Ocean Margin Research Articles

A new statistical approach for interpreting oceanic fCO2 data

Surface ocean CO2 fugacity (fCO2) often exhibits large fluctuations and heterogeneity because of multiple controlling factors that pose a challenge for trend analysis, especially in the ocean margins. We propose a new statistical approach, Generalized Additive Mixed Modeling (GAMM), to interpret oceanic fCO2 data in two ocean margins (Japan and Europe) and two open ocean areas. The latter included areas near the Hawaii Ocean Time-series (HOT) and the Bermuda Atlantic Time-series (BATS). This method utilizes day of year, sea surface salinity (SSS), sea surface temperature (SST), and sampling date as predictors. Using this method, we were able to derive multidecadal fCO2 trends with both improved precision and greater robustness to data gaps compared to an existing deseasonalization method used in the open ocean. The fCO2 trend derived by our method for the Japanese margin (1992–2013), the European margin (1989–2014), and the open ocean near HOT (1983–2013) were 2.1±0.6, 1.9±0.7, and 2.0±0.5μatmyear−1 (mean±standard deviation of multiple 1°×1° grids in margins and 5°×5° grids in open ocean), respectively, and the fCO2 trends were all close to the atmosphere CO2 trend (1.7–1.9μatmyear−1). Our analysis produced generally smaller standard errors (paired t-test, p≪0.001) than those obtained using the existing method based on the same dataset. In addition, our method was less sensitive to data gaps compared to this existing method. However, for regularly spaced fCO2 times-series data, for example, discrete bottle data collected in the BATS station in 1991–2011, this method was not advantageous over the existing method (1.9±0.2 vs. 2.0±0.2μatmyear−1). To test the broader applicability of this method, we compared fCO2 trends in the Southern Ocean derived using our method with those from a recently reported Markov Chain Monte Carlo method and found no significant difference between the two sets of values. Therefore, we recommend the application of our method in interpreting fCO2 data in different oceanic environments.

Tsunami hazard assessment in the Hudson River Estuary based on dynamic tsunami–tide simulations

This work is part of a tsunami inundation mapping activity carried out along the US East Coast since 2010, under the auspice of the National Tsunami Hazard Mitigation program (NTHMP). The US East Coast features two main estuaries with significant tidal forcing, which are bordered by numerous critical facilities (power plants, major harbors,...) as well as densely built low-level areas: Chesapeake Bay and the Hudson River Estuary (HRE). HRE is the object of this work, with specific focus on assessing tsunami hazard in Manhattan, the Hudson and East River areas. In the NTHMP work, inundation maps are computed as envelopes of maximum surface elevation along the coast and inland, by simulating the impact of selected probable maximum tsunamis (PMT) in the Atlantic ocean margin and basin. At present, such simulations assume a static reference level near shore equal to the local mean high water (MHW) level. Here, instead we simulate maximum inundation in the HRE resulting from dynamic interactions between the incident PMTs and a tide, which is calibrated to achieve MHW at its maximum level. To identify conditions leading to maximum tsunami inundation, each PMT is simulated for four different phases of the tide and results are compared to those obtained for a static reference level. We first separately simulate the tide and the three PMTs that were found to be most significant for the HRE. These are caused by: (1) a flank collapse of the Cumbre Vieja Volcano (CVV) in the Canary Islands (with a 80 km3 volume representing the most likely extreme scenario); (2) an M9 coseismic source in the Puerto Rico Trench (PRT); and (3) a large submarine mass failure (SMF) in the Hudson River canyon of parameters similar to the 165 km3 historical Currituck slide, which is used as a local proxy for the maximum possible SMF. Simulations are performed with the nonlinear and dispersive long wave model FUNWAVE-TVD, in a series of nested grids of increasing resolution towards the coast, by one-way coupling. Four levels of nested grids are used, from a 1 arc-min spherical coordinate grid in the deep ocean down to a 39-m Cartesian grid in the HRE. Bottom friction coefficients in the finer grids are calibrated for the tide to achieve the local spatially averaged MHW level at high tide in the HRE. Combined tsunami–tide simulations are then performed for four phases of the tide corresponding to each tsunami arriving at Sandy Hook (NJ): 1.5 h ahead, concurrent with, 1.5 h after, and 3 h after the local high tide. These simulations are forced along the offshore boundary of the third-level grid by linearly superposing time series of surface elevation and horizontal currents of the calibrated tide and each tsunami wave train; this is done in deep enough water for a linear superposition to be accurate. Combined tsunami–tide simulations are then performed with FUNWAVE-TVD in this and the finest nested grids. Results show that, for the 3 PMTs, depending on the tide phase, the dynamic simulations lead to no or to a slightly increased inundation in the HRE (by up to 0.15 m depending on location), and to larger currents than for the simulations over a static level; the CRT SMF proxy tsunami is the PMT leading to maximum inundation in the HRE. For all tide phases, nonlinear interactions between tide and tsunami currents modify the elevation, current, and celerity of tsunami wave trains, mostly in the shallower water areas of the HRE where bottom friction dominates, as compared to a linear superposition of wave elevations and currents. We note that, while dynamic simulations predict a slight increase in inundation, this increase may be on the same order as, or even less than sources of uncertainty in the modeling of tsunami sources, such as their initial water elevation, and in bottom friction and bathymetry used in tsunami grids. Nevertheless, results in this paper provide insight into the magnitude and spatial variability of tsunami propagation and impact in the complex inland waterways surrounding New York City, and of their modification by dynamic tidal effects. We conclude that changes in inundation resulting from the inclusion of a dynamic tide in the specific case of the HRE, although of scientific interest, are not significant for tsunami hazard assessment and that the standard approach of specifying a static reference level equal to MHW is conservative. However, in other estuaries with similarly complex bathymetry/topography and stronger tidal currents, a simplified static approach might not be appropriate.

