Christopher K. Sommerfield

Shelf record of climatic changes in flood magnitude and frequency, north-coastal California

Continental-shelf deposits off the Eel River, north-coastal California, document a recent increase in the magnitude and frequency of major hydrologic floods (≥10 yr recurrence interval). The shelf record reveals a sudden, three-fold increase in sedimentation rate ca. 1954 and a concomitant increase in the frequency of preserved flood beds. Comparison of sedimentary and river-discharge records reveals that major floods after ca. 1950 had a more pronounced effect on coastal sediment delivery and accumulation offshore than previous recorded events of similar magnitude, and that stratigraphic preservation of flood events is highly dependent on flood frequency and net sedimentation rate. We contend that this change in marine sedimentation is a response to documented climatic phenomena that have increased the frequency of major floods throughout the western United States during the past half century, together with intrabasinal impacts of extreme floods in 1955 and 1964. Anthropogenic increase in watershed-sediment production is a probable secondary factor.

Late Holocene sediment accumulation on the northern California shelf: Oceanic, fluvial, and anthropogenic influences

The late Holocene sedimentary record of the northern California continental shelf archives the combined influences of fluvial and oceanic mechanisms of land to ocean sediment flux, and provides perspective on the cumulative impacts of major floods and land-use change in the drainage basin during historical time. Piston cores collected on the shelf (50−150 m water depths) off the Eel and Klamath Rivers were analyzed to identify lithologies indicative of depositional processes over the past 5 k.y., and dated using 137Cs and 14C methods to resolve spatial patterns in sediment accumulation rates. Sediment accumulation rates averaged over the past several millennia display along- and across-shelf gradients that reflect interactions between the shelf flow field and antecedent tectonically produced topography. The highest rates (3−6 mm/yr; 0.4−0.8 g/cm2/yr) are associated with structural lows on the middle shelf, whereas the lowest rates (0.2−0.6 mm/yr; 0.03−0.05 g/cm2/yr) occur at structural highs and at the shelf edge. This accumulation pattern is spatially coincident with postglacial sediment thicknesses on the shelf, and with depocenters that form on shorter time scales in response to oceanic flood events. The sediment column exhibits an upward-fining sequence consistent with increased bypass of river silt and clay as delta plains progressively filled during the Holocene transgression. Sedimentary event beds indicative of major river floods, including two probable hyperpycnal discharges of the Eel River within the past 1 k.y., are preserved. Stratigraphic evidence of accelerated sediment delivery to the shelf during historical time, in the form of increased burial of fluvial mud relative to fine-grained sand, is superimposed on this natural record. This change is concordant with the regional history of forest clearing and related land-use practices in the watershed after the early 1800s, and appears to record the combined impacts of timber harvesting and several extreme floods after ca. A.D. 1950.

Across-shelf sediment transport since the Last Glacial Maximum, southern California margin

Correlation of continental shelf-slope stratigraphy in Santa Monica Bay (southern California) with Ocean Drilling Program records for nearby slope-basin sites has illuminated the timing and scale of terrigenous sediment dispersal on margin since the Last Glacial Maximum (LGM). Marine flooding surfaces preserved in a transgressive sequence on the Santa Monica Shelf provide a key link between base-level elevation and sediment transport across shelf. Sediment-accumulation rates at slope-basin sites were maximal ca. 15–10 ka, well after the LGM, decreased during the 12–9 ka transition from fluvial-estuarine to fully marine conditions on the shelf, and decelerated throughout the Holocene to 30%–75% of their values at the LGM. The deceleration is interpreted to manifest a landward shift in the margin depocenter with the onset of transgressive sedimentation beginning when sea level surmounted the shelf edge ca. 13 ka, as predicted by sequence-stratigraphic models. However, the records make clear that factors other than base level modulated slope-basin accumulation rates during the deglaciation.

