查看更多>>摘要：The pelagic sediments from Integrated Ocean Drilling Program (IODP) Site U1407 in the western North Atlantic are characterized by a sequence of organic-lean and organic-rich sediments representing a major global episode of organic matter (OM) burial during Oceanic Anoxic Event 2 (OAE2). The OM-rich deposits (up to 16% total organic carbon) consist primarily of radiolarians which are rare/absent in the organic-lean sediments. New nannofossil biostratigraphy and the correlation with the global delta C-13 curve were used to constrain the timing of paleoceanographic changes below, within, and above OAE2 sediments. Data were combined from elemental concentrations, biomarkers, and abundances of benthic foraminifera, biserial (heterohelicids) planktic foraminifera, and radiolarians to reconstruct the evolution of these changes. The delta C-13 curve records a global negative excursion within organic-lean sediments not typically recorded in other sections preceding the initial rise in isotopic values that signals OM burial worldwide. Maximum OM accumulation occurred during the second positive isotopic excursion, which is associated with higher terrigenous input (indicated by long-chain n-alkanes and terrestrially derived elements) and water column stratification (suggested by the paleoindicator gamma-cerane). In addition, sesquiterpene biomarkers derived from cypress and other conifers suggest flooding of gymnosperm forests from increased rainfall coupled with rising sea levels and resulting higher water tables. Within mid-OAE2 sediments, a sharp increase in radiolarians and fish teeth/debris and a decrease in benthic foraminifera implies deposition of OM from increased productivity/upwelling under low oxygen in the lower part of the water column. Upwelling and water column stratification occurring asynchronously at the surface resulted from changes in precipitation, with stratification developing during more humid/wet conditions. Greenhouse warming during OAE2 likely increased precipitation, weathering and nutrient inputs. However, prolonged anoxia caused buried phosphorus to be recycled as indicated by higher redox-sensitive trace element and biolimiting element concentrations, sustaining primary production and further enriching the sediments in OM. We suggest that increased marine paleoproductivity was the primary control on the OM-rich sediment accumulation during OAE2, while low-oxygen conditions resulting from stratification of surface waters were essential to its preservation.
查看更多>>摘要：Refined records of Holocene sea level and coastal evolution are needed to forecast changes in the Anthropocene. Beach ridge strandplains and other prograded barriers preserve an exceptional (and relatively untapped) archive of past sea level within their stratigraphy. Extracting these records using high-resolution geophysics can unearth sea-level indicators with the potential to resolve small fluctuations over large spatial- and temporal-scales. Brooke et al. (2019) construct a sea-level curve from Cowley strandplain and performs interesting modelling/analyses; but does so based on the beach ridge topography. Unfortunately, this methodological approach contradicts a long-held understanding that it is not appropriate to use ridge morphology or sediment analysis from cores/augers/pits to indicate sea level. This sets a misleading precedent if perpetuated in future research and does so at a critical time when extracting sea-level reconstructions from prograded barriers is becoming more established. While the Brooke et al. (2019) curve appears to correlate with data from this region, it is not precisely clear how it was constructed. Particularly perplexing is the most recent half of the curve, starting with the peculiar location of the modern analogue below mean sea level. Simply adjusting the existing curve to zero on the y-axis removes it from the tenuous correlation. This comment paper uses the geophysical and topographic data published in Brooke et al. (2019), as well as the protocol referenced, to construct relative sea-level curves. The results accentuate the difference between the reconstructions from the berm stratigraphy and the ridge morphology. Furthermore, despite the nature of this analysis utilizing a derivative dataset, the berm heights demonstrate remarkable correlation with the modelled sea-level curve for the region as well as the more traditional sea-level indicators. Ultimately, this glimpse provides insight on the possible extent of information that lies buried not just at Cowley and the 30+ prograded barriers around Australia, but at 300+ sites worldwide.
查看更多>>摘要：In this study, we investigated sedimentation differences between two distinctive cold seasons, in terms of hydmmeteorological and hydrodynamic conditions, in a coastal area of the Northern Baltic Sea in 2018-2020. A combination of sediment trap data, hydmmeteorological data and hydrodynamic modelling provided a unique set-up to discover differences in sedimentation rates and compositions. Our study shows that the averaged sedimentation accumulation rate (SAR) was nearly three times higher during warmer cold season (30.9 g m(-2) day(-1)), characterised by higher precipitation, especially rain and discharge, as well as snowless and open water conditions, compared to regular cold season (10.6 g m(-2) day(-1)). While sedimentation was higher during the warmer season, the mean sediment grain size (D-50) was higher during the regular cold season with permanent snow and ice cover. Similarly, while sediments of the regular cold season were organically rich, the total amount of organic matter accumulation was larger during the warmer cold season. Sediments consisted mostly of elastic matter (85-89%), of which the mean grain size varied from clay to fine silt (0.3-3.0 mu m). Sedimentation differences between the cold seasons can be explained by differences in precipitation, river flow, wind-induced resuspension and a low air pressure system forcing sea level changes. Sedimentation differences along the study bay were found to be connected to channel cross-sectional area and flow conditions caused by river input and sea level changes.
