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Long, J.H.; Hanebuth, T.J.J.; Durica, J.T., and Hawkes, A.D., 2024. Late Holocene stratigraphy and sedimentary facies distribution of an anthropogenically modified delta plain (Santee River Delta, South Carolina, U.S.A.). Journal of Coastal Research, 40(4), 623–646. Charlotte (North Carolina), ISSN 0749-0208. The Santee River of South Carolina has the second largest watershed in the eastern United States and forms the largest river-fed delta along the U.S. east coast. Anthropogenic modifications to the delta plain of the Santee River, and in many coastal environments within the region, have significantly altered the natural configurations of floodplains, channels, and shorelines. This study incorporated historic and modern state-of-the-art data sets and methods to evaluate the sediment distribution within the modern delta plain as well as the record of environmental change throughout the late Holocene as it is preserved within the subsurface stratigraphy. The study incorporated high-resolution seismo-acoustic and bathymetric data, detailed sediment core analysis, accelerator mass spectrometry 14C dating, micropaleontological analysis, and surface sediment samples to define geomorphic zones based on dominant depositional processes related to fluvial discharge, tides, and waves. Tidal- and wave-influenced conditions were established in the delta plain by around 3 kyr BP and continued into historic times, when the construction of rice fields across most of the delta plain fixed channel positions and isolated floodplains from flood-related sedimentation. Sediment distribution in the modern delta plain is significantly influenced by the maintained network of artificial canals and embankments associated with these historic fields. The influence of these modifications can also be seen within the stratigraphy beneath the delta plain, recording local changes in deposition and erosion that resulted from a modification in water circulation and sediment supply. [ABSTRACT FROM AUTHOR]

Megathrust earthquakes in western North America may be preceded by a precursor phase several years prior to megathrust, induced earthquakes. For example, on 27 March 1964, a 9.2 magnitude (on the Richter scale) earthquake occurred on the coast of Alaska. Changes in foraminifera and diatom assemblages at Girdwood Flats, Alaska, provide evidence of a precursor to this earthquake, thereby detailing a previously unknown sequence of events. We describe further evidence of precursor phases from marshes in Turnagain Arm, Alaska, United States, and farther south in Netarts Bay, Oregon, United States; this is the first time that two widely spaced locations have been examined for earthquake-related precursor stages. The Alaska earthquake offers the possibility to compare a modern sequence (A.D. 1964) of events with the geologic record. The Netarts Bay marsh has experienced no modern earthquake that could be used for comparison, but the nature of megathrust zones implies that the modern and ancient events should be physically similar. New cores examined from Turnagain Arm include both the 1964 earthquake and an event identified and dated at 1800 yr B.P. Prior to the 1964 event, the foraminifera and thecamoebian assemblages changed from a forest phase to a mildly brackish stage; this sequence was dated as being 15 yr or less in length, using [Pb.sup.210] and [Cs.sup.137] dating techniques. The event at 1800 yr B.P. was also associated with a similar precursor stage, indicating a small subsidence prior to the megathrust earthquake-related subsidence event. In Netarts Bay a new core was taken at a previously cored site to make use of carbon-14 dates and the assurance that at least four events were known to have occurred over the past 3000 yr. The new core contains four transitions, each identified by a mineralic deposit, often sharply bounded below and gradationally overlain by marsh peat. Transition in this context refers to the section of core analyzed, ~15 cm above and below each of the four sand layers. Foraminiferal assemblage analyses indicate that these units represented a high marsh prequake phase followed by a lower marsh precursor stage, the earthquake-related deposition (sand layer), and then a rebound back into postquake marsh deposits. Sand deposits with either no or few foraminifera are inferred as tsunami/ earthquake-related deposition stages. These transitions in two widely separated geographic areas (Alaska and Oregon) indicate that similar mechanisms operate for large megathrust earthquakes at subduction zones from Alaska to northern California in the Cascadia Subduction Zone, thus implying that precursor events also occur and can be detected by foraminiferai zonation all along this area. In a recent article (Dragert et al., 2001), scientists from the west coast suggested that 'slow' or 'silent' earthquakes they measured with continuous geographical positioning systems might be indicators of megathrust earthquakes. The transitions we document may be the prehistoric representations of these 'silent' quakes. Foraminiferal evidence may help by allowing, more accurate positioning of seismometers along the west coast of North America and thereby lead to more precise and timely earthquake prediction methods. Keywords: precursor, megathrust earthquake, foraminifera, thecamoebian, marsh/ forest transition.

