Biomarker records,organic carbon accumulation,and river discharge in the Holocene southern Kara Sea (Arctic Ocean)

Within the Russian–German research project on “Siberian River Run-off (SIRRO)” dealing with freshwater discharge and its influence on biological, geochemical, and geological processes in the Kara Sea, sedimentological and organic-geochemical investigations were carried out on two well-dated sediment cores from the Yenisei Estuary area. The main goal of this study was to quantify terrigenous organic carbon accumulation based on biomarker and bulk accumulation rate data, and its relationship to Yenisei river discharge and climate change through Holocene times. The biomarker data in both cores clearly indicate the predominance of terrigenous organic matter, reaching 70–100 and 50–80% of total organic carbon within and directly north of the estuary, respectively. During the last ca. 9 cal ka b.p. represented in the studied sediment section, siliciclastic sediment and (terrigenous) organic carbon input was strongly influenced by postglacial sea-level rise and climate-related changes in river discharge. The mid-Holocene Climatic Optimum is documented by maximum river discharge between 8.2 and 7.3 cal ka b.p. During the last 2,000 years, river discharge probably decreased, and accumulation of both terrigenous and marine organic carbon increased due to enhanced coagulation of fine-grained material.     

Late Glacial–Holocene ecostratigraphy of the south-eastern Aegean Sea, based on plankton and pollen assemblages

Quantitative analyses of coccolithophores, planktonic foraminifers, dinoflagellate cysts and pollen assemblages were carried out on shallow (NS-14) and deeper (NS-40) sediment cores from the south-eastern Aegean Sea. Nine coccolithophore (ACE 1–9) and nine planktonic foraminifer (APFE 1–9) ecozones, correlated with dinoflagellate cyst evidence, have been defined for the last ~14.5 cal. ka. Additionally, eight pollen assemblage zones (PAZ 1–8) have been recognised and correlated with the plankton ecozones. Although generally consistent with existing schemes for the central and eastern Mediterranean, the established high-resolution ecostratigraphy has led to an expanded palaeoecological reconstruction of the Late Glacial–Holocene archive in the south-eastern Aegean Sea, defining two warm and humid phases at 9.3–8.6 and 7.6–6.4 cal. ka b.p., associated with the deposition of the early Holocene sapropel S1, and a third one between 5.2 and 4.2 cal. ka b.p. The high sedimentation rates which characterise the study area enabled the detection of even minor and brief climatic events in the Aegean Sea during S1 deposition times. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mid to late Holocene constraints for continental shelf mud deposition in association with river input: the Guadiana Mud Patch (SW Iberia)

This study correlates Holocene sedimentation with regional environmental changes for the Guadiana middle shelf, SW Iberia, based on a detailed sedimentological analysis combined with radiocarbon and amino acid racemization dating. The sedimentary record of vibrocore CRIDA16 (307 cm) from the mud patch off the Guadiana River is characterized by a transgressive signature. Terrigenous sand–gravel dominates the core base up to ca. 5,000 cal. years b.p., enriched in schist/greywackes and non-reworked quartz. The shelf area was under the direct influence of Guadiana bed load discharges, as sea level was lower than today but rising, favouring the entrapment of fines during the Guadiana estuary infilling in the early Holocene. Core upwards, fines dominate the record. Sea-level stabilization at ca. 5,700–3,700 cal. years b.p. led to estuary infilling by coarser sediments and export of suspended sediment to the middle shelf. After ca. 900–700 cal. years b.p., the formation of the Guadiana Mud Patch signifies deposition in a marine environment dominated by suspended material from the Guadiana River. Comparison with earlier publications showed that a similar evolutionary mechanism of middle shelf mud deposition occurred in other parts of the Iberian Margin, pointing to a generalized establishment of this type of sedimentary body in Iberia since the mid to late Holocene. This suggests that the formation of mud belts on the north and south Iberian Margin was to a large extent synchronous. It is likely that the evolution of the Guadiana Mud Patch on the shelf was controlled foremost by the overall climate trends identified for several parts of Iberia during the Holocene (i.e. increased aridity punctuated by significant humidity stages), and less by human occupation of the region. Enhanced humidity stages led to higher rainfall and intensified floods, with a consequent increase of discharges to the shelf which ultimately were responsible for the Guadiana Mud Patch formation. It is expected that this pattern of sedimentary dynamics has been strongly disturbed since 2002, when the closing of the Alqueva Dam (the biggest on the Iberian Peninsula) was completed in the Guadiana River basin.     

