Where do coastlines stabilize following rapid retreat?

We present a numerical model that shows that the transgressing upper shoreline of wave-dominated estuaries (bayhead deltas), which commonly contain populous urban and industrial centers, stabilizes, and their rate of retreat decreases at tributary junctions. The decreased rate of retreat across a tributary junction is caused by a decrease in the total accommodation, while sediment supply remains conserved. Our model predicts that bayhead deltas from smaller systems will be located closer to tributary confluences than their larger counterparts. An examination of the modern bayhead deltas in Albemarle Sound, U.S. Atlantic Coast, reveals that bayhead deltas from smaller tributaries are located closer to tributary confluences than bayhead deltas associated with larger tributaries, supporting our model prediction. Our results highlight the importance of antecedent topography created during falling sea-levels on shaping the nature of transgression during the ensuing sea-level rise. In particular, tributary junctions act as pinning points during transgression. Key Points Bayhead deltas stabilize at tributary junctions during transgression Inherited topography impacts the nature of subsequent transgressions Smaller deltas retreat at slower rates within flooded valley networks

Bayhead deltas and shorelines: Insights from modern and ancient examples

Bayhead deltas are important components of the rock record as well as modern estuaries, hosting important hydrocarbon reservoirs and many coastal cities, ports and large expanses of wetlands. Despite their significance, few studies have summarized their occurrence and sedimentary characteristics. In this paper we review the stratigraphic, sedimentary, and geomorphic characteristics of 68 modern and ancient bayhead deltas. Bayhead deltas are found in incised valleys, structural basins, fjords, interdistributary bays of larger open-ocean deltas, and other backbarrier environments. Except for within fjords, they generally prograde into shallower and more brackish waters than their open-ocean equivalents. As a result, 80% of modern, 68% of Quaternary, and 67% of ancient bayhead deltas have clinoform thicknesses of 10 m or less with 73% of modern bayhead deltas having clinoform thicknesses of 5 m or less. Additionally, 89% of modern, 81% of Quaternary, and 77% of ancient bayhead deltas examined are fluvial dominated. We distinguish true bayhead deltas from their genetically similar bayhead shorelines, which are not constructional features but sites of enhanced marsh or estuarine sedimentation near river mouths with inadequate rates of sediment delivery to form distributary channels and prograde into the estuary or lagoon. We also distinguish confined bayhead deltas found in incised valleys, structural basins, and fjords from unconfined bayhead deltas found as incipient lobes of larger delta complexes and other back-barrier lagoons. The architecture of confined bayhead deltas is largely influenced by the limited accommodation brought about by the walls of the flooded valleys in which they are located. As such, confined bayhead-delta ontogeny is controlled by many autogenic interactions within these valley walls. Both confined and unconfined bayhead deltas are sensitive to sea-level rise, climate-controlled changes in sediment flux, and tectonics. Their relatively small size, connection with the terrestrial system, and protected nature make them the ideal depositional system to record Earth history including sea-level and climate changes

Late Holocene relative sea levels near Palmer Station, northern Antarctic Peninsula, strongly controlled by late Holocene ice-mass changes

