Clark R. Alexander, Jr

University of Georgia | UGA · Skidaway Institute of Oceanography

Bioturbation can increase time averaging by downward and upward movements of young and old shells within the entire mixed layer and by accelerating the burial of shells into a sequestration zone (SZ), allowing them to bypass the uppermost taphonomically active zone (TAZ). However, bioturbation can increase shell disintegration concurrently, neutral...

Shell ring archaeological sites are one of the most visible site types along the lower South Atlantic Coast of the United States. These cultural sites are large, circular to arcuate piles of mollusk shells with some reaching over three meters in elevation and over 100 m in diameter. They are comprised largely of mollusk shells (e.g., Eastern oyster...

People often modify the shoreline to mitigate erosion and protect property from storm impacts. The 2 main approaches to modification are gray infrastructure (e.g., bulkheads and seawalls) and natural or green infrastructure (NI) (e.g., living shorelines). Gray infrastructure is still more often used for coastal protection than NI, despite having mo...

Slump blocks are widely distributed features along marsh shorelines that can disturb marsh edge habitats and affect marsh geomorphology and sediment dynamics. However, little is known about their spatial distribution patterns or their longevity and movement. We employed an unoccupied aerial vehicle (UAV) to track slump blocks in 11 monthly images (...

The Flooding Dynamic Modeling Tools for Optimized Planning of CORE MPO Transportation Infrastructure Systems assess the vulnerability of the region’s stormwater infrastructure and surface transportation network to present and future flooding. This assessment aimed to develop a tool for optimal planning and design of resilient and equitable transpor...

Slump blocks are widely distributed features along marsh shorelines that can play an important role in marsh dynamics. However, little is known about their spatial distribution patterns, nor their longevity and movement. We employed an Unmanned Aerial Vehicle (UAV) to track slump blocks in 11 monthly images (March 2020–March 2021) of Dean Creek, a...

Efficient methods for shallow water data collection have been employed to create comprehensive bathymetric surfaces for 3 Georgia sounds. Survey areas were characterized using existing charted depths, aerial photographs, and lidar-derived elevation data to identify regions shallower than 3-m mean lower low water. Single-beam sonar missions were com...

Long, J.H.; Hanebuth, T.J.J.; Alexander, C.R., and Wehmiller, J.F., 0000. Depositional environments and stratigraphy of Quaternary paleochannel systems offshore of the Georgia Bight, southeastern USA. Journal of Coastal Research, 00(0), 000-000. Coconut Creek (Florida), ISSN 0749-0208. Paleochannels are common subsurface geological features beneath...

Burns, C.J.; Alexander, C.R., and Alber, M., 2021. Assessing long-term trends in lateral salt-marsh shoreline change along a U.S. East Coast latitudinal gradient. Journal of Coastal Research, 37(2), 291301. Coconut Creek (Florida), ISSN 0749-0208. Marshes are valuable intertidal habitats that respond to changes in their environment, and their perim...

Salt marshes rely on sufficient sediment inputs and room for lateral migration to maintain vertical and lateral stability under sea-level rise. As the global rate of sea-level rise accelerates, marshes unable to keep pace become vulnerable to drowning. We evaluated the long-term response of a salt marsh in Georgia, USA, to historical (1935–2018) an...

The eastern oyster ( Crassostrea virginica ) is an important proxy for examining historical trajectories of coastal ecosystems. Measurement of ~40,000 oyster shells from archaeological sites along the Atlantic Coast of the United States provides a long-term record of oyster abundance and size. The data demonstrate increases in oyster size across ti...

Salt marshes are valuable ecosystems, and there is concern that increases in the rate of sea level rise along with anthropogenic activities are leading to the loss of vegetated habitat. The area of vegetated marsh can change not only through advance and retreat of the open fetch edge, but also due to channel widening and contracting, formation and...

Rising sea levels and growing coastal populations are intensifying interactions at the land-sea interface. To stabilize upland and protect human developments from coastal hazards, landowners commonly emplace hard armoring structures, such as bulkheads and revetments, along estuarine shorelines. The ecological and economic consequences of shoreline...

