Marine Embayment Research Articles

Historic development of heavy metal contamination into the Firth of Thames, New Zealand

Near-coastal marine sediments often provide high-resolution records of various anthropogenic influences such as the release of heavy metals, which pose a potentially negative influence on aquatic ecosystems because of their toxicity and persistence. In places, the gradual onset of man-made heavy metal emission dates back to ~ 4500 years BP and is difficult to distinguish from potential natural sources. New Zealand offers a perfect setting for studies on anthropogenic impact due to its well-defined three-step development: pre-human era (until ~ 1300 CE), Polynesian era (~ 1300–1800 CE) and European era (since ~ 1840 CE). However, hardly any information exists about the degree of heavy metal input to New Zealand’s coastal areas and the ‘pristine’ natural background values. This study determines the natural background contents of lead (Pb) and zinc (Zn) in marine sediments of the Firth of Thames, a shallow marine embayment on New Zealand’s North Island, and investigates anthropogenic inputs in historic times. Eight sediment cores were analysed by X-ray fluorescence (XRF) for their element composition and temporally resolved by a pollen and radiocarbon-based stratigraphic framework. Sharp increases in Pb and Zn contents occurred simultaneously with the onset of goldmining activities (1867 CE) in the nearby catchment area. The contents of Zn (Pb) increase from very stable values around 60 (13) ppm in the older sediments, interpreted to reflect the natural background values, to an average maximum of 160 (60) ppm near the core top, interpreted to reflect a significant anthropogenic input. These findings unravel the history of contamination in the Firth of Thames and provide an urgently needed database for the assessment of its current ecological state.

Palaeoenvironmental interpretation of the Pliocene fan-delta system of the Vera Basin (SE Spain): Fossil assemblages, ichnology and taphonomy

The marine fossil assemblages of the Pliocene of south Spain constitute the record of the marine fauna that colonised the western part of the Mediterranean after the Messinian Salinity Crisis. This work focuses on the analysis of lithofacies and fossil assemblages including trace fossils, invertebrates, and vertebrates with special attention to taphonomic features, for interpreting palaeoenvironmental conditions in the Vera Basin (SE Spain). The sedimentary sequences of the northern region of the Vera Basin display diverse stratigraphical, sedimentological, and palaeontological features that correspond to the evolution of a fan-delta in a narrow basin. The Vera Basin was characterised by shallow-marine shelf conditions during the early-mid Pliocene (Cuevas Formation). The basin emergence with the development of Gilbert-type fan deltas (Vera Member), and a protected, partially-enclosed marine embayment (Almanzora Member) occurred during the mid-late Pliocene due to regional uplift and movements of the Palomares strike-slip Fault along the eastern basin margin. The progradation of the central fan-delta lobes and the interaction with marginal fan-delta resulted in the partitioning of the basin that formed a small sub-basin with restricted stagnant conditions that favoured a Konservat Fossil-Lagerstätte. The high input of siliciclasts due to the uplift context of the margins of the basin favoured a high sedimentation rate and the fast burial of vertebrate remains. Fossil marine mammals occurs from shallow shelf deposits (Cuevas Formation) to bottomset (Cuevas Formation-Vera Member transition) and lower part of the clinoforms in the foreset (Vera Member). Cetacean remains are usually recorded not only in the relatively deep-water silty marls and sandy marls of the outer shelf and distal facies of the fan-delta (Espiritu Santo Formation), but also in the shallower coarse sands and conglomerates (Cuevas Formation and Espiritu Santo Formation). Sirenian remains, in contrast, are only recorded in coarse sand facies (Cuevas Formation and Espiritu Santo Formation) associated to charcoal wood fragments deposited in shallow waters near the shoreline. This narrow and relatively protected basin is interpreted as an area of reproduction and nursery of juveniles on the basis of the presence of cetaceans.