Lateral variability of sediment transport in the Delaware Estuary

Lateral processes contribute significantly to circulation and material transport in estuaries. The mechanisms controlling transport may vary spatially such that shallow and deep regions of an estuary contribute differently to the total transport. An observational study was conducted to explore the importance of lateral variability in sediment transport mechanisms in the Delaware Estuary. Seven moorings were deployed across the channel in the region of the estuarine turbidity maximum (ETM) zone from April to August 2011. Time series of along-channel sediment transport reveal a consistent pattern of sediment export across the entire estuary during periods of high river discharge, followed by a transition to import within the channel and export on the flanks during low river flow. There is a persistent divergence of across-channel sediment fluxes on the Delaware side, where sediment from the flank is transported toward both the channel and wetland coast. Decomposition of the fluxes highlight that across-channel sediment transport is driven by mean lateral circulation, whereas along-channel transport is driven primarily by mean advection, with tidal pumping contributing to about 30% of total transport. The spatial and temporal variability of mean advection and tidal pumping were generally complementary, with both contributing to the observed sediment transport pathways. Tidal pumping, linked to tidal asymmetries in stratification and sediment resuspension, was shown to drive both ebb-driven export and flood-driven import depending on the tidal variability of stratification. The spatiotemporal patterns of sediment transport highlight the three-dimensional structure of the ETM and shed light on the variability of sediment transport mechanisms.

Tidal Marsh Record of Nutrient Loadings in Barnegat Bay, New Jersey

ABSTRACT Velinsky, D.J.; Paudel, B.; Belton, T.J., and Sommerfield, C.K., 2017. Tidal marsh record of nutrient loadings in Barnegat Bay, New Jersey. In: Buchanan, G.A.; Belton, T.J., and Paudel, B. (eds.), A Comprehensive Assessment of Barnegat Bay-Little Egg Harbor, New Jersey. Sediment and nutrient burial in tidal marshes of Barnegat Bay was investigated using age-dated sediment cores collected along a north-to-south transect. Measurements of radionuclides (210Pb and 137Cs) and stable isotopes (13C and 15N) were accompanied by nutrient and organic matter concentrations. Sediment accumulation rates, measured using 210Pb and 137Cs chronology, ranged from 48 to 81 mg cm2 y−1, whereas corresponding accretion rates ranged from 0.16 to 0.30 cm y−1. Sediment nitrogen (N) accumulation rates increased twofold at an upper bay site, starting in the mid-1950s, whereas at other locations, only small to no increases were seen with time. Phosphorus (P) accumulation was minimal with time. N and P accumulation rates were higher between the 1940s to 1950s at stations BB-1 and BB-3, while higher accumulation rate in the down-bay (BB-4) was identified in the early 1990s. Results indicate that bay marshes can sequester approximately 79 ± 11% of N and 54 ± 34% of P entering the Bay from upland sources; thus, these marshes perform an important ecosystem service in the form of nutrient sequestration. Marsh accretion rates at the coring sites fall at, to just below, rates of relative sea-level rise recorded by nearby tide gauges. These relatively low rates of accretion render the marsh vulnerable to inundation should the rate of sea-level rise accelerate in the future.

Suspended-Sediment Impacts on Light-Limited Productivity in the Delaware Estuary

The Delaware Estuary has a history of high anthropogenic nutrient loadings but has been classified as a high-nutrient, low-growth system due to persistent light limitation caused by turbidity. While the biogeochemical implications of light limitation in turbid estuaries have been well-studied, there has been minimal effort focused on the connectivity between hydrodynamics, sediment dynamics, and light limitation. Our understanding of sediment dynamics in the Delaware Estuary has advanced significantly in the last decade, and this study describes the impact of spatiotemporal variability of the estuarine turbidity maximum (ETM) on light-limited productivity. This analysis uses data from eight along-estuary cruises from March, June, September, and December 2010 and 2011 to evaluate the impact of the turbidity maximum on production. Whereas the movement of the ETM is controlled primarily by river discharge, the structure of the ETM is modulated by stratification, which varies with both river discharge and spring-neap conditions. We observe that the ETM’s location and structure control spatial patterns of light availability. To evaluate the relative contributions of river discharge and spring-neap variability to the location of phytoplankton blooms, we develop an idealized two-dimensional Regional Ocean Modeling System (ROMS) numerical model. We conclude that high river flows and neap tides can drive stratification that is strong enough to prevent sediment from being resuspended into the surface layer, thus providing light conditions favorable for primary production. This study sheds light on the role of stratification in controlling sediment resuspension and promoting production, highlighting the potential limitations of biogeochemical models that neglect sediment processes.