Evidence of a major environmental change recorded in a macrotidal bay (Marennes-Oléron Bay, France) by correlation between VHR seismic profiles and cores

New, very high-resolution (25 cm) seismic profiles have revealed the internal architecture of the infilling of a macrotidal bay, the Marennes-Oléron Bay, France. Within this geometry, a major seismic unconformity has been correlated with core data. This correlation provides evidence that the seismic unconformity corresponds to a sharp grain-size decrease. Both seismic and core data indicate that this change of grain size can be interpreted as a record of a recent (around 1,000 years b.p.) decrease in hydrodynamical energy with time and/or a larger supply of fine-grained material. This recent environmental change can be related to natural infilling of the Marennes Oléron Bay, and to tidal prism decrease, increasing human activities (e.g. land reclamation, deforestation, agricultural land use) and climate fluctuations during the late Holocene, such as the transition between the cold period of the Dark Ages (1,550–1,050 years b.p.) and the Medieval warm period (1,050–550 years b.p.).     

The Role of Suspended Oyster Culture on Nitrogen Cycle in Hiroshima Bay

The predominance of bivalves affects the cycle of materials in the coastal ecosystem. In the present study, the role of suspended oyster culture on the nitrogen cycle was demonstrated for the northern Hiroshima Bay. The nitrogen cycle was considered as two systems, (1) the primary production (PP) system and (2) the oyster culture (OC) system. The results show that about 26% of N productivity was supplied to process by cultured oysters in the OC system. This process varies seasonally due to the seasonal variations of PON, physiological activities and biomass of oysters. The N processing rates were found to be high in summer and low in winter. The biodeposition and excretion of N in the OC system are 3.0 and 2.1 ton N d^–1, while the natural sedimentation rate and N regeneration in the PP system are 8.3 and 18.0 ton N d^–1, which indicates that the PP system is a major system regenerating N in the water column. The release of total dissolved N from the bottom to the water column is about 8.3 ton N d^–1. The amount of N harvested as oyster product was about 1.3 ton N d^–1, which is about 10% of daily N loading in north Hiroshima Bay. According to the N cycle developed in the present study, the results suggest the significant role of suspended oyster culture on the nitrogen cycle in Hiroshima Bay. In addition, our results indicate that oyster production was efficiently harvested, suggesting that oyster culture could probably be used as a tool to remove N from Hiroshima Bay.     

Late glacial to Holocene sea-level changes in the Sea of Marmara: new evidence from high-resolution seismics and core studies

The late glacial to Holocene sedimentary record of the northern shelf of the Sea of Marmara (SoM) has been documented by detailed seismo-, chrono-, and biostratigraphic analyses using sub-bottom (Chirp) profiles and sediment cores. During MIS 3 and the main part of MIS 2 (60–15 ¹⁴C ka b.p.), disconnection from the Mediterranean and Black seas together with a dry climate resulted in a regression in the SoM, when the Sea was transformed into a brackish lake. The river incisions below 105 m water depth along the northern shelf took place during the last glacial maximum, when the lake level was modulated by stillstands at −98 and −93 m. The post-glacial freshwater transgressive stage of the Marmara ‘Lake’ occurred between 15 and 13.5 ¹⁴C ka b.p., leading to a rise in water level to −85 m by 13.0 ¹⁴C ka b.p., as evidenced by broad wave-cut terraces along the northern shelf. Since 12 ¹⁴C ka b.p., high-frequency sea-level fluctuations have been identified at the SoM entrance to the Strait of İstanbul (SoI). Thus, wave-cut terraces have been recorded at water depths of −76 and −71 m that, according to an age model for core MD04-2750, have ages of 11.5 and 10.5 ¹⁴C ka b.p., respectively. Ancient shoreline at −65 m along the northern shelf presumably formed soon after the Younger Dryas (YD) at ca. 10.1 ¹⁴C ka b.p. Moreover, there is compelling evidence of Holocene outflow from the Sea of Marmara to the Black Sea. At the SoM entrance to the SoI, the existence of bioherms on the reflector surface together with abundant Brizalina spathulata and Protoglobulimina pupoides in a core suggests a return to higher salinities due to strong Mediterranean water incursion into the SoM at ∼8.8 ¹⁴C ka b.p. This finding is consistent with earlier suggestions that, after the YD, the Black Sea was flooded by outflow from the SoM as a result of global sea-level rise.     

