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Abandoned river channels on alluvial floodplains represent areas where sediments, organic matter, and pollutants preferentially accumulate during overbank flooding. Theoretical models describing sedimentation in floodplain lakes recognize the different stages in their evolution, where the threshold for hydrological connectivity increases in older lakes as a plug‐bar develops. Sedimentary archives collected from floodplain lakes are widely used to reconstruct ecological and hydrological dynamics in riverine settings, but how floodplain lake evolution influences flow velocities and sedimentation patterns on an event scale remains poorly understood. Here we combine sediment samples collected in and around a floodplain lake with hydraulic modelling simulations to examine inundation, flow velocity, and sedimentation patterns in a floodplain lake along the Trinity River at Liberty, Texas. We focus our analyses on an extreme flood event associated with the landfall of Hurricane Harvey in August 2017 and develop a series of alternative lake bathymetries to examine the influence of floodplain lake evolution on flow velocity patterns during the flood. We find that sediments deposited in the lake after the Hurricane Harvey flood become thinner and finer with distance from the tie‐channel in accordance with simulated flow velocities that drop with distance from the tie‐channel. Flow velocity simulations from model runs with alternative plug‐bar geometries and lake depths imply that sedimentation patterns will shift as the lake evolves and infills. The integration of sediment sampling and hydraulic model simulations provides a method to understand the processes that govern sedimentation in floodplain lakes during flood events that will improve interpretations of individual events in sedimentary archives from these contexts. [ABSTRACT FROM AUTHOR]

Abstract: The Kacak Event in the Middle Devonian (Eifelian-Givetian (E-G) boundary) is a period of apparent global anoxia coincident with widespread deposition of black shale in hemipelagic, pelagic, and some [...]

Meridional shifts of the North Atlantic Subtropical High (NASH) western edge create a dipole that drives hydroclimate variability in the southeastern United States and Caribbean region. Southwest displacements suppress rainfall in the southern Caribbean. Northwest displacements drive southeast United States and northern Caribbean drying. Projections for the 21st century suggest a more meridionally displaced NASH, which jeopardizes Caribbean island communities dependent on rain‐fed aquifers. While recent work indicates that Atlantic and Pacific Ocean‐atmosphere variability influenced the NASH during the instrumental period, little is known about NASH behavior and subsequent hydroclimate responses over longer timescales. To address this limitation, we developed a ∼6000‐years long rainfall record through the analysis of calcite raft deposits archived within sediments from a coastal sinkhole in the northeast Bahamas (Abaco Island). Increased (decreased) calcite raft deposition provides evidence for increased (decreased) rainfall driven by NASH variability. We use simulations from the Community Earth System Model to support this interpretation. These simulations improve our understanding of NASH behavior on timescales congruous with the reconstruction and suggest an important role for the state of the Pacific Ocean. Furthermore, model simulations and a compilation of regional hydroclimate reconstructions reveal that the NASH‐driven dipole dominates northern and southern Caribbean rainfall on centennial timescales. These results bring Holocene Caribbean hydroclimate variability into sharper focus while providing important context for present and future changes to regional climate. Additionally, this study highlights the need for improved future predictions of the state of the Pacific Ocean to best inform water scarcity mitigation strategies for at‐risk Caribbean communities. Key Points: Centennial‐scale hydroclimate variability in the Northern Bahamas is anti‐phased with mid‐late Holocene hydroclimate over Grenada and BarbadosEvidence is presented for a Caribbean hydroclimate dipole caused by meridional displacements of the North Atlantic Subtropical HighCalcite raft sedimentation in sinkholes and caves on carbonate landscapes offers new opportunities to investigate rainfall variability [ABSTRACT FROM AUTHOR]