Many studies of Holocene relative sea-level (RSL) changes across Antarctica assume that their reconstructions record uplift from glacial isostatic adjustment caused by the demise of the Last Glacial Maximum (LGM) ice sheets. However, recent analysis of GPS observations suggests that mantle viscosity beneath the Antarctic Peninsula is weaker than previously thought, which would imply that solid Earth motion is not controlled by post-LGM ice-sheet retreat but instead by late Holocene ice-mass changes. If this hypothesis is correct, one might expect to find Holocene RSL records that do not reflect a monotonic decrease in the rate of RSL fall but show variations in the rate of RSL change through the Holocene. We present a new record of late Holocene RSL change from Torgersen Island near Palmer Station in the western Antarctic Peninsula that shows an increase in the rate of relative sea-level fall from 3.0 ± 1.2 mm/yr to 5.1 ± 1.8 mm/yr during the late Holocene. Independent studies of the glacial history of the region provide evidence of ice-sheet changes over similar time scales that may be driving this change. When our RSL records are corrected for sea-surface height changes associated with glacial isostatic adjustment (GIA), the rate of post-0.79 ka land uplift at Torgersen Island, 5.3 ± 1.8 mm/yr, is much higher than the rate of uplift recorded at a nearby GPS site at Palmer Station prior to the Larsen B breakup in 2002 AD (1998-2002 AD; <0.1 mm/yr), but similar to the rates observed after 2002 AD (2002-2013 AD; 6–9 mm/yr). This substantial variation in uplift rates further supports the hypothesis that Holocene RSL rates of change are recording responses to late Holocene and recent changes in local ice loading rather than a post-LGM signal across portions of the Antarctic Peninsula. Thus middle-to-late Holocene RSL data may not be an effective tool for constraining the size of the LGM ice sheet across portions of the Antarctic Peninsula underlain by weaker mantle. In addition, current global-scale GIA models are unable to predict our observed changes in late Holocene RSL. Complexities in Earth structure and neoglacial history need to be taken into consideration in GIA models used for correcting modern satellite-based observations of ice-mass loss

Recycling sediments between source and sink during a eustatic cycle: Systems of late Quaternary northwestern Gulf of Mexico Basin

The northwestern Gulf of Mexico Basin is an ideal natural laboratory to study and understand source-to-sink systems. An extensive grid of high-resolution seismic data, hundreds of sediment cores and borings and a robust chronostratigraphic framework were used to examine the evolution of late Quaternary depositional systems of the northwestern Gulf of Mexico throughout the last eustatic cycle (~125 ka to Present). The study area includes fluvial systems with a wide range of drainage basin sizes, climate settings and water and sediment discharges. Detailed paleogeographic reconstructions are used to derive volumetric estimates of sediment fluxes (Volume Accumulation Rates). The results show that the response of rivers to sea-level rise and fall varied across the region. Larger rivers, including the former Mississippi, Western Louisiana (presumably the ancestral Red River), Brazos, Colorado and Rio Grande rivers, constructed deltas that advanced across the shelf in step-wise fashion during Marine Isotope Stages (MIS) 5-2. Sediment delivery to these deltas increased during the overall sea-level fall due to increases in drainage basin area and erosion of sediment on the inner shelf, where subsidence is minimal, and transport of that sediment to the more rapidly subsiding outer shelf. The sediment supply from the Brazos River to its delta increased at least 3-fold and the supply of the Colorado River