Sources and sinks of sediment to the Amazon margin: The Amapa coast

Modern and Holocene muddy strata were studied along the shoreline adjacent to the Amazon river mouth using sedimentological, radiochemical, physical, and seismic methods. The present paper is a synthesis of the results, collected during the AmasSeds project, that is used to outline a regional shoreline sediment budget. Erosion of relict Amazon muds in southern Amapa supplies 10⁶ tons yr^–1 to the Amazon advective mud stream. Local rivers are sediment-poor (total suspended discharge ~ 1 × 10⁶ tons yr^–1), but form depositional sandflats on the shoreface downdrift of the river mouths. Mudflat accumulation in northern Amapa sequesters 10⁶–10⁷ tons yr^–1 by tidal-flat aggradation, alongshore mudcape accretion, and sediment trapping by mangroves. The processes temporarily store 1.5 × 10⁸ tons of Amazon mud in January–June.     

Character and distribution of exposed glaciodeltaic deposits off outer Cape Cod, Massachusetts, and their effects on hydrogeology and benthic habitats

Seabed outcrops of glaciodeltaic sediments were identified in four places east of Cape Cod, Massachusetts, during seismic-reflection, multibeam bathymetric and backscatter, bottom photographic, and sediment sampling surveys. These strata record coarser-grained ice-proximal glaciofluvial topset to finer-grained distal glaciolacustrine bottomset deposition within deltaic systems that prograded southwestward into glacial lakes from the South Channel lobe about 18 ka b.p. These beds are important because they (1) influence the outer Cape’s hydrogeologic framework, and (2) constitute relatively stable, locally rough habitats within an area of seafloor dominated by mobile sand and gravelly sediment, and benefit the benthic fauna by providing shelter and a substrate amenable to burrow construction.     

Introduced Pacific oysters Crassostrea gigas in South Africa: demographic change,genetic diversity and body condition

Successful aquaculture species are often chosen for their fast growth rates and fecundity, which are also characteristics of invasive species. The Pacific oyster Crassostrea gigas, which constitutes 80% of global oyster trade, has been confirmed as invasive in 17 of the 66 countries where it is cultured. The single study of its status in South Africa reported populations in six South Coast estuaries in 2001, dropping to three sites in 2003. We resurveyed these estuaries, visited others in the Eastern and Western Cape provinces, and sampled oyster tissue for molecular analyses of population structure. Pacific oysters have disappeared from Knysna and, following our collections, possibly also from the GouKou Estuary. Between 2003 and 2012, the Breede Estuary population decreased by 87%, from an estimated 184 206 to 23 760 individuals. Within this estuary, oysters 12 km upriver had denser shells and higher body condition indices than did those within 1.4 km of the river mouth, presumably reflecting higher availability of suspended organic matter. However, low salinity over most of the species’ range in the estuary probably inhibits recruitment. New populations of Pacific oysters in the Swartkops and Kaaimans estuaries urgently require monitoring and eradication. Haplotype (h) and nucleotide (π) diversities across all oyster populations sampled (h = 0.2300 [SD 0.0595], π = 0.0006 [SD 0.0007]) were lower than those of co-occurring indigenous Cape rock oysters Striostrea margaritacea from the GouKou and Breede estuaries (h = 0.9076 [SD 0.0386], π = 0.00589 [SD 0.00347]). Pacific oysters either have been introduced to South African estuaries infrequently, or have experienced genetic bottlenecks following river floods or human exploitation, or both. Populations growing outside culture infrastructure are restricted to estuaries in the Eastern and Western Cape provinces, with no evidence of occurrence in fully marine shelf environments. Given the species’ considerable socio-economic importance, estuarine and coastal surveillance coupled with aquaculture zoning are required to integrate biodiversity and food security considerations.     