Forty-two mineral sand samples from vibracores, collected offshore of Georgia (GA), South Carolina (SC), and North Carolina (NC), were investigated to determine the rare earth element (REE) concentration and corresponding rare earth minerals (REM). Geochemical analyses indicate that REE are present in low concentrations in the samples. For example,...

Oceanographic and evolutionary inferences based on fossil assemblages can be obscured by age offsets among co‐occurring shells (i.e., time averaging). To identify the contributions of sedimentation, mixing, durability, and production to within‐ and between‐species age offsets, we analyze downcore changes in the age‐frequency distributions of two bi...

Urbanization of coasts creates stresses on adjacent marine communities, but the full impact is seldom known owing to scarce historical records. Paleoecological analysis of sediment cores can be a powerful means of reconstructing baseline benthic communities, but is particularly challenging for continental shelves where dead-shell assemblages are pr...

Poster for Southeastern Geological Society of America Meeting

The article Effects of Small-Scale Armoring and Residential Development on the Salt Marsh-Upland Ecotone.

The article Generalizing Ecological Effects of Shoreline Armoring Across Soft Sediment Environments.

The Holocene upwelling history of the northern California continental slope is examined using the highresolution record of TN062-O550 (40.9°N, 124.6°W, 550 m water depth). This 7-m-long marine sediment core spans the last ~7500 years, and we use it to test the hypothesis that marine productivity in the California Current System (CCS) driven by coas...

Small-scale armoring placed near the marsh-upland interface to protect single-family homes is widespread but understudied. Using a nested, spatially blocked sampling design on the coast of Georgia, USA, we compared the biota and environmental characteristics of 60 marshes adjacent to either a bulkhead, a residential backyard with no armoring, or an...

Southeastern salt marshes are important repositories of sediment and carbon, and their formation is heavily dependent on deposition and accumulation of inorganic sediment. This study examined Groves Creek marsh near Savannah, GA, a typical Spartina alterniflora salt marsh of the southeastern US. Analyses were focused on the character, deposition an...

Despite its widespread use, the ecological effects of shoreline armoring are poorly synthesized and difficult to generalize across soft sediment environments and structure types. We developed a conceptual model that scales predicted ecological effects of shore-parallel armoring based on two axes: engineering purpose of structure (reduce/slow veloci...

Piston core TN062-O550, collected about 33 km offshore of Eureka, California, contains a high-resolution record of the climate and oceanography of coastal northernmost California during the past ~7.34 kyr. Chronology established by nine AMS ages on a combination of planktic foraminifers, bivalve shell fragments, and wood yields a mean sedimentation...

Alexander, C.R. and Windom, H.L., 2017. Material mobilization and transport in the Anadyr River-Estuarine system, eastern Siberia. Water and sediments samples collected from the Anadyr River, one of the most pristine Siberian watersheds, and its associated estuarine and coastal region were analyzed for a variety of inorganic constituents, trace met...

As development continues within the coastal zone of the southeastern U.S., the vulnerability of populations to shoreline erosion and flooding/inundation are becoming more of a focus of scientists and managers. Recently, a new software tool was developed called AMBUR-HVA (Hazard Vulnerability Assessment) to assess these threats while also incorporat...

Hurricane Sandy caused billions of dollars in damages to coastal communities along the east coast of the United States. Given the eventual likelihood of similar storms in the future, coastal communities have begun to develop strategies to increase their resilience to, and speed their recovery from, such an event. A detailed understanding of the dis...

A fundamental goal of the Earth Science community is to understand how perturbations on Earth's surface are preserved in the stratigraphic record. Recent Source to Sink (S2S) studies of the Waipaoa Sedimentary System (WSS), New Zealand, are synthesized herein to provide a holistic perspective of the processes that generate, transport and preserve s...

Sea level rise (SLR) may degrade habitat for coastal vertebrates in the Southeastern United States, but it is unclear which groups or species will be most exposed to habitat changes. We assessed 28 coastal Georgia vertebrate species for their exposure to potential habitat changes due to SLR using output from the Sea Level Affecting Marshes Model an...