Coupling remote sensing data with in-situ optical measurements to estimate suspended particulate matter under the Evros river influence (North-East Aegean sea, Greece)

ABSTRACTMonitoring the riverine output of Suspended Particulate Matter (SPM) distribution in marine embayment is a crucial factor for the water quality of neighbouring coastal regions. This study presents satellite-derived SPM calculations against in-situ measurements in the continental shelf of North-East Aegean surrounding the transboundary Evros river mouth. Surface SPM, Inherent Optical Properties (IOPs) and remote sensing reflectance (Rrs) data were collected in a field campaign during low river discharge period (June 2016). The relationship between the optical backscattering coefficient (bbp) and the in-situ SPM concentrations was investigated. Subsequently, an empirical single band model was applied for estimating SPM concentrations by using the Landsat-8 Operational Land Imager (L8/OLI) red band and the model was then locally tuned within the study area. Furthermore, a multi-band SPM-retrieval algorithm was developed using the in-situ surface reflectance Rrs for calibration and it was validated using the Leave-One-Out Cross Validation technique (LOOCV). The relationship between in-situ SPM and backscattering coefficient values showed good proportionality, thus, nominating the predominance of terrestrial mineral particles. Validation against field measurements indicated that the SPM concentrations derived from the newly-developed multi-band algorithm had an improved significance correlation (96%), compared to both the single band model (not-tuned) (coefficient of determination, R2 = 0.82) and its locally tuned version (R2 = 0.83). Most importantly, the generated multi-band model apart from exhibiting the best performance (R2 = 0.93), it revealed high SPM spots which were not detected by the locally tuned single band model, indicating additional processes originating from river outflows, coastal erosion and subaqueous thermal springs in the area. In contrast, the locally tuned single band model overestimated SPM values in offshore waters, where low concentrations are encountered under the influence of the clear Black Sea Water (BSW).

Sedimentation in a synclinal shallow‐marine embayment: Coniacian of the North Sudetic Synclinorium, SW Poland

AbstractThis study is a reconstruction of the Coniacian palaeogeographic and palaeoenvironmental development in the North Sudetic Basin, a synclinal trough within the Late Cretaceous Central European seaway linking the Boreal and Tethyan marine provinces. The basin formed as an early side effect of the Alpine orogeny combined with the mid‐Cretaceous eustasy, and crucial stages of its evolution occurred during the Coniacian. The basin in the early Coniacian was a long and narrow shallow‐marine embayment with a hypothetical (non‐preserved) bayhead strait funnelling tidal currents. Coalescing tidal sand ridges formed a littoral platform that prograded from the bayhead zone along the basin axis, impinged on laterally by the basin‐margin shoreface and local river deltas. A mid‐Coniacian forced marine regression and closure of the bayhead strait, attributed to the Alpine tectonism combined with eustasy, brought about a dramatic change in the basin, whereby the basin‐wide littoral sand platform emerged and turned briefly into a denudated coastal plain. The late Coniacian eustatic marine transgression formed an in‐place growing coastal sand barrier at the outer edge of the former littoral platform, sheltering a paralic limno‐lagoonal plain with peat‐forming mires. The coastal barrier was eventually drowned by the sea and maximum marine flooding occurred, followed by a normal regression recorded as a rapidly upwards‐shallowing succession of offshore‐transition to fluvio‐deltaic deposits. This case study of the sedimentation pattern in an evolving, tectonically controlled marine embayment contributes to the existing facies models for estuarine embayments formed by a passive marine drowning of large fluvial or glacial valleys.

Tidal strait to embayment: Seismic stratigraphy and evolution of a rock-bounded embayment in the context of Holocene sea level change

Elongate, rock-framed Santa Catarina Island partly encloses a low-energy marine embayment that communicates with the ocean via narrow, rock-bounded inlets with ebb and flood-tide deltas. The bay shoreline consists of low-energy sandy beaches, strandplains, mangroves and bayhead deltas while the exposed Atlantic shoreline comprises a series of headland-embayment littoral cells in a linked, north-directed longshore drift system.New and previously published seismic stratigraphic investigations enable assessment of the palaeogeographic/geomorphic evolution of the area since the early Holocene. At a sea level of ca. −50 m a sandy mainland-attached shoreline complex (dune and beach) developed on a coastal plain setting some 6–7 km offshore of the present shoreline. North of the island, a barrier island and tombolo were formed in a coastal re-entrant adjacent to the northern entrance to Florianopolis Bay. The open coast beach/dune system was overstepped by rapid sea-level rise during MWP-1B whereas the barrier island and tombolo continued to accrete until MWP-1C when they too were overstepped.The back-island embayment contains up to 40 m of sediment overlying an irregular bedrock surface. It was initially flooded during MWP-1C and the early muddy sedimentation suggests a distal, sheltered marine embayment. The subsequent development of many large bioherms on bedrock and mud suggests a change to more fully marine conditions as sea-level continued to rise. In the North Bay, these were subsequently enveloped by tidal dunes migrating north from the mid-bay. The vertical stacking of 3-D and 2-D sandy bedforms suggests that a tidal strait had developed with strong tidal currents radiating from a constriction in the vicinity of modern Florianopolis City. The strong tidal currents may be linked to the Holocene sea-level high that is well documented for this region.A fall in sea level from this highstand, coupled with accumulation of tidal strait sediments, reduced the tidal prism and strength of tidal currents in the bay. At the same time, regressive beachridge plains developed in open coast and bay environments. This coincided with development of the contemporary tidal inlet complexes at the north and south outlets of the bay. As a consequence, the upper surface of the tidal strait sand deposits experienced tidal ravinement and the bay switched to a sheltered back-barrier environment dominated by deposition of mud and silt which blanket the contemporary bay floor. The tidal inlets sequestered sand from the littoral drift system in ebb and flood-tide deltas, which created breaks in the longshore drift system, causing littoral sediment to accumulate at the north of the island in a series of spits, and promoting strandplain development south of the island.