Biological hot spots and the accumulation of marine dissolved organic matter in a highly productive ocean margin

AbstractConcentrations of dissolved organic carbon (DOC) and major biochemicals (amino acids and carbohydrates) were measured during five cruises (2009–2010) to the Louisiana margin in the northern Gulf of Mexico. Concentrations of amino acids and carbohydrates were elevated at mid‐salinities and were indicative of plankton production of dissolved organic matter (DOM) in surface waters. Hot spots of two compositionally distinct types of labile DOM were identified based on the relative abundances of amino acids and carbohydrates. Amino acid‐rich hot spots occurred sporadically in regions of high phytoplankton biomass and were mostly observed between dusk and dawn, reflecting a grazing source. In contrast, carbohydrate‐rich hot spots were more widespread and were often found in nutrient‐poor waters, indicating the production of carbon‐rich DOM associated with nutrient limitation. Major biochemical indicators and bioassay experiments indicated labile DOM comprised a relatively small fraction of the DOC. Most DOM was degraded and had a semi‐labile nature. Substantial accumulations of marine (plankton‐derived) DOC were observed in surface waters, particularly at mid‐salinities during the summer. Microbial alteration of marine DOC and nutrient limitation of microbial utilization of carbon‐rich DOM appeared largely responsible for the accumulation of DOC. The reservoir of accumulated marine DOC in the shelf surface mixed layer ranged from 0.11 Tg C to 0.23 Tg C, with the lowest and highest values occurring during winter and summer. Substantial cross‐shelf export of semi‐labile marine DOM occurred during the summer and provided a major carbon and energy subsidy to microbial food webs in offshore waters.

Salt tectonics at the margins of young oceans

The paper is devoted to salt tectonics in marginal oceanic salt-dome basins and is based on a wide synthesis of the literature and the author’s data. For the first time, the general pattern of global distribution of these basins has been illustrated by a map. Their localization and structure, tectonic position and evolution, and peculiar morphokinematic features of salt tectonics are characterized and compared with the attributes of salt tectonics inherent to continental regions. The geodynamic settings of the initial formation of marginal oceanic basins and their present-day arrangement have been refined, as well as the onset of salt tectonics therein, manifested in various styles. It has been shown that the geodynamic type of basin and stages of its geodynamic evolution determine the morphokinematic type of salt tectonics, character of its manifestation, and dislocations in host sedimentary complexes, and, therefore, they are auxiliary indicators of geodynamic regimes. The mechanisms of salt tectonics, its effect on the structure of overlying sedimentary sequences, and localization of hydrocarbon fields are discussed.