Lithology and microfossil biostratigraphy beneath the marshes of a central Oregon estuary limit geophysical models of Cascadia megathrust rupture during successive earthquakes by ruling out >0.5 m of coseismic coastal subsidence for the past 2000 yr. Although the stratigraphy in cores and outcrops includes as many as 12 peat-mud contacts, like those commonly inferred to record subsidence during megathrust earthquakes, mapping, qualitative diatom analysis, foraminiferal transfer function analysis, and 14C dating of the contacts failed to confirm that any contacts formed through subsidence during great earthquakes. Based on the youngest peat-mud contact's distinctness, >400 m distribution, ~0.6 m depth, and overlying probable tsunami deposit, we attribute it to the great 1700 CE Cascadia earthquake and(or) its accompanying tsunami. Minimal changes in diatom assemblages from below the contact to above its probable tsunami deposit suggest that the lower of several foraminiferal transfer function reconstructions of coseismic subsidence across the contact (0.1-0.5 m) is most accurate. The more limited stratigraphic extent and minimal changes in lithology, foraminifera, and(or) diatom assemblages across the other 11 peat-mud contacts are insufficient to distinguish them from contacts formed through small, gradual, or localized changes in tide levels during river floods, storm surges, and gradual sea-level rise. Although no data preclude any contacts from being synchronous with a megathrust earthquake, the evidence is equally consistent with all contacts recording relative sea-level changes below the ~0.5 m detection threshold for distinguishing coseismic from nonseismic changes. [ABSTRACT FROM AUTHOR]

Coseismic subsidence along the Cascadia subduction zone causes abrupt relative sea-level (RSL) rise that is recorded in coastal stratigraphy and foraminiferal assemblages. RSL reconstructions therefore provide insight into the magnitude, nature, and frequency of great earthquakes that can constrain deformation models and quantify the seismic risk faced by coastal populations. These reconstructions are commonly generated using transfer functions that are calibrated from counts of modern (surface) foraminifera and corresponding elevation measurements. We developed four transfer functions of increasing complexity to explore how and why the composition of the modern dataset and the choice of transfer-function type affects subsidence reconstructions. Application of these four models to stratigraphic contacts (mud abruptly overlying peat or soil) representing the A.D. 1700 Cascadia earthquake and a field experiment that simulated subsidence show that a Bayesian transfer function (BTF) calibrated using a large modern dataset (19 sites from California to Vancouver Island) and incorporating prior information from stratigraphic context produces systematically larger subsidence estimates than a weighted-averaging transfer function calibrated using a smaller modern dataset (8 sites in Oregon) that does not leverage stratigraphic context. This difference arises from (1) training set composition, (2) taxa-elevation relationships in the BTF that are not assumed to be unimodal, and (3) stratigraphic prior information that compensates for postdepositional, downward mixing of postearthquake foraminifera into pre-earthquake sediment, which biases reconstructions at some sites toward smaller subsidence. Our reconstructions support a heterogeneous rupture model for the A.D. 1700 earthquake, but indicate that slip estimates in patches from Alsea Bay to Netarts Bay (Oregon) and from Netarts Bay to Vancouver Island should be increased. [ABSTRACT FROM AUTHOR]

A new regional dataset comprising 425 intertidal diatom taxa from 175 samples from 11 ecologically diverse Oregon and Washington estuaries illustrates the importance of compiling a large modern dataset from a range of sites. Cluster analyses and detrended correspondence analysis of the diatom assemblages identify distinct vertical zones within supratidal, intertidal and subtidal environments at six of the 11 study sites, but the abundance of some of the most common species varies widely among and within sites. Canonical correspondence analysis of the regional dataset shows relationships between diatom species and tidal exposure, salinity and substratum (grain size and organic content). Correspondence analyses of local datasets show higher values of explained variation than the analysis of the combined regional dataset. Our results emphasize that studies of the autecology of diatom species require many samples from a range of modern environments to adequately characterize species–environment relationships. [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]