Deglaciation chronology of the Donegal Ice Centre, north‐west Ireland

ABSTRACTDuring the Last Glacial Maximum, Donegal in north‐west Ireland functioned as an independent centre of ice dispersal that separated and fed into the Donegal Bay Ice Lobe (sourced in the Irish Midlands) to the south and the Hebrides/Malin Sea Ice Stream to the north. We report geochronological data that demonstrate marked contrasts in the timing and rate of deglaciation in northern and southern Donegal. In northern Donegal, which occupied an inter‐ice‐stream/lobe location, decoupling from the Hebrides/Malin Sea Ice Stream resulted in formation of a marine embayment along the north coast by ∼22–21 ka, and subsequent slow (∼4 ± 1 m a−1) climatically driven inland retreat of the ice margin to mountain source areas by ∼17 ka. By contrast, in southern Donegal, which lay near the axis of the Donegal Bay Ice Lobe, deglaciation was delayed until ∼18 ka following readvance of ice to a moraine in outer Donegal Bay. The ice margin subsequently underwent net retreat, apparently uninterrupted by readvances, at a net rate of ∼ 18 ± 6 m a−1. A mean terrestrial cosmogenic nuclide age of ∼15.0 ka obtained for samples from the foothills of the Blue Stack Mountains in south‐east Donegal indicates that ice persisted in valley heads and cirques at the beginning of the Lateglacial Interstadial, suggesting that these and nearby mountains supported the last remnants of the Irish Ice Sheet before complete deglaciation of Ireland, and that almost all the shrinkage of the ice sheet in this sector occurred under stadial conditions before the onset of interstadial warming at ∼14.7 ka.

Environmental extremes drive population structure at the northern range limit of Atlantic salmon in North America.

Conservation of exploited species requires an understanding of both genetic diversity and the dominant structuring forces, particularly near range limits, where climatic variation can drive rapid expansions or contractions of geographic range. Here, we examine population structure and landscape associations in Atlantic salmon (Salmo salar) across a heterogeneous landscape near the northern range limit in Labrador, Canada. Analysis of two amplicon-based data sets containing 101 microsatellites and 376 single nucleotide polymorphisms (SNPs) from 35 locations revealed clear differentiation between populations spawning in rivers flowing into a large marine embayment (Lake Melville) compared to coastal populations. The mechanisms influencing the differentiation of embayment populations were investigated using both multivariate and machine-learning landscape genetic approaches. We identified temperature as the strongest correlate with genetic structure, particularly warm temperature extremes and wider annual temperature ranges. The genomic basis of this divergence was further explored using a subset of locations (n=17) and a 220K SNP array. SNPs associated with spatial structuring and temperature mapped to a diverse set of genes and molecular pathways, including regulation of gene expression, immune response, and cell development and differentiation. The results spanning molecular marker types and both novel and established methods clearly show climate-associated, fine-scale population structure across an environmental gradient in Atlantic salmon near its range limit in North America, highlighting valuable approaches for predicting population responses to climate change and managing species sustainability.