How well would modern‐day oceanic property distributions be known with paleoceanographic‐like observations?

Author(s): Gebbie, G; Streletz, GJ; Spero, HJ | Abstract: Compilations of paleoceanographic observations for the deep sea now contain a few hundred points along the oceanic margins, mid-ocean ridges, and bathymetric highs, where seawater conditions are indirectly recorded in the chemistry of buried benthic foraminiferal shells. Here we design an idealized experiment to test our predictive ability to reconstruct modern-day seawater properties by considering paleoceanographic-like data. We attempt to reconstruct the known, modern-day global distributions by using a state estimation method that combines a kinematic tracer transport model with observations that have paleoceanographic characteristics. When a modern-like suite of observations (Θ, practical salinity, seawater δ18O, δ13CDIC, PO4, NO3, and O2) is used from the sparse paleolocations, the state estimate is consistent with the withheld data at all depths below 1500 m, suggesting that the observational sparsity can be overcome. Physical features, such as the interbasin gradients in deep δ13CDIC and the vertical structure of Atlantic δ13CDIC, are accurately reconstructed. The state estimation method extracts useful information from the pointwise observations to infer distributions at the largest oceanic scales (at least 10,000 km horizontally and 1500 m vertically) and outperforms a standard optimal interpolation technique even though neither dynamical constraints nor constraints from surface boundary fluxes are used. When the sparse observations are more realistically restricted to the paleoceanographic proxy observations of δ13C, δ18O, and Cd/Ca, however, the large-scale property distributions are no longer recovered coherently. At least three more water mass tracers are likely needed at the core sites in order to accurately reconstruct the large-scale property distributions of the Last Glacial Maximum.

Near-constant apparent hydrogen isotope fractionation between leaf wax n-alkanes and precipitation in tropical regions: Evidence from a marine sediment transect off SW Africa

A transect of marine surface sediment samples from 1°N to 28°S off southwest Africa was analysed to verify the application of hydrogen isotope compositions of terrestrial plant-wax n-alkanes preserved in ocean sediments as a proxy for continental hydrological conditions. Conditions on the adjacent continent range from humid evergreen forests to deciduous forests, wood- and shrub land and further to arid grasslands and deserts. The hydrogen isotope values for the dominant n-alkane homologues (C29, C31 and C33) vary from −123‰ to −141‰ VSMOW and correlate with the modelled hydrogen isotope composition of mean annual and growing season precipitation of postulated continental source areas (r up to 0.8, p<0.01). The apparent hydrogen isotope fractionation between alkanes and mean annual precipitation is remarkably uniform (−109‰ on average, σ⩽5‰, n=27). Potentially, effects of aridity on the apparent hydrogen isotope fractionation are concealed by the contribution of different plants (C3 dicotyledons vs C4 grasses). Thus, isotope ratios of leaf wax n-alkanes preserved in ocean margin sediments in these and similar tropical regions may be directly converted to δD ratios of ancient precipitation by employing a constant hydrogen isotope fractionation.

CO2 Saturation and Trophic Shift Induced by Microbial Metabolic Processes in a River-Dominated Ocean Margin (Tropical Shallow Lagoon, Chilika, India)