Over short periods of time, extreme storms can significantly alter barrier island morphology, increasing the vulnerability of coastal habitats and communities relative to future storms. These impacts are complex and the result of interactions between oceanographic conditions and the geomorphic, geological, and ecological characteristics of the island. A 2D XBeach model was developed and compared to observations in order to study these interactions along an undeveloped barrier island near the landfall of Hurricane Florence in 2018. Beachface water levels during the storm were obtained from two cross-shore arrays of pressure sensors for comparison to model hydrodynamics. Aerial drone imagery was used to derive pre-storm and post-storm elevation data in order to quantify spatially varying erosion and overwash. Sediment grain size was measured in multiple locations, and we estimated spatially varying friction by using Sentinel-2 satellite imagery. The high spatial and temporal resolution of satellite imagery provided an efficient method for incorporating pre-storm spatially varying land cover. While previous studies have focused on the use of spatially varying friction, we found that the utilization of local median grain sizes and full directional wave spectra was critical to reproducing observed overwash extent. [ABSTRACT FROM AUTHOR]

Previous research has documented the usefulness of Lidar data to derive a variety of topographic products (e.g., DEM, DTM, canopy and forest structure, and urban infrastructure). Lidar has been used to map coastal environments and geomorphology; however, there is no comprehensive model to derive coastal geomorphology. Therefore, the purpose of this project was to build on existing research and develop an automated modeling approach to classify coastal geomorphology across barrier islands. The model was developed and tested at four sites in North Carolina including two undeveloped and two developed islands. Barrier island geomorphology is shaped by natural coastal processes, such as storms and longshore sediment transport, as well as human influences, such as beach nourishment and urban development. The model was developed to classify ten geomorphic features over four time-steps from 1998 to 2014. Model results were compared to compute change through time and derived the rate and direction of feature movement. Tropical storms and hurricanes had the most influence in geomorphic change and movement. On the developed islands, there was less influence of storms due to the inability of features to move because of coastal infrastructure. From 2005 to 2010, beach nourishment was the dominant influence on developed beaches because this activity ameliorated the natural tendency for an island to erode. Understanding how natural and anthropogenic processes influence barrier island geomorphology is critical to predicting an island's future response to changing environmental factors such as sea-level rise. The development of an automated model enables it to be replicated in other locations where policy makers and coastal managers may use this information to make development and conservation decisions. [ABSTRACT FROM AUTHOR]

Salt-marsh sediments provide precise and near-continuous reconstructions of Common Era relative sea level (RSL). However, organic and low-density salt-marsh sediments are prone to compaction processes that cause post-depositional distortion of the stratigraphic column used to reconstruct RSL. We compared two RSL reconstructions from East River Marsh (Connecticut, USA) to assess the contribution of mechanical compression and biodegradation to compaction of salt-marsh sediments and their subsequent influence on RSL reconstructions. The first, existing reconstruction (‘trench’) was produced from a continuous sequence of basal salt-marsh sediment and is unaffected by compaction. The second, new reconstruction is from a compaction-susceptible core taken at the same location. We highlight that sediment compaction is the only feasible mechanism for explaining the observed differences in RSL reconstructed from the trench and core. Both reconstructions display long-term RSL rise of ∼1 mm/yr, followed by a ∼19th Century acceleration to ∼3 mm/yr. A statistically-significant difference between the records at ∼1100 to 1800 CE could not be explained by a compression-only geotechnical model. We suggest that the warmer and drier conditions of the Medieval Climate Anomaly (MCA) resulted in an increase in sediment compressibility during this time period. We adapted the geotechnical model by reducing the compressive strength of MCA sediments to simulate this softening of sediments. ‘Decompaction’ of the core reconstruction with this modified model accounted for the difference between the two RSL reconstructions. Our results demonstrate that compression-only geotechnical models may be inadequate for estimating compaction and post-depositional lowering of susceptible organic salt-marsh sediments in some settings. This has important implications for our understanding of the drivers of sea-level change. Further, our results suggest that future climate changes may make salt marshes more susceptible to the impacts of RSL rise by enhancing sediment compressibility. We stress, however, that the cause of the softening remains enigmatic. Until this is better constrained, it is premature to widely extrapolate our findings to existing core-based reconstructions of Holocene RSL. [ABSTRACT FROM AUTHOR]