The ‘tunnel’ valleys of East Anglia and Fenland, UK: subaerial, not subglacial

Integrated field surveys, continuously cored boreholes and seismic-reflection surveys in Fenland and the adjacent marine embayment of The Wash have revealed a network of concealed valleys. These are interpreted as a pre-glacial, cold-climate fluvial system that entered the North Sea via a gap in the Chalk escarpment at the mouth of The Wash. The pre-glacial Fenland valley system was overridden by an ice sheet during the Anglian Stage (Marine Isotope Stage (MIS) 12) when it became infilled with glacial deposits, mostly till and varved clay. East of the Chalk escarpment, a pre-glacial network of dipslope valleys drained directly to the North Sea. This was also overridden by Anglian ice and locally over-deepened, presumably by subglacial meltwater. The over-deepened parts have been referred to as tunnel valleys by various authors, but they can be more accurately described as glacially modified fluvial valleys. The analysis of high-resolution seismic-reflection profiles in the North Sea and adjacent areas has revealed complex anastomosing networks of tunnel valleys that were formed subglacially without a fluvial precursor or superimposed on possible fluvial systems that were glacially modified to the extent that they cannot now be confidently identified as fluvial in origin. In comparison, the concealed fluvial valleys of East Anglia and Fenland have been subject to minor glacial modifications, and are morphologically distinct from the North Sea examples. They should not be referred to as tunnel valleys. Prior to the Anglian glaciation, the rivers that flowed from the Midlands to the North Sea passed through one of two gaps in the Chalk escarpment, the Goring Gap in the south and a similar gap that is now concealed beneath the Wash. The field evidence does not support the presence of an additional gap in the intervening Bury St Edmunds area. A wide embayment to the north of this in the Thetford to Swaffham area in which the crest of the Chalk escarpment has been displaced eastwards by up to 20 km is an Anglian glaciation erosion feature.

Sedimentary responses to Holocene sea-level change in a shallow marine environment of southern China

This study has examined how the sedimentary processes in the prevalent non-deltaic coastal marine environments along the southeast coast of China have responded to sea-level change and other environmental drivers during the Holocene. Three sediment cores collected from a shallow marine embayment of the Lantau Island, Hong Kong, on the northern coast of the South China Sea, were analyzed. The results of particle size, total organic carbon content, organic carbon and nitrogen ratios, organic carbon stable isotope ratios, abundance of alkenones and radiocarbon dating of these cores have helped reconstruct the sedimentary history of this site. This history shares the similarity in sedimentary processes within the deltaic environment for the early Holocene, because palaeo-incised channels exist in both environments. The first 1500 years of deposition involved rapid channel infill caused by the rapidly rising sea level and strong monsoon-driven discharge. This was followed by a period of decelerated sedimentation under strong marine influence as the sea flooded the wide accommodation space in the subsequent two millennia. Major differences between the two environmental settings occur in the past 7000 years: vertical accretion was seen in the non-deltaic coastal environment, whilst lateral shoreline advance was the dominant process in the deltaic basin. The progressively slowing down in sedimentation rate in the non-deltaic site during the middle and late Holocene was caused by the reduced monsoon discharge, intensified disturbance from tropical cyclone activity and coastal currents.

Molecular systematics and historical biogeography of the genus Gerrhonotus (Squamata: Anguidae)

AbstractAimMultiple geological and climatic events have created geographical or ecological barriers associated with speciation events, playing a role in biological diversification in Mexico. Here, we evaluate the influence of Neogene geological events and of Pleistocene climate change in the diversification of the genus Gerrhonotus using molecular dating and ancestral area reconstruction.LocationMexico and south‐central United States.MethodsA multilocus sequence dataset was generated for 86 individuals of Gerrhonotus from most Mexican biogeographical provinces and belonging to five of the seven currently recognized species, as well as two putative undescribed species. Phylogeographical structure was explored using Poisson‐Tree‐Processes molecular species delimitation. Divergence events were estimated based on the fossil record using a relaxed uncorrelated lognormal clock. Ancestral areas were estimated at divergence events across the tree using a probabilistic Bayesian approach.ResultsExtensive geographical structure was evident within three well‐supported clades. These clades probably diverged from each other in the early to mid‐Miocene, and their divergence was followed by six divergences in the late Miocene and eight divergences in the Pliocene. The ancestral origin of Gerrhonotus with keeled dorsal scales (keeled‐scale Gerrhonotus) was reconstructed to be across the Pacific Coast Province. Our phylogenetic analyses did not support the monophyly of Gerrhonotus.Main conclusionsMiocene and Pliocene geomorphology, perhaps in conjunction with climate change, appears to have induced allopatric divergence on a relatively small spatial scale in this genus. The late Miocene–Pliocene reduction in the highlands along the Tehuantepec fault probably created a large marine embayment that led to an early divergence in a clade of Gerrhonotus. Our analysis suggests uplifting of the Trans‐Mexican Volcanic Belt during this same time period resulted in additional diversification. This was followed by more recent, independent colonization events in the Pliocene from the Mexican Plateau to the Sierra Madre Oriental, Sierra Madre Occidental, Tamaulipas and Edwards Plateau provinces. A genus Gerrhonotus with the keeled‐scale species in addition to Coloptychon rhombifer (= G. rhombifer) is strongly supported. Inclusion of the smooth dorsal‐scale species in the genus is uncertain and maintained only tentatively.