Modelling the influence of environmental factors on the physiological status of the Pacific oyster Crassostrea gigas in an estuarine embayment; The Baie des Veys (France)

It is well known that temporal changes in bivalve body mass are strongly correlated with temporal variations in water temperature and food supply. In order to study the influence of the year-to-year variability of environmental factors on oyster growth, we coupled a biogeochemical sub-model, which simulates trophic resources of oysters (i.e. phytoplankton biomass via chlorophyll a), and an ecophysiological sub-model, which simulates growth and reproduction (i.e. gametogenesis and spawning), using mechanistic bases. The biogeochemical sub-model successfully simulated phytoplankton dynamics using river nutrient inputs and meteorological factors as forcing functions. Adequate simulation of oyster growth dynamics requires a relevant food quantifier compatible with outputs of the biogeochemical sub-model (i.e. chlorophyll a concentration). We decided to use the phytoplankton carbon concentration as quantifier for food, as it is a better estimator of the energy really available to oysters. The transformation of chlorophyll a concentration into carbon concentration using a variable chlorophyll a to carbon ratio enabled us to improve the simulation of oyster growth especially during the starvation period (i.e. autumn and winter). Once validated, the coupled model was a suitable tool to study the influence of the year-to-year variability of phytoplankton dynamics and water temperature on the gonado-somatic growth of the Pacific oyster. Four years with highly contrasted meteorological conditions (river inputs, water temperature and light) 2000, 2001, 2002 and 2003, were simulated. The years were split into two groups, wet years (2000 and 2001) and dry years (2002 and 2003). Significant variability of the response of oysters to environmental conditions was highlighted between the four scenarios. In the wet years, an increase in loadings of river nutrients and suspended particulate matter led to a shift in the initiation and the magnitude of the phytoplanktonic spring bloom, and consequently to a shift in oyster growth patterns. In contrast, in the dry years, an increase in water temperature—especially during summer—resulted in early spawning. Thus, the gonado-somatic growth pattern of oysters was shown to be sensitive to variations in river loadings and water temperature. In this context, the physiological status of oysters is discussed using a relevant indicator of energy needs.     

Terrestrial n-alkane signatures in the middle Okinawa Trough during the post-glacial transgression: control by sea level and paleovegetation confounded by offshore transport

We report records of land plant-derived long-chain n-alkanes since 37 ka b.p. from a sediment core in the middle Okinawa Trough, East China Sea. The data show the content and carbon preference index (CPI; degree of freshness) of n-alkanes generally decreasing as sea level rose, which could be explained by coastline retreat resulting in an increased transport route of n-alkanes toward the study site. The n-alkane CPI returned to higher values during the Holocene highstand, however, suggesting sea-level rise was not the only cause for the decline of freshness of n-alkanes. Paleovegetation changes in terms of C₃ vs. C₄ contributions inferred from n-alkane δ¹³C are overall consistent with published marine and terrestrial records during two distinct intervals: from 37.0 to 15.2 ka b.p., and from 7.6 ka b.p. to the present. However, n-alkane δ¹³C excursions from 15.2 to 7.6 ka b.p. are difficult to reconcile with terrestrial signatures. This disagreement, along with other possible causes for the decline of freshness of n-alkanes, and the higher-energy sedimentary environment inferred from increased mean grain sizes and silt/clay ratios during this time period, is consistent with existing knowledge of offshore transport of materials previously stored on the extensive continental shelf during the post-glacial transgression. We therefore suggest that n-alkane records from the Okinawa Trough should be used only cautiously to infer deglacial vegetation and sea-level changes.     