ABSTRACTAn effort has been made for the first time in Asia's largest brackish water lagoon, Chilika, to investigate the spatio-temporal variability in primary productivity (PP), bacterial productivity (BP), bacterial abundance (BA), bacterial respiration (BR) and bacterial growth efficiency (BGE) in relation to partial pressure of CO2 (pCO2) and CO2 air–water flux and the resultant trophic switchover. Annually, PP ranged between 24 and 376 µg C L−1 d−1 with significantly low values throughout the monsoon (MN), caused by light limitation due to inputs of riverine suspended matter. On the contrary, BP and BR ranged from 11.5 to 186.3 µg C L−1 d−1 and from 14.1 to 389.4 µg C L−1 d−1, respectively, with exceptionally higher values during MN. A wide spatial and temporal variation in the lagoon trophic status was apparent from BP/PP (0.05–6.4) and PP/BR (0.10–18.2) ratios. The seasonal shift in net pelagic production from autotrophy to heterotrophy due to terrestrial organic matter inputs via rivers, enhanced the bacterial metabolism during the MN, as evident from the high pCO2 (10,134 µatm) and CO2 air–water flux (714 mm m−2 d−1). Large variability in BGE and BP/PP ratios especially during MN led to high bacteria-mediated carbon fluxes which was evident from significantly high bacterial carbon demand (BCD >100% of PP) during this season. This suggested that the net amount of organic carbon (either dissolved or particulate form) synthesized by primary producers in the lagoon was not sufficient to satisfy the bacterial carbon requirements. Lagoon sustained low to moderate autotrophic–heterotrophic coupling with annual mean BCD of 231% relative to the primary production, which depicted that bacterioplankton are the mainstay of the lagoon biogeochemical cycles and principal players that bring changes in trophic status. Study disclosed that the high CO2 supersaturation and oxygen undersaturation during MN was attributed to the increased heterotrophic respiration (in excess of PP) fuelled by allochthonous organic matter. On a spatial scale, lagoon sectors such as south sector, central sector and outer channel recorded “net autotrophic,” while the northern sector showed “net heterotrophic” throughout the study period.

GEOCHEMICAL FINGERPRINTS OF CLIMATIC OSCILLATIONS DURING THE LATE HOLOCENE IN RIA DE VIGO (N SPAIN)

This work studies mineralogical data from X-Ray Diffraction Techniques (XRD) and geochemical data analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) of fine fraction from samples collected along the Ocean Margin Exchange Project (OMEX) core KSGX 24. This core was recovered in the external sector of Ria de Vigo, N Spain. Radiocarbon ages allowed the estimation of an age about 3 ka cal BP for the core base. Core KSGX 24 is a muddy sedimentary sequence with two main sections where the sediments are finer: ≈134-90 cm and ≈26-0 cm. These finer sections are characterized by a relatively low sedimentation rate, and by changes in the geochemical and mineralogical composition of the sediments and their magnetic susceptibility. Basically, they are relatively depleted in carbonates and Ca content, and relatively enriched in detrital minerals, namely phyllosilicates, and in Al, Co and Sc. The slightly coarser grained intercalated sections are marked by the increase of Ce, La and Th content. These results suggests that, despite the prevalence of a weak hydrodynamic regime in the study area during the last 3 ka, it has undergone some changes. The mineralogical and geochemical deviations may have been conditioned by several factors, such as: authigenetic and diagenetic reactions; anthropogenic activities in the nearby continental region; the natural tendency for sea level rise; changes in hydrodynamic conditions regulated by modifications in the pattern of prevailing winds; and alterations in the rainfall regime. The two last factors are related to climate oscillations and probably with the tendency of the North Atlantic Oscillation (NAO) to remain more positive or negative during longer periods in the past than at present. doi: 10.12957/jse.2016.21642

Effects of eustatic sea-level change, ocean dynamics, and nutrient utilization on atmospheric &amp;lt;i&amp;gt;p&amp;lt;/i&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and seawater composition over the last 130 000 years: a model study

Abstract. We have developed and employed an Earth system model to explore the forcings of atmospheric pCO2 change and the chemical and isotopic evolution of seawater over the last glacial cycle. Concentrations of dissolved phosphorus (DP), reactive nitrogen, molecular oxygen, dissolved inorganic carbon (DIC), total alkalinity (TA), 13C-DIC, and 14C-DIC were calculated for 24 ocean boxes. The bi-directional water fluxes between these model boxes were derived from a 3-D circulation field of the modern ocean (Opa 8.2, NEMO) and tuned such that tracer distributions calculated by the box model were consistent with observational data from the modern ocean. To model the last 130 kyr, we employed records of past changes in sea-level, ocean circulation, and dust deposition. According to the model, about half of the glacial pCO2 drawdown may be attributed to marine regressions. The glacial sea-level low-stands implied steepened ocean margins, a reduced burial of particulate organic carbon, phosphorus, and neritic carbonate at the margin seafloor, a decline in benthic denitrification, and enhanced weathering of emerged shelf sediments. In turn, low-stands led to a distinct rise in the standing stocks of DIC, TA, and nutrients in the global ocean, promoted the glacial sequestration of atmospheric CO2 in the ocean, and added 13C- and 14C-depleted DIC to the ocean as recorded in benthic foraminifera signals. The other half of the glacial drop in pCO2 was linked to inferred shoaling of Atlantic meridional overturning circulation and more efficient utilization of nutrients in the Southern Ocean. The diminished ventilation of deep water in the glacial Atlantic and Southern Ocean led to significant 14C depletions with respect to the atmosphere. According to our model, the deglacial rapid and stepwise rise in atmospheric pCO2 was induced by upwelling both in the Southern Ocean and subarctic North Pacific and promoted by a drop in nutrient utilization in the Southern Ocean. The deglacial sea-level rise led to a gradual decline in nutrient, DIC, and TA stocks, a slow change due to the large size and extended residence times of dissolved chemical species in the ocean. Thus, the rapid deglacial rise in pCO2 can be explained by fast changes in ocean dynamics and nutrient utilization whereas the gradual pCO2 rise over the Holocene may be linked to the slow drop in nutrient and TA stocks that continued to promote an ongoing CO2 transfer from the ocean into the atmosphere.