Mapping a coastal transition in braided systems: an example from the Precipice Sandstone, Surat Basin

ABSTRACTThe Precipice Sandstone is traditionally interpreted as a braided fluvial deposit that transitions upwards into meandering channel deposits responding to a rise in base level that eventually deposits the overlying alluvial to lacustrine Evergreen Formation. This study found sedimentary evidence of tidal to marine influence within the Precipice Sandstone coincident with avulsion and diversion of the system from southward to northward-flowing channels as the system was transgressed. The north-flowing channels are interpreted to debouch into a shallow restricted marine embayment with tide and wave influence, which provides an alternative insight into this unit and suggests a Lower Jurassic north or northeasterly marine connection. The Precipice Sandstone is a regional aquifer, in places hosts hydrocarbons and has been considered as a storage unit for CO2 geosequestration. Outcrop analogues can provide geometries to accompany facies interpreted from sedimentary structures that are observable in core, to assist in characterising reservoir heterogeneity.

Palynological assemblage from the Deccan Volcanic Province, central India: Insights into early history of angiosperms and the terminal Cretaceous paleogeography of peninsular India

Sedimentary deposits of the Deccan Volcanic Province (DVP), of central India are rich in fossil flora and fauna. Based on a new palynological study of Infratrappean deposits from a borehole succession located in the Ashtona village, Yeotmal District, Maharashtra (Wardha Basin), this paper attempts a reconstruction of the Late Cretaceous paleovegetation, paleoecology, depositional environment and paleogeography of central India prior to Deccan volcanism, and also seeks to evaluate the early history and timing of the angiosperm diversification relative to the Cretaceous-Paleogene (K-Pg) transition. The stratigraphic age of the Infratrappean sequence was determined on the basis of several index palynomorphs (Aquilapollenites, Jigansupollis), Azolla cretacea, Gabonisporis sp, Scollardia conferta of Maastrichtian age. Two new pollen species (Dipterocarpuspollenites cretacea sp. nov., Retiacolpites pigafettaensis sp. nov., and one new dinoflagellate species (Pierceites deccanensis sp. nov.) are described in this study. The numerical abundance of the mangrove pollen Spinizonocolpites pollen (Nypa), combined with the dinoflagellate cyst Pierceites deccanensis, indicates a brackish marine depositional setting close to low-lying coastal forests in central India during the Late Cretaceous. Furthermore, the new evidence presented here from an inland location in the Deccan province strongly suggests the existence of a shallow marine embayment in central India during the late Maastrichtian, which extended from the east coast through the Godavari rift zone, thus adding significantly to the previously described marine signatures from the Upper Cretaceous sequence of Rajahmundry area on the south-eastern coast of peninsular India. The Late Cretaceous fossil pollen of Dipterocarpaceae documented here provides compelling evidence for the evolution of these tropical rain forest elements in the insular India and their dispersal to south east Asia after the initiation of India-Asia collision, consistent with the Out of India hypothesis. The new palynomorph assemblage also has significant implications for our understanding of the floral response to the initiation of Deccan volcanism.

Holocene environmental change along the southern Cape coast of South Africa – Insights from the Eilandvlei sediment record spanning the last 8.9 kyr