合浦珠母贝代谢产物对异枝麒麟菜生长的促进作用

于１９９５年４月间在海南省三亚市、陵水县分别采集合浦珠母贝和异枝麒麟菜并进行混养实验，测定贝释放的三氮（ＮＨ４－Ｎ，ＮＯ３－Ｎ，ＮＯ２－Ｎ）以及藻类对贝释放的三氮的吸收及其生长，在期探讨合浦珠母贝代谢产物促进异枝麒麟菜生长的原因。结果表明，贝释放物中以ＮＨ４－Ｎ为主，ＮＯ３－Ｎ次之，ＮＯ２－Ｎ含量较少；藻对三氮的吸收 依此顺序递减。实验组藻体的生长速率明显高于对照组的，在模拟实验组中，通过加入无机     

Reductions in TBT concentrations in UK estuaries following legislation in 1986 and 1987

In 1986 the Government of the United Kingdom (UK) introduced legislation to control the sale of tribytltin (TBT)-based paints. From 1986 to 1989 monitoring was undertaken, whereby samples of water, sediment, oysters (Crassostrea gigas) and mussels (Mytilus edulis) were collected from six estuaries and water samples from five marinas/harbours, and analysed for TBT. In 1989 the concentrations of TBT in water, oysters and mussels were generally only one-third to one-quarter of those observed in 1986. The most marked decrease in concentrations of TBT in oysters and mussels occurred in 1987–1988, following the 1987 ban on the use of TBT-based paints on small boats and mariculture equipment. As the concentrations of TBT in oyster tissues had decreased there has been improvement in oyster growth in terms of both meat production and growth of thin (i.e. normally shaped) shells. Oysters with a normal shell shape can now be grown in five of the six estuaries which were monitored. In contrast to the reduction in concentrations of TBT noted in water and bivalves, changes in concentrations of TBT in sediments showed no clear trend from 1986 to 1989.     

Effects of polynuclear aromatic hydrocarbons on fishes and shellfish: An overview of research in virginia

This paper discusses the following topics related to polynuclear aromatic hydrocarbon pollution in estuaries: (1) the use of oysters (Crassostrea virginica), hard clams (Mercenaria mercenaria) and brackish water clams (Rangia cuneata) in residue monitoring; (2) the effects of elevated PAH residues on oyster condition; (3) three years of field studies relating PAH sediment contamination to abnormalities in fishes; and (4) laboratory bioassays for effects and bioconcentration modeling. Oysters, hard clams and Rangia have been shown, from 3 years of field studies, to be good monitors of pollution inputs as one proceeds along salinity gradients from 25‰ to 0·5‰. Effects of increased body burdens of PAHs are shown by a lowering of the oysters' condition index, as measured by lipid levels. Fishes inhabiting the Elizabeth River, VA, which is highly contaminated with PAHs, have abnormalities (cataracts, skin lesions, abnormal fins, etc.). These abnormalities have higher incidence in regions of the river where the sediments are more heavily contaminated. Laboratory studies utilizing contaminated sediments have reproduced some of the abnormalities observed in the field. Bioconcentration of PAHs from sediments has been studied with oysters and hard clams. Oysters generally accumulate three times the body burdens of clams exposed to the same suspensions. A faster depuration rate (k₂) for clams appears to be responsible for the higher equilibrium body burden of oysters.     

Differential recruitment of introduced Pacific oysters and native mussels at the North Sea coast: coexistence possible?

