarlCaldas, Luciano H. O....
10页
查看更多>>摘要：This study aims to reconstruct the Holocene evolution of a semiarid tidal channel system in response to the deglacial sea-level rise and modern coastal processes. With a maximum water depth of approximately 10 m, this very shallow system is in the easternmost part of the Brazilian equatorial margin along a tide-dominated coast in a semidiurnal, mesotidal regime. The channel system is interconnected with the Atlantic Ocean through a tidal inlet formed by the Galinhos spit. This work comprised ship and land-based fieldwork with sedimentological and stratigraphic approaches. Four seismic units (U1-U4) and seismic horizons (H1-H4) were identified by reconstructing the stratigraphic architecture. The results show that: (1) the formation of paleochannels in the current spit may have started before the middle Holocene; (2) there was a network of pre-existing channels with a different configuration filled by units U2 and U3, constituting the transgressive and highstand systems tracts; (3) spit migration is the key factor to open and close inlets; (4) internal configuration patterns show a less energy and monotonous depositional environment above H4, within unit U4 (which started at similar to 5000 cal yrs. BP); (5) the current system configuration has been active since the formation of the U3 unit since the closure of the paleochannels; and (6) the Galinhos region shows progressive sedimentary infilling of a lagoon behind a coastal spit via sediment accumulation from marine sources and its ultimate transformation into a coastal swamp with tidal channels.
Armstrong, ChristianHowe, John A.Allen, ChristopherWatson, Peter...
16页
查看更多>>摘要：The Pentland Firth between mainland Scotland and Orkney is an area of the UK continental shelf that experiences extreme tidal flows (>3 m/s). This study presents a time series of bathymetric data coupled with a tidal flow model to examine hydrodynamics and bedform response at the eastern approaches to the Pentland Firth. These observations were additionally augmented by sediment grain type and ADCP data to validate the numerical flow model. Tidal flows of the Pentland Firth result from a tidal phase difference between the east and west approaches to the channel. A resulting barotropic pressure gradient leads to flow accelerations that locally exceed 5 m/s. The extreme tidal setting of the Pentland Firth's eastern approach was found to promote distinct bedforms that are spatially varied in geomorphology with distance from the Pentland Firth. Sediment analysis also showed a decreasing grain size trend also with distance from the Pentland Firth. The modelled residual tidal current shows strong agreement with the sediment transport pathways, supported by the bedform migration direction. The energetic tidal flows of the Pentland Firth's eastern approach interacts with the highly irregular coastline, generating residual tidal counter currents leeward of flow obstructions. These counter currents (i.e. residual tidal current vortices) were reflected in bedform migrations and thence the sediment transport pathways. As the Pentland Firth is considered as a bedload parting site, the residual tidal current vortices are expected to influence the rate of erosion at the bedload parting site, by recirculating sediment back upstream as a counter current inshore of the main flow. The modification of sediment transport pathways by residual tidal current vortices may affect the development of nearshore and offshore engineering, and should be considered in any initial site assessment.
Wang, ShuoGe, JianzhongMeadows, Michael E.Wang, Zhanghua...
15页
查看更多>>摘要：Using geological evidence to extend the history of typhoon activity beyond the instrumental period and understand the possible linkage between future climate change and increasing storm intensity is of great significance in preparing for coastal risks in a warming world. In this study, an extreme storm event that occurred on the southern Yangtze coast, East China, during the latter stage of Neolithic Liangzhu Culture were estimated based on sedimentary records and numerical modeling. We first investigated the spatial distribution of peat layers formed during the time of the Liangzhu Culture to produce a high-resolution map of the paleo-coastline of the Ningbo Plain. Inverse modeling using the grain size of storm deposits recovered at the end of Liangzhu Culture at the Neolithic Yushan site was applied to calculate the instantaneous water level including wave run-up and storm surge caused by the extreme event. Numerical simulations of multiple typhoon scenarios combined with precipitation and sea-level rise constrained by the paleo-coastline and paleo-topography were then used to estimate the storm surge and inundation on the coast of Hangzhou Bay. Through comparison of results between the inverse model and numerical simulations, we infer that the intensity of the storm recorded at the Yushan site was likely greater than any typhoon in the local instrumental records and that the storm deposits were formed by a Super Typhoon, which moved close to or made landfall directly on the southeast coast of Hangzhou Bay. Our modeling further suggests that the extreme storm event was coupled with an abrupt relative sea-level rise at ca. 4.5 cal kyr BP, which indicates a warmer west Pacific Ocean at that time.
查看更多>>摘要：A multi-proxy analyses was applied on the sediment core from the Gulf of Saros (GoS) to identify and characterize climate and hydrological changes during the middle-to-late Holocene. The formation of two discrete Holocene sapropel layers in the GoS sediments was documented for the first time in the sediment core based on total organic carbon analysis. According to our paleo-proxy records, the lower Holocene sapropel was deposited under warm and humid climate conditions that gave rise to high delivery of terrestrial organic matter by numerous rivers in the northern catchment of the GoS. Biomarker and mu-XRF data were used to decipher climate variations during the middle to late Holocene. The general trends of sea-surface temperature records from the GoS and Sea of Marmara (SoM) at the beginning of late Holocene are in good agreement, underlying the influence of the Black Sea inflow. A relatively warm and wet climate together with a high sedimentation rate during the mid-Holocene Climatic Optimum resulted in high organic productivity and ensuing formation of the younger Holocene sapropel between 5.4 and 3.0 cal ka BP. Late Holocene European climate periods are evident in the Saros core records. The Roman Humid Period is represented by high variation in climate, indicating an earlier (2.5-2.3 cal ka BP) dry and a later (2.3-1.55 cal ka BP) wet periods. The abrupt return to drier condition during the Dark Ages Cold Period (1.6-1.3 cal ka BP) was followed by a wetter Medieval Climate Anomaly (1.1-0.7 cal ka BP). The paleo-proxy record of the core indicates a passage from a wetter to drier climate during the cold Little Ice Age period (730-110 cal yr BP), and highlights the influence of deforestation in the catchment of the GoS as a result of human activities during the last three centuries.
NSTL
Elsevier