The fine details of overmarsh circulation remain largely unexplored and yet they are typically assumed to control many attributes of salt marsh material cycling, transport and accretion. We characterized the spatial and temporal variability in overmarsh circulation at a 2 km2 Georgia, USA salt marsh using field observations, dye tracer and numerica...

The documented presence of the pesticide DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane) and its metabolites DDE (1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene), and DDD (1,1-dichloro-2,2-bis(p-chlorophenyl)ethane), collectively referred to as ΣDDT, in sediments from the Monterey Submarine Canyon, Monterey Fan Channel and Monterey Fan indicates...

It is increasingly apparent that stressors associated with anthropocentric climate change are likely to have dramatic effects on future human settlement patterns. Although sea-level rise is one of the best understood implications of climate change, geographically precise estimation of potential population displacement due to tidewater inundation ha...

Although this volume covers a broad range of temporal and methodological topics, the chapters are unified by a geographic focus on the archaeology of the Georgia Bight. The various research projects span multiple time periods (including Archaic, Woodland, Mississippian, and contact periods) and many incorporate specialized analyses (such as petrogr...

Submarine landslide complexes and mass-transport deposits (MTD's) occur widely across active margins affected by many forcing factors including earthquakes; sediment loading; gas hydrates; tectonic deformation and slope undercutting. The Hikurangi Margin is no exception, reflecting the tectonically dynamic nature of the margin and a voluminous terr...

Sedimentation on the Pleistocene New Jersey (NJ) shelf is complex, and results from the interaction of processes chiefly driven by glacioeustatic change. Erosion, non-deposition, downcutting and infilling combine to produce a complicated set of reflectors and sedimentary units that are best interpreted in the shallow subsurface with the aid of high...

Foraminiferal assemblages were used to investigate the nature of sedimentation on the technically active Poverty continental margin (PCM) of New Zealand. Recent research around the world has been focused on understanding the sedimentary functioning of small mountainous rivers to the global sediment budget, and this study is part of a greater effort...

This study was conducted in order to determine if the source of contaminants at formerly used defense sites (FUDS) in Alaska were deposited as a result of military occupancy or from long-distance transport. This determination largely influences whether remediation will occur, and, if so, to what extent. For this reason, plant samples (rinsed and un...

Archaeological sites in beach and estuarine environments are continually threatened by diverse natural marine processes. Shoreline erosion, bluff retreat, and sea level rise all present poten-tial for site destruction. Using historic maps, aerial imagery, and field survey methods in a GIS, 21 potentially significant archaeological sites on Georgia...

We present geophysical and core evidence showing how subsidence caused by forearc shortening has accommodated Late Pleistocene and Holocene sediments supplied to the tectonically active Waipaoa shelf (NZ), limiting off-shelf export during the early sea level highstand. The last glacioeustatic fall and subsequent rise exposed and then flooded a shel...

The Poverty margin was sampled in 2005 and 2006 as part of an international initiative to examine the terrestrial and marine sedimentary response to natural and human impacts on dispersal systems at mud-dominated coasts: the NSF MARGINS Source-to-Sink Initiative. 210Pb accumulation rates generally decrease from ~ 1 g/cm2 y in the mid-shelf depocent...

During the 1964 M9.2 great Alaskan earthquake, submarine-slope failures resulted in the generation of highly destructive tsunamis at Port Valdez, Alaska. A high-resolution, mini-sparker reflection profiler was used to image debris lobes, which we attribute to slope failures that occurred both during and prior to the 1964 megathrust event. In these...

Most of the west coast of Korea is fronted by broad intertidal sand- and mud-flats formed in a macrotidal environment that is seasonally subjected to monsoonal winds and intense winter storm surges. Field studies, conducted in about 60 km2 of Namyang Bay, Korea, were undertaken for the dual purpose of describing tidal flat sedimentary facies (surfi...

The sedimentology and stratigraphy of river-fed continental margins reflect a diverse range of tectonic, climatic and hydrodynamic conditions that moderate the supply, transport and accumulation of terrigenous sediment in the coastal ocean. This paper describes a study of the modern and late Holocene northern California shelf and slope. The aim is...