Diversity, Abundance, and Niche Differentiation of Ammonia-Oxidizing Prokaryotes in Mud Deposits of the Eastern China Marginal Seas.

The eastern China marginal seas (ECMS) are prominent examples of river-dominated ocean margins, whose most characteristic feature is the existence of isolated mud patches on sandy sediments. Ammonia-oxidizing prokaryotes play a crucial role in the nitrogen cycles of many marine environments, including marginal seas. However, few studies have attempted to address the distribution patterns of ammonia-oxidizing prokaryotes in mud deposits of these seas. The horizontal and vertical community composition and abundance of ammonia-oxidizing archaea (AOA) and bacteria (AOB) were investigated in mud deposits of the South Yellow Sea (SYS) and the East China Sea (ECS) by using amoA clone libraries and quantitative PCR. The diversity of AOB was comparable or higher in the mud zone of SYS and lower in ECS when compared with AOA. Vertically, surface sediments had generally higher diversity of AOA and AOB than middle and bottom layers. Diversity of AOA and AOB showed significant correlation with latitude. Nitrosopumilus and Nitrosospira lineages dominated AOA and AOB communities, respectively. Both AOA and AOB assemblages exhibited greater variations across different sites than those among various depths at one site. The abundance of bacterial amoA was generally higher than that of archaeal amoA, and both of them decreased with depth. Niche differentiation, which was affected by dissolved oxygen, salinity, ammonia, and silicate (SiO), was observed between AOA and AOB and among different groups of them. The spatial distribution of AOA and AOB was significantly correlated with δ15NTN and SiO, and nitrate and δ13C, respectively. Both archaeal and bacterial amoA abundance correlated strongly with SiO. This study improves our understanding of spatial distribution of AOA and AOB in ecosystems featuring oceanic mud deposits.

Clavadoce (Annelida: Phyllodocidae) from Australia.

The first records of the phyllodocid genus Clavadoce are provided from Australia, where the fifth species in the genus is now known: Clavadoce dorsolobata (Hartmann-Schröder, 1987) comb. nov. which is widely distributed in intertidal habitats in southeastern Australia. Clavadoce dorsolobata was described as Eumida (Sige) dorsolobata Hartmann-Schröder, 1987 and herein transferred to Clavadoce. Five species of Clavadoce are now known world wide, four of which are from different regions on the Pacific Ocean margin, while Clavadoce cristata is from the North Atlantic. The Australian species is the first record of Clavadoce for the southern hemisphere.

Analysis of Low-Biomass MicrobialCommunities in theDeep Biosphere.

Over the past few decades, the subseafloor biosphere has been explored by scientific ocean drilling to depths of about 2.5km below the seafloor. Although organic-rich anaerobic sedimentary habitats in the ocean margins harbor large numbers of microbial cells, microbial populations in ultraoligotrophic aerobic sedimentary habitats in the open ocean gyres are several orders of magnitude less abundant. Despite advances in cultivation-independent molecular ecological techniques, exploring the low-biomass environment remains technologically challenging, especially in the deep subseafloor biosphere. Reviewing the historical background of deep-biosphere analytical methods, the importance of obtaining clean samples and tracing contamination, as well as methods for detecting microbial life, technological aspects of molecular microbiology, and detecting subseafloor metabolic activity will be discussed.