Abstract This study investigates Holocene sediments from Eilandvlei, a coastal lake located within the Wilderness embayment at the southern Cape coast of South Africa. The evolution of the present estuarine/coastal lake system is reconstructed based on seismic data as well as a multi-proxy approach on a 30.5 m sediment core spanning the last ~8.9 kyr. Geochemical (Ca, TOC/S, Br/TOC) and micropalaeontological data (diatoms, foraminifera) reflect changes in the degree of marine influence at the core site. The embayment likely developed via distinct phases of connectivity to the Indian Ocean caused by sea level changes and dune progradation. Marine conditions prevailed at the core site from ~8900 to 4700 cal BP. The rapid sea level rise during the early Holocene caused the inundation of a palaeovalley that most likely had formed at lower sea levels during the Pleistocene. Towards the mid-Holocene the sea level exceeded its present height around ~7500 cal BP creating a marine embayment. At ~4700 cal BP, the embayment became distinctly more disconnected from the ocean turning into a lagoon system that persisted until ~1200 cal BP. Subsequently, the marine influence further decreased and the present estuarine/coastal lake system was established. Grain size and geochemical data (Fe, Si/Al, chemical index of alteration (CIA)) further reflect changes in the deposition of terrigenous sediments at the core site. While the sedimentation of fine-grained (

Sedimentation in isolated glaciomarine embayments during glacio-isostatically induced relative sea level fall (northern Champlain Sea basin)

The nature of glaciomarine sediments deposited during ice margin retreat can vary according to physiographic setting and relative sea level fluctuations. To understand the effects of these two parameters on sedimentation, we analyzed the sediment records of four lakes located within former isolated glaciomarine embayments of the northern Champlain Sea basin. These lakes were initially inundated by marine water of the Champlain Sea, following deglaciation, and have subsequently experienced basin isolation owing to glacio-isostatic rebound. Three of these lakes reveal a common litho- and acoustic stratigraphic succession, characterized by an IRD-free glaciomarine to marine facies consisting of homogeneous to faintly laminated clayey silts grading into well-laminated silts with rapidly deposited layers. These two units recorded the transitional environment from glaciomarine sedimentation below multiyear shorefast ice to increased terrestrial runoff and rapid glacio-isostatic rebound once the ice margin retreated inland. During ice margin retreat, relative sea level fell concomitantly resulting in the deposition of coarser sediments in marine embayments. Upon the complete retreat of the ice margin, the supply of terrestrial sediments diminished and lake isolation, driven by relative sea level fall, led to higher biogenic content and increased bioturbation. This study provides a framework for sedimentation in isolated glaciomarine embayments which differs from deep-water sedimentation owing to the presence of shorefast sea-ice and their protected location from major ice-stream outlets.

Punctuated Sediment Discharge during Early Pliocene Birth of the Colorado River: Evidence from Regional Stratigraphy, Sedimentology, and Paleontology

The Colorado River in the southwestern U.S. provides an excellent natural laboratory for studying the origins of a continent-scale river system, because deposits that formed prior to and during river initiation are well exposed in the lower river valley and nearby basinal sink. This paper presents a synthesis of regional stratigraphy, sedimentology, and micropaleontology from the southern Bouse Formation and similar-age deposits in the western Salton Trough, which we use to interpret processes that controlled the birth and early evolution of the Colorado River. The southern Bouse Formation is divided into three laterally persistent members: basal carbonate, siliciclastic, and upper bioclastic members. Basal carbonate accumulated in a tide-dominated marine embayment during a rise of relative sea level between ~6.3 and 5.4Ma, prior to arrival of the Colorado River. The transition to green claystone records initial rapid influx of river water and its distal clay wash load into the subtidal marine embayment at ~5.4–5.3Ma. This was followed by rapid southward progradation of the Colorado River delta, establishment of the earliest through-flowing river, and deposition of river-derived turbidites in the western Salton Trough (Wind Caves paleocanyon) between ~5.3 and 5.1Ma. Early delta progradation was followed by regional shut-down of river sand output between ~5.1 and 4.8Ma that resulted in deposition of marine clay in the Salton Trough, retreat of the delta, and re-flooding of the lower river valley by shallow marine water that deposited the Bouse upper bioclastic member. Resumption of sediment discharge at ~4.8Ma drove massive progradation of fluvial-deltaic deposits back down the river valley into the northern Gulf and Salton Trough.These results provide evidence for a discontinuous, start-stop-start history of sand output during initiation of the Colorado River that is not predicted by existing models for this system. The underlying controls on punctuated sediment discharge are assessed by comparing the depositional chronology to the record of global sea-level change. The lower Colorado River Valley and Salton Trough experienced marine transgression during a gradual fall in global sea level between ~6.3 and 5.5Ma, implicating tectonic subsidence as the main driver of latest Miocene relative sea-level rise. A major fall of global sea level at 5.3Ma outpaced subsidence and drove regional delta progradation, earliest flushing of Colorado River sand into the northern Gulf of California, and erosion of Bouse basal carbonate and siliciclastic members. The lower Colorado River valley was re-flooded by shallow marine waters during smaller changes in global sea level~5.1–4.8Ma, after the river first ran through it, which requires a mechanism to stop delivery of sand to the lower river valley. We propose that tectonically controlled subsidence along the lower Colorado River, upstream of the southern Bouse study area, temporarily trapped sediment and stopped delivery of sand to the lower river valley and northern Gulf of California for ~200–300 kyr. Massive progradation of the fluvial-deltaic system back down the river valley into the Salton Trough starting ~4.8–4.5Ma apparently was driven by a huge increase in sediment discharge that overwhelmed the sediment-storage capacity of sub-basins along the lower river corridor and established the fully integrated river channel network.