increased at least 6-fold by the late stages of sea-level fall through the lowstand. Repeated filling and purging of fluvial valleys from ~119-22 ka contributed to the episodic growth of falling-stage deltas.During the MIS 2 lowstand (~22-17 ka), the Mississippi River abandoned its falling-stage fluvial-deltaic complex on the western Louisiana shelf and drained to the Mississippi Canyon. Likewise, the Western Louisiana delta was abandoned, presumably due to merger of the Red River with the Mississippi River, terminating growth of the Western Louisiana delta. The Brazos River abandoned its MIS 3 shelf margin delta to merge with the Trinity, Sabine and Calcasieu rivers and together these rivers nourished a lowstand delta and slope fan complex. The Colorado and Rio Grande rivers behaved more as point sources of sediment to thick lowstand delta-fan complexes.Lowstand incised valleys exhibit variable morphologies that mainly reflect differences in onshore and offshore relief and the time intervals these valleys were occupied. They are deeper and wider than falling stage channel belts and are associated with a shelf-wide surface of erosion (sequence boundary).During the early MIS 1 (~17 ka to 7~10 ka) sea-level rise, the offshore incised valleys of the Calcasieu, Sabine, Trinity, Brazos, Colorado, and Rio Grande rivers were filled with sediment. The offshore valleys of smaller rivers of central Texas would not be filled until the late Holocene, mainly by highstand mud. The lower, onshore portions of east Texas incised valleys were filled with sediment mainly during the Holocene, with rates of aggradation in the larger Brazos and Colorado valleys being in step with sea-level rise. Smaller rivers filled their valleys with back-stepping fluvial, estuarine and tidal delta deposits that were offset by flooding surfaces. In general, the sediment trapping capacity of bays increased as evolving barrier islands and peninsulas slowly restricted tidal exchange with the Gulf and valley filling led to more shallow, wider bays. A widespread period of increased riverine sediment flux and delta growth is attributed to climate change during MIS 1, between ~11.5 and 8.0 ka, and occurred mainly under cool-wet climate conditions.Relatively small sea-level oscillations during the MIS 1 transgression (~17 ka to ~4.0 ka) profoundly influenced coastal evolution, as manifested by landward stepping shorelines, on the order of tens of kilometers within a few thousand years. The current barriers, strand plains and chenier plains of the study area formed at different times over the past ~8 ka, due mainly to differences in sand supply and the highly variable relief on the MIS 2 surface on which these systems formed.Modern highstand deposition on the continental shelf formed the Texas Mud Blanket, which occurs on the central Texas shelf and records a remarkable increase in fine-grained sediment supply. This increase is attributed to greater delivery of sediments from the Colorado and Brazos rivers, which had filled their lower valleys and abandoned their transgressive deltas by late Holocene time, and to an increase in westward directed winds and surface currents that delivered suspended sediments from the Mississippi River to the Texas shelf.Collectively, our results demonstrate that source-to-sink analyses in low gradient basin settings requires a long-term perspective, ideally a complete eustatic cycle, because most of the sediment that was delivered to the basin by rivers underwent more than one cycle of erosion, transport and sedimentation that was regulated by sea-level rise and fall. Climate was a secondary control. The export of sediments from the hinterland to the continental shelf was not directly in step with temperature change, but rather varied between different fluvial-deltaic systems