Late Triassic paleogeographic reconstruction along the Neo–Tethyan Ocean margins, southern Tibet

Sandstone petrographic and U–Pb detrital zircon analyses of Upper Triassic sedimentary rocks from the northern margin of India (Tethyan Himalaya Sequence) and southern margin of Eurasia (Lhasa terrane) provide new constraints on the Mesozoic paleogeography of Neo–Tethyan Ocean basins. The Upper Triassic Nieru Formation of the Tethyan Himalaya Sequence (THS) near Lazi city (∼29°N, 87.5°E) is dominated by Indian-affinity, Precambrian detrital zircons, which are typical of the majority of the THS. However, the Upper Triassic Langjiexue Formation of the THS exposed to the east (at 90–93°E longitude) includes significant populations of Permian to Early Jurassic (291–184 Ma) detrital zircons for which there is no known Indian source. In addition, the Upper Triassic Nieru Formation near Kangma town (∼28.5°N, 90°E), located ∼200 km to the southeast of Lazi city, yielded detrital zircon age spectra that are similar to those of Langjiexue Formation. Based on detrital zircon age spectra comparisons, we propose that both the Langjiexue and Nieru formations were derived from continental crustal fragments that were adjacent to the northwestern margin of Australia. Furthermore, we suggest that these THS units, and age-equivalent strata in Northwest Australia, West Sulawesi, Timor and West Papua, comprised a Late Triassic submarine fan along the northern Australian shelf. The Upper Triassic Mailonggang Formation in the southern Lhasa terrane (35 km northeast of Lhasa city, ∼30°N, 91.5°E) is dominated by Permian detrital zircons, which were likely derived from proximal Lhasa terrane sources. The Mailonggang Formation differs from all age-equivalent strata in the Tethyan Himalaya; therefore we interpret that it was separated from Greater India by the Neo–Tethyan Ocean.

Distribution, input pathway and mass inventory of black carbon in sediments of the Gulf of Thailand, SE Asia

The coastal margins around Southeast Asia (SE Asia) may serve as an ideal location to study the source-sink process of sedimentary black carbon (BC) because SE Asia has been identified as one of the major BC emission source regions in the world. This study provides an extensive picture of recent regional-scale sedimentary BC sequestration in the Gulf of Thailand (GOT), a tropical marine system in SE Asia. Generally, the sedimentary BC concentrations (0.07–3.99 mg/g) were in the low to moderate ranges of those obtained in other coastal sediments around the world. Regional variability of the BC and its correlation with the sediment grain size and total organic carbon (TOC) content indicated a general hydrodynamic constraint on BC occurrence in the lower Gulf in contrast to the upper Gulf with a more source dependence due to the direct land-based input. BC/TOC% values and the varied BC components (char and soot), as well as their correlations suggested that char was the predominant constituents of sedimentary BC both in the upper and lower Gulf, which could be mainly derived from biomass burning and entered into the nearshore region through direct fluvial transport and surface run-off. The estimated BC burial flux (∼212 μg/cm2/y) and mass inventory (∼200 Gg/y) in the GOT on the hundred-year timescale were of the same order of magnitude compared with other oceanic margins, and thus the tropical shelf sediments from SE Asia could serve as an important sink of land-emitted BC.

African humid periods triggered the reactivation of a large river system in Western Sahara.

The Sahara experienced several humid episodes during the late Quaternary, associated with the development of vast fluvial networks and enhanced freshwater delivery to the surrounding ocean margins. In particular, marine sediment records off Western Sahara indicate deposition of river-borne material at those times, implying sustained fluvial discharges along the West African margin. Today, however, no major river exists in this area; therefore, the origin of these sediments remains unclear. Here, using orbital radar satellite imagery, we present geomorphological data that reveal the existence of a large buried paleodrainage network on the Mauritanian coast. On the basis of evidence from the literature, we propose that reactivation of this major paleoriver during past humid periods contributed to the delivery of sediments to the Tropical Atlantic margin. This finding provides new insights for the interpretation of terrigenous sediment records off Western Africa, with important implications for our understanding of the paleohydrological history of the Sahara.