Pacific oysters (Crassostrea gigas Thunberg 1793) have been introduced into the Wadden Sea (North Sea), where they settle on native mussel beds (Mytilus edulis L.), which represent the only extensive insular hard substrata in this soft-sediment environment. As abundances of C. gigas rose, some mussel beds became increasingly overgrown with oysters, whereas others did not. Field experiments revealed that recruitment of C. gigas was higher in the lower intertidal than in the upper subtidal zone, that it was higher on conspecifics than on mussels, and that it was not affected by barnacle epigrowth except when settling on mussels. Mussel recruitment is known from inter- and subtidal zones. It occurred equally on oyster and mussel shells but showed a clear preference for barnacle epigrowth over clean shells. Assuming that settlement and recruitment are key processes for species abundances on the North Sea coast, it is predicted that the positive feedback in oyster settlement will lead to rapid reef formation of this invader at the expense of mussel beds. Mussels, however, may escape competitive exclusion by settling between or on the larger oysters especially when barnacles are abundant. Experimental patches with mussels were more often covered by fucoid algae (Fucus vesiculosus forma mytili Nienburg) than patches with oysters, and oyster recruitment was poor underneath such algal canopies. Thus, fucoids may provide the native mussels with a refuge from the invading oysters and the two bivalves may coexist, provided food is not limiting.     

Facilitative effects of introduced Pacific oysters on native macroalgae are limited by a secondary invader,the seaweed Sargassum muticum

Introduced habitat-providing organisms such as epibenthic bivalves may facilitate the invasion and expansion of further non-native species which may modify the effects of the primary invader on the native system. In the sedimentary intertidal Wadden Sea (south-eastern North Sea) introduced Pacific oysters (Crassostrea gigas) have overgrown native blue mussel beds (Mytilus edulis). These oyster beds are now providing the major attachment substratum for macroalgae. Recently, oysters have expanded their distribution into the shallow subtidal zone of the Wadden Sea, and there support a rich associated species community including the Japanese seaweed Sargassum muticum, which has been presumably introduced together with the oysters. With a block designed field experiment, we explored the effects of S. muticum on the associated community of soft-bottom C. gigas beds in the shallow subtidal. Replicated oyster plots of 1 m² were arranged with a density of 0, 7, 15 or 45 S. muticum m^? 2, respectively. We found no effects of different S. muticum densities on associated epi- and endobenthic community compositions associated to the oyster plots. However, the overall coverage of sessile organisms settling on the oyster shells was significantly reduced at high S. muticum densities. The occurrence of abundant native macro-algal species such as Polysiphonia nigrescens, Antithamnion plumula and Elachista fucicola decreased with increasing S. muticum densities. Sessile invertebrates, by contrast, were only marginally affected and we found no effects of S. muticum canopy on diversity and abundance of endofauna organisms. We conclude that increasing densities of S. muticum on C. gigas beds in the shallow subtidal zone of the Wadden Sea limit the occurrence of native macroalgae which otherwise would benefit from the additional hard substratum provided by the oysters. Thus, a secondary invader may abolish the effects of the primary invader for native species by occupying the new formed niche.     

Origin of rippled scour depressions associated with cohesive sediments in a shoreface setting (eastern Bay of Seine, France)

Side-scan sonar investigations in the eastern part of the macrotidal Bay of Seine have revealed the presence of numerous rippled scour depressions (RSDs) at water depths of 5–9 m. The sediments in these depressions consist essentially of coarse-grained shell hash derived from underlying Holocene sediments dated at roughly 6,500 years BP, and arranged in large wave-generated ripples. The shallow marine area where these features occur consists of a wave-generated ravinement surface produced during the marine flooding of the late Holocene transgression. It can be shown that, during the last 20 years at least, erosion of the muddy sand and sandy seabed has exposed underlying relict sediments. These consist of stiff clays, silts and a layer of shell debris which, when exposed, cover the bottom of large scour depressions which appear to be in equilibrium with the local hydrodynamic regime. Morphological and hydrodynamic data suggest that the RSDs are generated by strong cross-shore bottom currents flowing parallel to the features in the direction of the prevailing waves, and probably associated with storm-induced downwelling events.     