Seasonal variability of the inorganic carbon system in a large coastal plain estuary

Carbonate geochemistry research in large estuarine systems is limited. More work is needed to understand how changes in land-use activity influence watershed export of organic and inorganic carbon, acids, and nutrients to the coastal ocean. To investigate the seasonal variation of the inorganic carbon system in the Delaware Estuary, one of the largest estuaries along the US east coast, dissolved inorganic carbon (DIC), total alkalinity (TA), and pH were measured along the estuary from June 2013 to April 2015. In addition, DIC, TA, and pH were periodically measured from March to October 2015 in the nontidal freshwater Delaware, Schuylkill, and Christina rivers over a range of discharge conditions. There were strong negative relationships between river TA and discharge, suggesting that changes in HCO−3 concentrations reflect dilution of weathering products in the drainage basin. The ratio of DIC to TA, an understudied but important property, was high (1.11) during high discharge and low (0.94) during low discharge, reflecting additional DIC input in the form of carbon dioxide (CO2), most likely from terrestrial organic matter decomposition, rather than bicarbonate (HCO−3 ) inputs due to drainage basin weathering processes. This is also a result of CO2 loss to the atmosphere due to rapid water transit during the wet season. Our data further show that elevated DIC in the Schuylkill River is substantially different than that in the Delaware River. Thus, tributary contributions must be considered when attributing estuarine DIC sources to the internal carbon cycle versus external processes such as drainage basin mineralogy, weathering intensity, and discharge patterns. Longterm records in the Delaware and Schuylkill rivers indicate shifts toward higher alkalinity in estuarine waters over time, as has been found in other estuaries worldwide. Annual DIC input flux to the estuary and export flux to the coastal ocean are estimated to be 15.7± 8.2× 109 molC yr−1 and 16.5± 10.6× 109 molCyr−1, respectively, while net DIC production within the estuary including inputs from intertidal marshes is estimated to be 5.1× 109 molCyr−1. The small difference between riverine input and export flux suggests that, in the case of the Delaware Estuary and perhaps other large coastal systems with long freshwater residence times, the majority of the DIC produced in the estuary by biological processes is exchanged with the atmosphere rather than exported to the sea.

Qualities and Limitations of Fluvial Suspended Sediment Data Published by the United States Geological Survey

ABSTRACT Sommerfield, C.K., 2016. Qualities and limitations of fluvial suspended sediment data published by the United States Geological Survey. Fluvial suspended sediment concentration (SSC) data published by the United States Geological Survey (USGS) are frequently used to address questions of sediment delivery to estuaries and coasts. Despite the availability of USGS reports detailing methods of sample collection and data processing, many end users remain misinformed about the characteristics and limitations of these data. Two types of USGS-SSC data are available for stream gauging stations: (1) discrete samples and (2) daily record. Discrete samples are laboratory measurements of SSC from water collected at a stream gauging station. In contrast, daily-record data are interpolations of SSC from a smaller number of discrete samples available for a station. A potential source of confusion arises because both types of USGS-SSC data are given the identical description (suspended sediment concentration, mg/L) and parameter code (80154) in the national database. Although the daily-record series for a station is derived from actual samples, discrete and daily SSC values for the same date of record are not always strongly correlated because of factors related to sampling and data processing. As shown in this paper, discrete-sample SSC data series for some stations are inhomogeneous and biased because of changes in USGS sampling practices over the decades, thereby limiting the possible end uses of the data. Ultimately it is up to users to determine which type of SSC data are best suited for a particular application and whether the statistical properties of a data set are conducive to applications such as trend analysis.

Effects of Northeaster Storms on Water Level and Turbidity in a Delaware Bay Subestuary

ABSTRACT Moskalski, S.M. and Sommerfield, C.K., 2013. Effects of northeaster storms on water level and turbidity in a Delaware Bay subestuary. Coastal storms have a major influence on the ecology and geomorphology of U.S. Atlantic estuaries and tidal marshes. The purpose of this study was to determine which types of storms are most effective in flooding the marsh platform with high-turbidity water, a condition conducive for sedimentation. Eleven years (2000–10) of continuous water level and turbidity data for the St. Jones River National Estuarine Research Reserve, a subestuary of Delaware Bay, were analyzed and compared to weather events registered in the National Climate Data Center Storm Events Database. Statistically significant water-level and turbidity reference values were established and used to identify storm-produced events in the data records. Results indicate that northeasters were responsible for most (41%) of all concurrent water-level and turbidity events; other types of weather conditions, including continental lows, northern highs, and frontal storms, produced mostly isolated water level and turbidity peaks. Northeasters coincident with a high-pressure system over the north Atlantic consistently produced the highest water levels and surges, but northeaster intensity was not strongly correlated with surge height. A particular combination of remote wind forcing, intense rainfall, and river runoff distinguishes northeasters among other types of coastal storms in generating flooding events of significance to marsh sedimentation in the St. Jones estuary and, by extension, other subestuaries of Delaware Bay.

Author Correction: Accuracy and Precision of Tidal Wetland Soil Carbon Mapping in the Conterminous United States

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.