Linkages among fluorescent dissolved organic matter, dissolved amino acids and lignin-derived phenols in a river-influenced ocean margin

Excitation emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC) is commonly used to investigate the dynamics of dissolved organic matter (DOM). However, a lack of direct comparisons with known biomolecules makes it difficult to substantiate the molecular composition of specific fluorescent components. Here, coincident surface-water measurements of EEMs, dissolved lignin, and total dissolved amino acid (TDAA) acquired in the northern Gulf of Mexico were used to investigate the relationships between specific fluorescent components and DOM biomolecules. Two terrestrial humic-like components identified by EEM-PARAFAC using samples obtained from river to offshore waters were strongly linearly correlated with dissolved lignin concentrations. In addition, changes in terrestrial humic-like abundance were correlated with those in lignin phenol composition, suggesting such components are largely derived from lignin and its alteration products. By applying EEM-PARAFAC to offshore samples, two protein-like components were obtained. The tryptophan-like component was strongly correlated with TDAA concentrations, corroborating the suggested protein/peptide origin of this component. The ratios of tryptophan-like component to tyrosine-like component or dissolved organic carbon (DOC) concentrations were also correlated with DOC-normalized yields of TDAA, suggesting these proxies are useful indicators of the bioavailability of DOM in marine waters of the studied ecosystem.

U-Th isotope constraints on gas hydrate and pockmark dynamics at the Niger delta margin

The application of uranium-thorium dating methods to authigenic carbonates provides unique constraints on the temporal evolution of methane seeps at ocean margins. In this study, we report U-Th isotope measurements for carbonate breccias collected from within a hydrate-bearing pockmark located at the Niger Delta margin. These concretions were extracted from a carbonate-rich layer in the upper two meters of a sediment core (N2-KS-44; ~1200m water depth), well above the present-day sulphate-methane transition zone (about 3m depth) and the presence of gas hydrates in the sediment. The stratigraphy of core N2-KS-44 was established by tuning its downcore Al/Ti profile to a well-dated nearby reference core, and carbonate 230Th/U ages were calculated using isochron methods.Our results indicate that a major event of aragonite precipitation occurred between about 13 and 2.5ka at the studied location. Comparison of sediment accumulation rates at both core N2-KS-44 and the nearby reference site suggests that the initial stage of carbonate precipitation, between 13 and 10ka, was associated with sediment winnowing, probably related to intense fluid seepage. In contrast, our data indicate that sedimentation rates rapidly increased within the pockmark after 7ka. In agreement with the presence of carbonate breccias exhibiting U-Th ages older than their corresponding stratigraphic age, this observation would suggest that sediment reworking took place after that time, possibly caused by erosion of the surrounding sediment within the pockmark.We hypothesize that the period of carbonate formation between 13 and 2.5ka was related to an upward migration of gas-hydrate reservoirs to the near seafloor environment. After this pulse of enhanced fluid flow, the diminution of methane fluxes at the base of the local gas-hydrate occurrence zone would have led to hydrate dissolution in sub-surface sediments and pockmark formation, thereby explaining the progressive increase in sedimentation rates, the absence of recent carbonate concretions and the deepening of the sulphate-methane transition zone at site N2-KS-44 inferred from pore water data. Overall, these results provide further constraints about the relationship between gas hydrate dynamics and the evolution of pockmarks at ocean margins through time.

Lacustrine turbidites produced by surficial slope sediment remobilization: A mechanism for continuous and sensitive turbidite paleoseismic records