Coastal karst aquifers have an upper meteoric water mass stratified from a saline groundwater mass below that increasingly mix and discharge towards the ocean. Preserved sediment in caves and sinkholes provide an opportunity to evaluate the long-term hydrodynamics in the local coastal karst aquifer (e.g., salinity, oxygenation, vertical displacement in response to sea-level forcing). Here we evaluate the response of shallow (<2 m water depth) benthic foraminifera to water level variability from tidal forcing in Bermudian caves. Intertidal habitats had a higher proportion of brackish-tolerant agglutinated and hyaline taxa (e.g., Trochammina inflata , Entzia macrescens , Miliammina fusca , Helenina anderseni , and Trichohyalus aguayoi). Subtidal habitats proximal to cave entrances with higher proportion of terrestrially-derived organic carbon had an increased abundance of hyaline and infaunal taxa (e.g., Bolivina spp., Melonis barleeanum). In contrast, porcelaneous (e.g., Spirophthalmidium , Sigmoilina) and hyaline genera (e.g., Spirillina , Patellina , Mychostomina) dominated subtidal habitats deeper into the cave, where marine organic carbon sources were dominant. Lastly, similar benthic foraminiferal assemblages were determined with both >45 μm versus >63 μm mesh sieve fractions. These modern analog results can be used to interpret subfossil benthic foraminiferal records from coastal karst aquifers, especially during early cave inundation by concomitant groundwater and sea-level rise. • Recent benthic foraminifera analyzed in caves flooded by coastal karst aquifer • An intertidal assemblage develops from water table variability in caves • Two subtidal assemblages were differentiated by sediment texture and organic matter • Results provide modern analogs to help calibrate coastal karst aquifer reconstructions • No difference observed when processing foraminifera with 45 vs 63 μm sieves [ABSTRACT FROM AUTHOR]

In the mid-20th century, an inland brackish pond from Bermuda, known as Eve's Pond, was filled with marine sediment from an adjacent coastal lagoon. At this time, an eyewitness reported "...sediment billowing out of the Green Bay Cave for days...", which is a marinedominated anchialine cave located proximal to the former location of Eve's Pond (~200 m). The purpose of this study was to evaluate the potential impact of this infilling event on cave sedimentation and benthic meiofaunal communities, as proxied by the unicellular protists foraminifera that remain preserved in the sediment record. Eight sediment cores were collected from an underwater passage in Green Bay Cave in a transect towards the location where Eve's Pond was surveyed in 1901 CE. The sediment cores were analyzed for visual and density changes (photography, X-radiography), textural variability, benthic foraminifera fauna and diversity, and radiocarbon dating. The recovered sediment cores mostly sampled a late Holocene carbonate mud facies that had been described during previous research in the cave, with benthic foraminiferal assemblages post-dating the onset of seawater circulating between the saline groundwater flooding the cave and the adjacent Harrington Sound ~1,900 years ago. However, two cores located further into the cave (cores 13 and 17) contain a carbonate sand layer with lagoon foraminifera that is anomalous with respect to the Holocene depositional history of the cave and is most likely related to the mid-20th century infilling of Eve's Pond. Examination of these two cores showed that after the infilling event, the community of benthic foraminifera rapidly reverted to pre-impact assemblages with foraminiferal stygophiles (e.g., Spirophthalmidium emaciatum, Sigmoilina tenuis), which were not displaced by new colonizers introduced into the cave by the dredge spoils. We caution that the results cannot be extrapolated to the pelagic crustacean community, but the results suggest that this physical sedimentary disturbance only minimally impacted the benthic foraminifera community in the cave passages that were sampled. [ABSTRACT FROM AUTHOR]

Exceptionally well-preserved organic remains of thecamoebians (testate amoebae) were preserved in marine sediments that straddle the greatest extinction event in the Phanerozoic: the Permian-Triassic Boundary. Outcrops from the Late Permian Zewan Formation and the Early Triassic Khunamuh Formation are represented by a complete sedimentary sequence at the Guryul Ravine Section in Kashmir, India, which is an archetypal Permian-Triassic boundary sequence []. Previous biostratigraphic analysis provides chronological control for the section, and a perspective of faunal turnover in the brachiopods, ammonoids, bivalves, conodonts, gastropods and foraminifera. Thecamoebians were concentrated from bulk sediments using palynological procedures, which isolated the organic constituents of preserved thecamoebian tests. The recovered individuals demonstrate exceptional similarity to the modern thecamoebian families Centropyxidae, Arcellidae, Hyalospheniidae and Trigonopyxidae, however, the vast majority belong to the Centropyxidae. This study further confirms the morphologic stability of the thecamoebian lineages through the Phanerozoic, and most importantly, their apparent little response to an infamous biological crisis in Earth’s history. [ABSTRACT FROM AUTHOR]