Li, ZhanhaiJia, JianjunWang, Ya PingZhang, Guoan...
12页
查看更多>>摘要：In many estuaries, current velocity, suspended sediment concentration (SSC), and water depth have notable flood-ebb asymmetries in magnitude or duration. Such asymmetries can make marked impact on estuarine sediment transport and require in-depth analysis of the underlying mechanism. Based on field measurement over a neap-spring cycle in the Changjiang Estuary in January 2015, net suspended sediment transport modulated by multiple asymmetries were analyzed. The results showed that there were notable flood-ebb asymmetries in the current velocity, current duration, SSC, and water depth. The former two were ebb-dominated, while the latter two were flood-dominated. The net suspended sediment transport was landward and exhibited remarkable tidal fluctuation in magnitude. Ebb-dominated velocity and duration asymmetries favored seaward transport, but flood-dominated SSC and water depth asymmetries promoted landward transport. The relative contribution of these asymmetries to the net sediment transport was different and closely related to their asymmetry intensities. The magnitude of net sediment transport was highly modulated by the combined effects of these asymmetries. Further analyses revealed that the ebb-dominated asymmetries in velocity and duration were controlled by river flow, tidal wave deformation, and compensation flow of Stokes drift. The flood-dominated water depth asymmetry was related to the progressive tidal wave. The flood-dominated SSC asymmetry was mainly attributed to the lower SSCs in early ebbs, notable sediment supply limitation in late ebbs, wind wave height asymmetry, and gravitational circulation. Additionally, it is worth noting that the progressive tidal wave can yield strong current asymmetry, notable water depth asymmetry, and obvious wind wave height asymmetry during strong wind periods.
查看更多>>摘要：Coastal currents play a key role in regulating alongshore sediment transport, and their relationships with winter storms (burst of the East Asian winter monsoon) and formation of the coastal mud deposits on the eastern China shelf have been established, based on which the evolution of the East Asian winter monsoon has been widely explored. Unlike that of coastal deposits, the formation of offshore deposits on the eastern China shelf is very complex and highly debated, leading to enormous challenges on paleoclimate reconstructions based on these deposits. In this study, cross-front sediment transport under a variable Yellow Sea Warm Current (YSWC) was explored using remote sensing imagery and sedimentary records (seismic profile and sediment grain size) from the North Yellow Sea (NYS). The results indicate that, although the offshore mud deposit in the western NYS is formed by winter storms through triggering cross-front transport of coastal sediment around the Shandong Peninsula, the winter storm signal was completely obscured by that of the YSWC, which might determine the flux of cross-front sediment transport on a millennial scale. The 1500 y sub-orbital climate variability cycle could also be observed in distal muds after 2.8 ka. By comparing the sediment composition within the same mud deposit or between adjacent mud deposits in the NYS, we found that the response of cross-front sediment transport to the YSWC varied spatially, which may be a reason for discrepancies observed in previous winter monsoon reconstructions on the eastern China shelf. Therefore, as indicated by this study, offshore deposits on the eastern China shelf are not suitable for inversions of the winter monsoon. In addition, mud deposits on the eastern China shelf are also dramatically affected by other dynamic factors, including typhoons and river flooding; thus, deeply exploring the dynamic characteristics and formation mechanism of these deposits are prerequisites for paleoclimate reconstruction.
Beaubouef, Chelsea E.Liu, Kam-biuCulligan, NicholasRiedlinger, Lance...
9页
查看更多>>摘要：This study tests the ability of a novel approach to identifying washover beds in coastal lakes. Combined X-Ray Fluorescence (XRF) and cluster analysis was used to identify hurricane washover beds in sediment cores from Clam Lake on the McFaddin National Wildlife Refuge in southeastern Texas. The lake is known to contain washover beds from recent hurricanes, but the washover sediment has similar microfossil, loss-on-ignition and textural characteristics to non-washover sediment and is not readily distinguishable. Sediment cores taken from marshes surrounding the lake do contain visually-recognizable sandy washover beds of Hurricanes Ike, Rita, Carla and Audrey. XRF analysis of these washover beds, combined with cluster analysis, was used to construct "elemental fingerprints" with the potential to detect washover beds in the lake. Results are promising: multiple washover beds were detected in the lake and tentatively attributed to recent hurricanes. In some lake cores, washover beds likely to be present were not detected by the XRF/clustering technique; in other lake cores, up to nine washover beds were detected. The variation in the number of washover beds probably resulted from bio-turbation, identification of two or more washover beds in a single washover deposit, and washover beds resulting from smaller storms. Valuable outcomes of this study are; 1) it confirms the presence of washover beds in the lake; 2) it provides greater insight into the number, stratigraphic position and thickness of washover deposits; 3) it identifies periods of heightened and diminished overwash activity, and 4) it provides a means of estimating the contribution of washover deposition to sedimentation in the lake. An additional unexpected finding is that long-term sedimentation rates derived from the lake and marsh cores closely match the rate of local sea-level rise, suggesting that sea-level rise may drive sedimentation in the study area.
NSTL
Elsevier