Turbidite records along ocean margins and in lake basins are increasingly used as paleoseismic proxies. However, the slope remobilization processes that formed the earthquake-related turbidity currents are poorly understood and, therefore, it is difficult to assess the consistency of turbidite paleoseismic records. We analyzed the sedimentary imprint of the giant AD 1960 (Mw 9.5) Chile earthquake in four South-Central Chilean lakes. Mass-transport deposits and turbidites were compared by interpreting reflection-seismic profiles, sedimentary facies, volume balances and proxies for turbidite composition (i.e. grain size, radionuclides, sediment color, organic matter). Sediment lightness was used as a proxy for organic matter content. Comparison of lightness of turbidites and the underlying sediments shows that earthquake-triggered turbidites in the four lakes result from remobilization of a thin veneer (on average about 5cm) of slope sediments and do not result from disintegration of subaquatic landslides. Statistical analysis of 34 turbidites in a new 5200year sedimentary record does not show any correlation between inter-event time and turbidite thickness (and thus volume). This means that inter-event slope recharging is not a dominant factor governing turbidite recurrence and that initiation of turbidity currents by remobilization of surficial sediments is different to slope failure related to subaquatic landslides. We conclude that surficial slope sediment remobilization forms a valid mechanism that explains why turbidite paleoseismic records can be of excellent continuity and high sensitivity. Moreover, based on correlations between seismic intensity, turbidite volume and stratigraphic remobilization depth, we propose that surficial slope sediment remobilization allows for turbidite records containing information about paleo-earthquake intensity.

Palaeoceanographic controls on spatial redox distribution over the Yangtze Platform during the Ediacaran–Cambrian transition

AbstractThe Ediacaran–Cambrian interval was an eventful transitional period, when dynamic interactions between the biosphere and its physical environment allowed the Earth System to cross into a new state, characterized by the presence of metazoans, more equable climates and more expansive oxygenation of the oceans. Due to the retreat of widespread sulphidic conditions, redox‐sensitive trace‐metals could accumulate to a greater extent in ‘black shales’ deposited in localized anoxic/euxinic environments, such as highly productive ocean margins. This study investigates the concentrations of the redox‐sensitive trace‐metals molybdenum and vanadium in organic‐rich sedimentary rocks from seven sections of the Yangtze Platform, slope and basin. Iron speciation analyses were carried out in order to distinguish oxic, anoxic‐ferruginous and anoxic‐sulphidic settings, while sulphur and nitrogen isotope ratios were measured to gain insight into sulphate and nitrate availability, respectively, in the context of changing redox conditions. The data herein demonstrate an overall increase in redox‐sensitive trace‐metal contents in black shales across the Ediacaran–Cambrian transition, but with marked temporal and spatial variability. Euxinia is evident in South China before 551 Ma in the Ediacaran, and again in the early Cambrian. However, some time‐equivalent sections are not enriched in redox‐sensitive trace‐metals, and also exhibit contrasting S‐isotope and N‐isotope systematics. A more complex configuration of the Yangtze Platform, for example with vast intra‐shelf basins, together with changing (generally rising) eustatic sea‐level may account for this variability. In this regard, it is proposed that a mid‐depth sulphidic wedge, caused by nutrient upwelling over the south‐east platform margins, migrated over time (but generally landward), leading to spatially variable redox conditions determined by sea‐level, currents and bathymetric constraints. The changing extents of anoxia and euxinia appear to have limited the distribution of emerging Ediacaran and Cambrian ecosystems.

Nutrient dynamics across the river‐sea interface in the Changjiang (Yangtze River) estuary—East China Sea region

AbstractA total of 13 cruises were performed in different seasons between July 2010 and July 2012, and in various subsystems of the Changjiang (Yangtze River) Estuary and the adjacent East China Sea (ECS). Multiple vessels were deployed to conduct transect sampling, simultaneous and consecutive sampling, sampling with buoys, and sampling during phytoplankton bloom events, which allowed us to track detached patches of water masses and to collect discrete and time‐series samples in different patches. We found that the river‐derived nutrients from the Changjiang could be transported through the western ECS and reach as far as the northeastern ECS during summer, but in the form of isolated patches rather than continuous plumes. The concentrations of decreased with increasing seaward distance of these patches, showing largely nonconservative mixing behavior and the ECS being a sink for nutrients. During winter, conversely, all nutrient species showed mostly conservative behavior, with correlative coefficients (R2) even reaching &gt; 0.97 in the salinity‐nutrient correlations for both and during the western ECS cruise in February 2012 and during the Changjiang mouth cruise in March 2012. In addition, a large variability in nutrient concentrations was observed within the Changjiang freshwaters and could be partly attributed to inputs from local tributaries. This study highlights the importance of simultaneous field sampling with multiple vessels for investigating the biogeochemical behavior of nutrients in highly dynamic ocean margin areas such as the Changjiang Estuary‐ECS region.