Esterification of vertebrate-like steroids in the eastern oyster (Crassostrea virginica)

The esterification of two model vertebrate steroid hormones – estradiol (E2) and dehidroepiandrosterone (DHEA) – was studied in the oyster Crassostrea virginica. The activity of acyl-CoA:steroid acyltransferase was characterized in microsomal fractions isolated from oyster digestive glands. The apparent K_m and V_max values changed with the fatty acid acyl-CoA used (C20:4, C18:2, C18:1, C16:1, C18:0 or C16:0), and were in the range of 9–17 μM, and 35–74 pmol/min/mg protein for E2, and in the range of 45–120 μM, and 30–182 pmol/min/mg protein for DHEA. Kinetic parameters were also assessed in gonadal tissue. The enzyme saturated at similar concentrations, although conjugation rates were lower than in digestive gland. Preliminary data shows that tributyltin (TBT) in the low μM range (1–50) strongly inhibits E2 and DHEA esterification, the esterification of E2 being more sensitive to inhibition than that of DHEA. Overall, results indicate that apolar conjugation occurs in oysters, in both digestive gland and gonads, at a very similar rate to mammals, suggesting that this is a well conserved conjugation pathway during evolution. Esterification, together with other mechanisms, can modulate endogenous steroid levels in C. virginica, and might be a target for endocrine disrupters, such as TBT.     

Variability of organic δC and C/N in the Mersey Estuary, U.K. and its implications for sea-level reconstruction studies

Microfossil analysis (e.g. diatoms, foraminifera and pollen) represents the cornerstone of Holocene relative sea-level (RSL) reconstruction because their distribution in the contemporary inter-tidal zone is principally controlled by ground elevation within the tidal frame. A combination of poor microfossil preservation and a limited range in the sediment record may severely restrict the accuracy of resulting RSL reconstructions. Organic δ¹³C and C/N analysis of inter-tidal sediments have shown some potential as coastal palaeoenvironmental proxies. Here we assess their viability for reconstructing RSL change by examining patterns of organic δ¹³C and C/N values in a modern estuarine environment. δ¹³C and C/N analysis of bulk organic inter-tidal sediments and vegetation, as well as suspended and bedload organic sediments of the Mersey Estuary, U.K., demonstrate that the two main sources of organic carbon to surface saltmarsh sediments (terrestrial vegetation and tidal-derived particulate organic matter) have distinctive δ¹³C and C/N signatures. The resulting relationship between ground elevation within the tidal frame and surface sediment δ¹³C and C/N is unaffected by decompositional changes. The potential of this technique for RSL reconstruction is demonstrated by the analysis of part of an early Holocene sediment core from the Mersey Estuary. Organic δ¹³C and C/N analysis is less time consuming than microfossil analysis and is likely to provide continuous records of RSL change.     

Large-scale environmental influences on the benthic macroinfauna of the southern Gulf of Mexico

The influence of large-scale natural disturbance from winter storms (‘northers’) and river runoff on the macrobenthic community structure of the southern Gulf of Mexico was investigated in both carbonate and transitional carbonate–terrigenous sedimentary environments. Samples of the infauna were obtained in three seasons from 13 stations from two 250 km transects along 80–170 and 20–50 m water depth. Samples after the northers season had the lowest total number of families and individuals, 114 and 2940, respectively, compared to the dry and rainy seasons with 129 and 132 families and 11580 and 15266 individuals, respectively. Spatial patterns of macroinfauna composition varied across and along the shelf as a response to sedimentary environments and depth. Coarser sediments from the carbonate area harboured the highest mean densities per station with 500–24,000 individuals m⁻² and 108–122 families in total, compared to the transitional sediment with 500–8200 individuals m⁻² and 56–74 families across the three seasons. Univariate and multivariate statistical techniques demonstrated that low densities and number of taxa were associated with winter storms, but storm influence was dependent on depth and sediment type. Multiple linear regression analysis and BIOENV analysis indicated that sediment mean grain size, percentage of clay and organic matter best explained the macroinfauna spatial patterns, although BIOENV indicated that depth has an overriding role. An increase in densities of opportunistic taxa (numerous polychaetes of small sizes) was observed four months after the ‘northers’ and this was more evident in the area of carbonate sediment. Additionally a combined disturbance from northers and river runoff is suspected to be responsible for a naturally impoverished macroinfauna community in the transitional sedimentary environment.