Abstract Salt marshes respond to sea-level rise through a series of complex and dynamic bio-physical feedbacks. In this study, we found that sea-level rise triggered salt marsh habitat restructuring, with the associated vegetation changes enhancing salt marsh elevation resilience. A continuous record of marsh elevation relative to sea level that includes reconstruction of high-resolution, sub-decadal, marsh elevation over the past century, coupled with a lower-resolution 1500-year record, revealed that relative sea-level rose 1.5 ± 0.4 m, following local glacial isostatic adjustment (1.2 mm/yr). As sea-level rise has rapidly accelerated, the high marsh zone dropped 11 cm within the tidal frame since 1932, leading to greater inundation and a shift to flood- and salt-tolerant low marsh species. Once the marsh platform fell to the elevation favored by low-marsh Spartina alterniflora , the elevation stabilized relative to sea level. Currently low marsh accretion keeps pace with sea-level rise, while present day high marsh zones that have not transitioned to low marsh have a vertical accretion deficit. Greater biomass productivity, and an expanding subsurface accommodation space favorable for salt marsh organic matter preservation, provide a positive feed-back between sea-level rise and marsh platform elevation. Carbon storage was 46 ± 28 g C/m2/yr from 550 to 1800 CE, increasing to 129 ± 50 g C/m2/yr in the last decade. Enhanced carbon storage is controlled by vertical accretion rates, rather than soil carbon density, and is a direct response to anthropogenic eustatic sea-level rise, ultimately providing a negative feedback on climate warming. Graphical abstract Image 1 Highlights • New England salt marshes transition from high to low marsh habitat under the highest rates of sea-level rise in 1500 years. • Low marsh transgression results in resilient platform elevation due to greater productivity and organic matter accretion. • Accelerating accretion in response to sea-level rise results in enhanced carbon storage rates in salt marshes. [ABSTRACT FROM AUTHOR]

Abstract Several processes contributing to coastal relative sea-level (RSL) change in the North Atlantic Ocean are observed and/or predicted to have distinctive spatial expressions that vary by latitude. To expand the latitudinal range of RSL records spanning the past ∼3000 years and the likelihood of recognizing the characteristic fingerprints of these processes, we reconstructed RSL at two sites (Big River and Placentia) in Newfoundland from salt-marsh sediment. Bayesian transfer functions established the height of former sea level from preserved assemblages of foraminifera and testate amoebae. Age-depth models constrained by radiocarbon dates and chronohorizons estimated the timing of sediment deposition. During the past ∼3000 years, RSL rose by ∼3.0 m at Big River and by ∼1.5 m at Placentia. A locally calibrated geotechnical model showed that post-depositional lowering through sediment compaction was minimal. To isolate and quantify contributions to RSL from global, regional linear, regional non-linear, and local-scale processes, we decomposed the new reconstructions (and those in an expanded, global database) using a spatio-temporal statistical model. The global component confirms that 20th century sea-level rise occurred at the fastest, century-scale rate in over 3000 years (P > 0.999). Distinguishing the contributions from local and regional non-linear processes is made challenging by a sparse network of reconstructions. However, only a small contribution from local-scale processes is necessary to reconcile RSL reconstructions and modeled RSL trends. We identified three latitudinally-organized groups of sites that share coherent regional non-linear trends and indicate that dynamic redistribution of ocean mass by currents and/or winds was likely an important driver of sea-level change in the North Atlantic Ocean during the past ∼3000 years. Highlights • Relative sea level reconstructions spanning ∼3000 years from 2 sites in Newfoundland. • Database of sea level reconstructions decomposed using a spatio-temporal model. • 20th century rate of global rise was fastest in past ∼3000 years. • Ocean/atmosphere circulation likely drove regional non-linear sea-level change. • Local scale processes (including compaction) had small influence on sea-level trends. [ABSTRACT FROM AUTHOR]