A community-based geological reconstruction of Antarctic Ice Sheet deglaciation since the Last Glacial Maximum

A robust understanding of Antarctic Ice Sheet deglacial history since the Last Glacial Maximum is important in order to constrain ice sheet and glacial-isostatic adjustment models, and to explore the forcing mechanisms responsible for ice sheet retreat. Such understanding can be derived from a broad range of geological and glaciological datasets and recent decades have seen an upsurge in such data gathering around the continent and Sub-Antarctic islands. Here, we report a new synthesis of those datasets, based on an accompanying series of reviews of the geological data, organised by sector. We present a series of timeslice maps for 20 ka, 15 ka, 10 ka and 5 ka, including grounding line position and ice sheet thickness changes, along with a clear assessment of levels of confidence. The reconstruction shows that the Antarctic Ice sheet did not everywhere reach the continental shelf edge at its maximum, that initial retreat was asynchronous, and that the spatial pattern of deglaciation was highly variable, particularly on the inner shelf. The deglacial reconstruction is consistent with a moderate overall excess ice volume and with a relatively small Antarctic contribution to meltwater pulse 1a. We discuss key areas of uncertainty both around the continent and by time interval, and we highlight potential priorities for future work. The synthesis is intended to be a resource for the modelling and glacial geological community

Whole-genome sequencing reveals host factors underlying critical COVID-19

Critical COVID-19 is caused by immune-mediated inflammatory lung injury. Host genetic variation influences the development of illness requiring critical care1 or hospitalization2,3,4 after infection with SARS-CoV-2. The GenOMICC (Genetics of Mortality in Critical Care) study enables the comparison of genomes from individuals who are critically ill with those of population controls to find underlying disease mechanisms. Here we use whole-genome sequencing in 7,491 critically ill individuals compared with 48,400 controls to discover and replicate 23 independent variants that significantly predispose to critical COVID-19. We identify 16 new independent associations, including variants within genes that are involved in interferon signalling (IL10RB and PLSCR1), leucocyte differentiation (BCL11A) and blood-type antigen secretor status (FUT2). Using transcriptome-wide association and colocalization to infer the effect of gene expression on disease severity, we find evidence that implicates multiple genes—including reduced expression of a membrane flippase (ATP11A), and increased expression of a mucin (MUC1)—in critical disease. Mendelian randomization provides evidence in support of causal roles for myeloid cell adhesion molecules (SELE, ICAM5 and CD209) and the coagulation factor F8, all of which are potentially druggable targets. Our results are broadly consistent with a multi-component model of COVID-19 pathophysiology, in which at least two distinct mechanisms can predispose to life-threatening disease: failure to control viral replication; or an enhanced tendency towards pulmonary inflammation and intravascular coagulation. We show that comparison between cases of critical illness and population controls is highly efficient for the detection of therapeutically relevant mechanisms of disease

Ogof Draenen : speleogenesis of a hydrological see-saw from the karst of South Wales

Discovered in 1994, Ogof Draenen is currently the longest cave in Britain and among the thirty longest caves in the World, with a surveyed length in excess of 70km. Like other great caves, Ogof Draenen has had a complex multiphase history. This interpretation of the genesis of the cave is based on speleo-morphological observations throughout the system. Evidence of at least four phases of cave development can be identified, associated with major shifts in resurgence location and changes in flow direction of up to 180°. Joints have had a dominant influence on passage genesis. In particular joints have facilitated the development of maze networks and remarkably shallow horizontal phreatic conduits. The amplitude of these conduits is much shallower than predicted by models based on flow path length and stratal dip. Here, we suggest that presence of laterally extensive open joints, orientated perpendicular to the regional neo-tectonic principal stress field, determines the depth of flow in the aquifer, rather than fissure frequency per se as suggested in Ford’s Four State Model. We argue that the rate of base-level lowering, coupled with the depth of karstification determines whether a cave responds by phreatic capture or vadose incision. Maze cave networks within Ogof Draenen were probably initiated by bedrock-hosted sulphide oxidation and sulphuric acid speleogenesis

Balancing the last glacial maximum (LGM) sea-level budget

Estimates of post-Last Glacial Maximum (LGM) sea-level rise are not balanced by the estimated amount of ice melted since the LGM. We quantify this “missing ice” by reviewing the possible contributions from each of the major ice sheets. This “missing ice” amounts to 18.1 ± 9.6 m of global sea-level rise. Ocean expansion accounts for 2.4 ± 0.3 m of this discrepancy while groundwater could contribute a maximum of another 1.4 m to this offset. After accounting for these two potential contributors to the sea-level budget, the shortfall of 15.6 ± 9.6m suggests that either a large reservoir of water (e.g. a missing LGM ice sheet) has yet to be discovered or current estimates of one or more of the known LGM ice sheets are too small. Included within this latter possibility are potential inadequacies of current models of glacial isostatic adjustment

Local and Regional Constraints on Relative Sea-Level Changes in Southern Isle of Skye, Scotland, since the Last Glacial Maximum

New relative sea-level (RSL) data constrain the timing and magnitude of RSL changes in the southern Isle of Skye following the Last Glacial Maximum (LGM). We identify a marine limit at ~23 m OD, indicating RSL ~20 m above present c. 15.1 ka. Isolation basin data, supported by terrestrial and marine limiting dates, record an RSL fall to 11.59 m above present by c. 14.2 ka. This RSL fall occurs across the time of global Meltwater Pulse 1A, supporting recent research on the sources of ice melting. Our new data also help to resolve some of the chronological issues within the existing Isle of Skye RSL record and provide details of the sub-Arctic marine environment associated with the transition into Devensian Lateglacial climate at c. 14.5 k cal a bp, and the timing of changes in response to the Loch Lomond Stadial climate. Glacio-isostatic adjustment (GIA) model predictions of RSL deviate from the RSL constraints and reflect uncertainties in local and global ice models used within the GIA models. An empirical RSL curve provides a target for future research