How climate controls hurricane variability has critical implications for society is not well understood. In part, our understanding is hampered by the short and incomplete observational hurricane record. Here we present a synthesis of intense-hurricane activity from the western North Atlantic over the past two millennia, which is supported by a new, exceptionally well-resolved record from Salt Pond, Massachusetts ( USA). At Salt Pond, three coarse grained event beds deposited in the historical interval are consistent with severe hurricanes in 1991 (Bob), 1675, and 1635 C.E., and provide modern analogs for 32 other prehistoric event beds. Two intervals of heightened frequency of event bed deposition between 1400 and 1675 C.E. (10 events) and 150 and 1150 C.E. (23 events), represent the local expression of coherent regional patterns in intense-hurricane-induced event beds. Our synthesis indicates that much of the western North Atlantic appears to have been active between 250 and 1150 C.E., with high levels of activity persisting in the Caribbean and Gulf of Mexico until 1400 C.E. This interval was one with relatively warm sea surface temperatures (SSTs) in the main development region (MDR). A shift in activity to the North American east coast occurred ca. 1400 C.E., with more frequent severe hurricane strikes recorded from The Bahamas to New England between 1400 and 1675 C.E. A warm SST anomaly along the western North Atlantic, rather than within the MDR, likely contributed to the later active interval being restricted to the east coast. [ABSTRACT FROM AUTHOR]

Abstract: The recent report from Working Group II of the Intergovernmental Panel on Climate Change (IPCC) called for an increased understanding of how groundwater interacts with the ocean-atmospheric system. This remains a prerequisite to better understand how groundwater will respond to climate change. However, achieving this goal is hampered by several problems, including: (1) our minimal understanding of how current groundwater conditions evolved and responded to previous climatic perturbations, and (2) the scarcity of techniques available for evaluating prehistoric groundwater. Subfossil benthic foraminifera in underwater caves are an overlooked method of evaluating prehistoric groundwater on coastal carbonate terrain because they provide a proxy for groundwater salinity and circulation. Twelve radiocarbon dates and foraminiferal paleoecology from two sediment cores obtained from Green Bay Cave (GBC), Bermuda, provide the first Holocene-scale groundwater reconstruction. First, the cave floor (−20.7mbsl) was flooded at 7.7ka as North Atlantic sea level and groundwater rose in near synchrony. Over the Holocene, groundwater conditions (salinity, circulation and degree of oxygenation) shifted a minimum of five times, primarily in response to sea-level rise. Most importantly, we provide evidence that a prehistoric meteoric lens collapsed at 1.6ka as sea-level rise breached a local sill and completely altered Bermudian coastal circulation. The complete loss, or retreat, of a groundwater mass associated with a minor sea-level change validates some numerical predictions that small changes in sea level can destabilize coastal groundwater resources, when coastal geometry and circulation are considered. [Copyright &y& Elsevier]

The risk posed by intensification of North Atlantic hurricane activity remains controversial, in part due to a lack of available storm proxy records that extend beyond the relatively stable climates of the late Holocene. Here we present a record of storm-triggered turbidite deposition offshore the Dry Tortugas, south Florida, USA, that spans abrupt transitions in North Atlantic sea-surface temperature and Atlantic Meridional Overturning Circulation (AMOC) during the Younger Dryas (12.9-11.7 ka). Despite potentially hostile conditions for cyclogenesis in the tropical North Atlantic at that time, our record and numerical experiments suggest that strong hurricanes may have regularly affected Florida. Less severe surface cooling at midlatitudes (~20°-40°N) than across much of the tropical North Atlantic (~10°-20°N) in response to AMOC reduction may best explain strong hurricane activity during the Younger Dryas near the Dry Tortugas and possibly along the entire southeastern coast of the United States. [ABSTRACT FROM AUTHOR]