The spatial variability of Holocene relative sea-level (RSL) change influences the capacities of coastal environments to accommodate a sedimentary record of paleoenvironmental change. In this study we couch a specific investigation in more general terms in order to demonstrate the applicability of the relative sea-level history approach to paleoseismic investigations. Using subsidence stratigraphy, we trace the different modes of coastal sedimentation over the course of time in the eastern Indian Ocean where RSL change evolved from rapidly rising to static from 8000 yr ago to present. Initially, the coastal sites from the Aceh, Sumatra, coastal plain, which are subject to repeated great earthquakes and tsunamis, built up a sedimentary sequence in response to a RSL rise of 1.4 mm/yr. The sequence found at 2 sites 8 km apart contained 3 soils of a mangrove origin (Rhizophora, Bruguiera/Ceriops, Avicennia pollen, and/or intertidal foraminifera) buried by sudden submergence related to coseismic subsidence and 6 tsunami sands that contain pristine subtidal and planktic foraminifera. After 3800 cal yr B.P. (years before A.D. 1950), sea level stabilized and remained such to the present. The stable relative sea level reduced accommodation space in the late Holocene, suggesting that the continued aggradation of the coastal plain was a consequence of periodic coastal inundation by tsunamis. [ABSTRACT FROM AUTHOR]

Late Holocene relative sea-level (RSL) reconstructions can be used to estimate rates of land-level (subsidence or uplift) change and therefore to modify global sea-level projections for regional conditions. These reconstructions also provide the long-term benchmark against which modern trends are compared and an opportunity to understand the response of sea level to past climate variability. To address a spatial absence of late Holocene data in Florida and Georgia, we reconstructed ~ 1.3 m of RSL rise in northeastern Florida (USA) during the past ~ 2600 years using plant remains and foraminifera in a dated core of high salt-marsh sediment. The reconstruction was fused with tide-gauge data from nearby Fernandina Beach, which measured 1.91 ± 0.26 mm/year of RSL rise since 1900 CE. The average rate of RSL rise prior to 1800 CE was 0.41 ± 0.08 mm/year. Assuming negligible change in global mean sea level from meltwater input/removal and thermal expansion/contraction, this sea-level history approximates net land-level (subsidence and geoid) change, principally from glacio-isostatic adjustment. Historic rates of rise commenced at 1850–1890 CE and it is virtually certain ( P = 0.99) that the average rate of 20th century RSL rise in northeastern Florida was faster than during any of the preceding 26 centuries. The linearity of RSL rise in Florida is in contrast to the variability reconstructed at sites further north on the U.S. Atlantic coast and may suggest a role for ocean dynamic effects in explaining these more variable RSL reconstructions. Comparison of the difference between reconstructed rates of late Holocene RSL rise and historic trends measured by tide gauges indicates that 20th century sea-level trends along the U.S. Atlantic coast were not dominated by the characteristic spatial fingerprint of melting of the Greenland Ice Sheet. [ABSTRACT FROM AUTHOR]

We assessed the utility of δ13C and bulk geochemistry (total organic content and C:N) to reconstruct relative sea-level changes on the Cascadia subduction zone through comparison with an established sea-level indicator (benthic foraminifera). Four modern transects collected from three tidal environments at Siletz Bay, Oregon, USA, produced three elevation-dependent groups in both the foraminiferal and δ13C/bulk geochemistry datasets. Foraminiferal samples from the tidal flat and low marsh are identified by Miliammina fusca abundances of >45%, middle and high marsh by M. fusca abundances of <45% and the highest marsh by Trochamminita irregularis abundances >25%. The δ13C values from the groups defined with δ13C/bulk geochemistry analyses decrease with an increasing elevation; −24.1±1.7‰ in the tidal flat and low marsh; −27.3±1.4‰ in the middle and high marsh; and −29.6±0.8‰ in the highest marsh samples. We applied the modern foraminiferal and δ13C distributions to a core that contained a stratigraphic contact marking the great Cascadia earthquake of AD 1700. Both techniques gave similar values for coseismic subsidence across the contact (0.88±0.39m and 0.71±0.56m) suggesting that δ13C has potential for identifying amounts of relative sea-level change due to tectonics. [ABSTRACT FROM AUTHOR]