All s ediment that is delivered to the seabed is subject to post-depositional alteration before it becomes part of the preserved stratigraphic record. Physical and biological processes occurring in the upper few decimetres of the seabed alter newly deposited sediment, thereby creating the fine-scale sedimentary record. The geographical region of fo...

Sediment discharged into the portion of the Southern California Bight extending from Santa Barbara to Dana Point enters a complex system of semi-isolated coastal cells, narrow continental shelves, submarine canyons, and offshore basins. On both the Santa Monica and San Pedro margins, 210Pb accumulation rates decrease in an offshore direction (from...

Recent multibeam mapping, sub-bottom profiling, and coring in Port Valdez, Alaska have disclosed massive debris flow deposits and large blocks displaced during the 1964 M9.2 Alaska earthquake. These failures led to some of the highest tsunami wave runups ever documented (~50 m). Landslide volumes have been calculated using available bathymetry from...

1] Small mountainous rivers (SMRs) export substantial amounts of sediment into the world's oceans. The concomitant yield of organic carbon (OC) associated with this class of rivers has also been shown to be significant and compositionally unique. We report here excessively high loadings of polycyclic aromatic hydrocarbons (PAHs), lignin, and levogl...

Continental margin sediments contain a wealth of information on changing climatic, terrestrial and oceanographic conditions over time, but their interpretation depends on our ability to understand the myriad controls on sedi- ment dispersal and accumulation. Traditional models consider sea level to be the primary control on the partition- ing of se...

The New Jersey margin is an ideal location for the study of sedimentary response to glacioeustatic forcing because this passive continental edge is both wide and stable. Although the region has been intensively imaged and mapped geophysically, it is still far from being understood stratigraphically because of a lack of samples to constrain timing a...

Beginning in the early 1990s, an increase of coastal storm activity brought about a growing concern over the protection of cultural resources along, or within close proximity to, Cumberland Island, Georgia's back-barrier shore. Such storm activity has exacerbated the erosion problem along several segments of the shoreline already experiencing net l...

1] Submarine canyons are major geomorphologic features on the Earth's surface. Their formation has received considerable debate, but their demise has received less attention. Research of modern canyons with cores and moorings has documented active sediment transport and deposition, but extrapolation of these local observations over larger areas is...

Paleoenvironmental data from a stalagmite from western Belize provide a 3300-year record of droughts that impacted the Maya civilization at least four times across a span of 1500 years, and the most sustained period of drought coincided with the collapse of Classic Maya civilization. The stalagmite, which comes from Macal Chasm in the Vaca Plateau,...

The M9.2 Alaska earthquake of 1964 caused major damage to the port facilities and town of Valdez, most of it the result of submarine landslide and the consequent tsunamis. Recent bathymetric multibeam surveys, high-resolution subbottom profiles, and dated sediment cores in Port Valdez supply new information about the morphology and character of the...

The outer shelf and slope seaward of the Waipaoa River, New Zealand has experienced considerable morphological change in the late Quaternary. The complexion of the margin has evolved as a result of sedimentation affected by sea level, oceanographic, and tectonic forcings. Integration of seismic, core and multibeam data indicate that the modern seab...

The Anadyr River, which has an arctic-subarctic watershed, discharges into the Bering Sea. The watershed and the area surrounding its estuary has a human population of approximately 11,000 and mining of metalliferous and coal deposits has been the only significant human activity which may have impacted the geochemistry of this region. To assess the...

On the eastern Raukumara Ranges of the New Zealand East Coast, active tectonics, vigorous weather systems, and human colonisation have combined to cause widespread erosion of the mudstone- and sandstone-dominated hinterland. The Waipaoa River sedimentary dispersal system is an example that has responded to environmental change, and is now New Zeala...

A new research program focusing on sediment dispersal across the active margin of the New Zealand east coast has provided the foundation for a holistic understanding of the transport and fate of terrestrial materials in the coastal ocean. Field studies began in January 2005 with two acoustic mapping and shallow seabed sampling expeditions to the sh...