The genetic structure of a marine teleost, Chrysophrys auratus, in a large, heterogeneous marine embayment

There is considerable interest in the processes that generate genetic divergence in marine species and the spatial and temporal scales over which these processes operate. Shark Bay, a large embayment (~13,000 km2) on the arid west coast of Australia, has been described as a focal point for genetic divergence in marine species due to its heterogeneous environment. This study represents the first DNA-based analysis of the genetic structure of a marine species with pelagic early life stages (ELS) in this region. Twelve microsatellite loci were used to compare the genetic composition of the teleost Chrysophrys auratus from five areas within Shark Bay, and from near-by shelf waters approximately 250 km to the south. The results suggest that the microsatellite composition of C. auratus is homogeneous across most of Shark Bay and near-by shelf waters. This genetic homogeneity is probably maintained via small amounts of contemporary gene flow, which may reduce the potential for C. auratus to locally adapt to the Shark Bay environment. A weakly differentiated assemblage in Freycinet Estuary in the Western Gulf of Shark Bay provided an important exception to the otherwise homogeneous microsatellite composition of C. auratus in the study area. This weak differentiation probably reflects restrictions to gene flow into and/or out of this site due to the temporal isolation of breeding adults, selective mortality of immigrants and/or the presence of hydrological barriers to larval transport. Each of these factors can be linked to environmental heterogeneity in Shark Bay.

Gypsum stromatolites from Sawda Nathil: relicts from a southern coastline of Qatar

Sawda Nathil is one of a series of inland depressions (inland sabkhas and salinas) that extend nearly continuously along the Southern Qatar border with Saudi Arabia. Six to eight thousand years ago, these depressions were marine embayments that separated the peninsula of Qatar from the mainland. These embayments in-filled rapidly with marine sediments and dune sands, blown southeastward across Qatar. Over time, they have become progressively more evaporitic. Inland depressions like Sawda Nathil host a set of unique environments within Qatar. Most depressions are close to or below sea level, bringing water near the surface in the driest parts of Qatar. Evaporation to salt saturation creates thick gypsum and halite crusts (sabkhas), as well as shallow hypersaline lakes/ponds (salinas) with spectacular domal microbial gypsum stromatolites. The present ground surface is a mosaic of relict marine facies, deflated dune sands, and inland sabkhas and salinas. Sediment of four short push cores was photographed, described, and sampled for petrographic thin-section and X-ray diffraction (XRD) analyses. Radiocarbon (AMS) as well as optically stimulated luminescence (OSL) age-dating were carried out on three samples. Scanning electron microscopy (SEM), carried out on samples from a gypsum stromatolite, was used to resolve the microbial-influence on gypsum precipitation. Radiocarbon dating of marine shells provides ages of approximately 6600 un-calibrated 14C years before present (year BP), coinciding with a well-documented sea-level highstand, approximately 2–4 m higher than present. During that time, Qatar was mostly an island, only connected in the south to the Arabian Peninsula by narrow land bridges (isthmuses). SEM examinations of gypsum stromatolites show gypsum crystals developing in close spatial association with microbial biofilms (filamentous structures). Whether this is purely a passive microbial-influenced gypsum mineralization process or an example where microorganisms actively control the gypsum crystal morphology to obtain ecological advantages, remains to be evaluated.

Vase‐shaped microfossils from the Tonian Callison Lake Formation of Yukon, Canada: taxonomy, taphonomy and stratigraphic palaeobiology

AbstractVase‐shaped microfossils (VSMs), interpreted as the remains of testate amoebae, are found in late Tonian sedimentary rocks around the world. Here we explore the taxonomy, taphonomy and stratigraphical occurrence of VSMs from the Callison Lake Formation of the Coal Creek inlier, Yukon, Canada. Found in silicified black shale horizons and stromatolitic dolostone, sedimentological data suggest these VSMs inhabited a series of marine embayments characterized by lagoonal and/or shelf interior depositional environments. The fossiliferous strata have recently been dated with Re–Os geochronology at c. 753–740 Ma, which indicates they are not only coeval with diverse VSM assemblages described in the Chuar Group of Grand Canyon, Arizona, but also provides supportive evidence for the early diversification of eukaryotic clades prior to the Sturtian age Snowball Earth event (c. 717–660 Ma). Petrographic examination of well‐preserved specimens reveal taxa comparable to those from the Chuar Group, as well as two previously undescribed species. Species overlapping with Chuar Group VSMs are Bonniea dacruchares, Bonniea pytinaia, Cycliocyrillium simplex, Cycliocyrillium torquata, Melanocyrillium hexodiadema and Palaeoarcella athanata. New taxa described here are Bonniea makrokurtos and Cycliocyrillium rootsi. Energy dispersive x‐ray spectroscopic data reveal that the Callison Lake microfossils are preserved through a variety of taphonomic pathways, including silicification, infilling, authigenic mineralization and dolomitization. We explore the utility of M. hexodiadema as a latest Tonian biostratigraphical marker and examine the role of Callison Lake sequence stratigraphy as a control on the distribution and abundance of VSMs in the Coal Creek inlier and other global sedimentary successions.

Greenhouse gases and submarine groundwater discharge in a Sydney Harbour embayment (Australia)

We investigated whether submarine groundwater discharge (SGD) traced by radon (222Rn, a natural groundwater tracer) may drive carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in surface waters in Chowder Bay, a marine embayment in Sydney Harbour, Australia. A radon mass balance revealed significant groundwater discharge rates into the bay (8.7 ± 5.8 cm d−1). The average CO2, CH4, and N2O concentrations in the subterranean estuary were 3.5, 7.2, and 2.8 times higher than the average surface water concentrations, indicating the possibility of coastal groundwater as a source of greenhouse gases to the bay. SGD-derived fluxes of greenhouse gases were 5.02 ± 2.28 mmol m−2 d−1, 5.63 ± 2.55 μmol m−2 d−1, and 1.72 ± 0.78 μmol m−2 d−1 for CO2, CH4 and N2O, respectively. The average CO2 evasion rate from surface water was 2.29 ± 0.46 mmol m−2 d−1 while CH4 and N2O evasion rates were 12.89 ± 3.05 and 1.23 ± 0.25 μmol m−2 d−1 respectively. Therefore, groundwater-derived greenhouse gas fluxes accounted for >100% CO2 and N2O and ∼43% of CH4 surface water evasion, indicating SGD is likely an important source of greenhouse gases to surface waters. However, this may be due to observations being performed near the SGD source, which may overestimate its contribution to the wider Sydney Harbour. Over a 20-year time frame, the combined emissions of CH4 and N2O from surface waters to the atmosphere accounted for 25% of the total CO2-equivalent emissions. Although this study gives preliminary insight into SGD and greenhouse gas dynamics in Sydney Harbour, more spatial and temporal resolution sampling is required to fully constrain these processes.

Stepped Holocene sea-level rise and its influence on sedimentation in a large marine embayment: Maputo Bay, Mozambique

The sedimentary record in large marine embayments holds important information on Holocene environmental and sea-level changes. This paper investigates the evolution of a large subtropical marine embayment, Maputo Bay, in southern Mozambique. Using a combination of sediment cores, stable isotope data, seismic reflection profiling and 14C dating, we show complex back-barrier changes during the Holocene and link these to stepped rises in sea level. Our data reveal a sea-level “slowstand” followed by an abrupt rise that we correlate with the 8.2 ka global rise in sea level. Sedimentological, isotope and seismic evidence point to a subsequent slowstand interspersed by two punctuated rises in sea level that formed clear tidal ravinement surfaces. Sedimentary changes in the embayment are primarily forced by sea-level change, whereby tidal ravinement surfaces sandwich normal regressive packages formed via marginal progradation and shallowing of the system during “slow-” or still-stands. Large marine embayment systems hold great potential in unravelling relative sea-level change. Axiomatically, this reduces their utility as archives of climate-change.