Dr. Chris Jenkins

DOI: 10.1073/pnas.1804250115

Abstract:

Oceanic uptake of anthropogenic CO₂ leads to decreased pH, carbonate ion concentration, and saturation state with respect to CaCO₃ minerals, causing increased dissolution of these minerals at the deep seafloor. This additional dissolution will figure prominently in the neutralization of man-made CO₂. However, there has been no concerted assessment of the current extent of anthropogenic CaCO₃ dissolution at the deep seafloor. Here, recent databases of bottom-water chemistry, benthic currents, and CaCO₃ content of deep-sea sediments are combined with a rate model to derive the global distribution of benthic calcite dissolution rates and obtain primary confirmation of an anthropogenic component. By comparing preindustrial with present-day rates, we determine that significant anthropogenic dissolution now occurs in the western North Atlantic, amounting to 40–100% of the total seafloor dissolution at its most intense locations. At these locations, the calcite compensation depth has risen ∼300 m. Increased benthic dissolution was also revealed at various hot spots in the southern extent of the Atlantic, Indian, and Pacific Oceans. Our findings place constraints on future predictions of ocean acidification, are consequential to the fate of benthic calcifiers, and indicate that a by-product of human activities is currently altering the geological record of the deep sea.

DOI: 10.1073/pnas.1802379115

Abstract:

Bottom trawlers land around 19 million tons of fish and invertebrates annually, almost one-quarter of wild marine landings. The extent of bottom trawling footprint (seabed area trawled at least once in a specified region and time period) is often contested but poorly described. We quantify footprints using high-resolution satellite vessel monitoring system (VMS) and logbook data on 24 continental shelves and slopes to 1,000-m depth over at least 2 years. Trawling footprint varied markedly among regions: from <10% of seabed area in Australian and New Zealand waters, the Aleutian Islands, East Bering Sea, South Chile, and Gulf of Alaska to >50% in some European seas. Overall, 14% of the 7.8 million-km² study area was trawled, and 86% was not trawled. Trawling activity was aggregated; the most intensively trawled areas accounting for 90% of activity comprised 77% of footprint on average. Regional swept area ratio (SAR; ratio of total swept area trawled annually to total area of region, a metric of trawling intensity) and footprint area were related, providing an approach to estimate regional trawling footprints when high-resolution spatial data are unavailable. If SAR was ≤0.1, as in 8 of 24 regions, there was >95% probability that >90% of seabed was not trawled. If SAR was 7.9, equal to the highest SAR recorded, there was >95% probability that >70% of seabed was trawled. Footprints were smaller and SAR was ≤0.25 in regions where fishing rates consistently met international sustainability benchmarks for fish stocks, implying collateral environmental benefits from sustainable fishing.

DOI: 10.1038/srep01057

Abstract:

Sharks and rays' abundance can decline considerably with fishing. Community changes, however, are more complex because of species interactions and variable vulnerability and exposure to fishing. We evaluated long-term changes in the elasmobranch community of the Adriatic Sea, a heavily exploited Mediterranean basin where top-predators have been strongly depleted historically and fishing developed unevenly between the western and eastern side. Combining and standardizing catch data from five trawl surveys from 1948–2005, we estimated abundance trends and explained community changes using life histories, fish-market and effort data and historical information. We identified a highly depleted elasmobranch community. Since 1948, catch rates have declined by >94% and 11 species ceased to be detected. The exploitation history and spatial gradients in fishing pressure explained most patterns in abundance and diversity, including the absence of strong compensatory increases. Ecological corridors and large-scale protected areas emerged as potential management options for elasmobranch conservation.

DOI: 10.2110/jsr.2018.15

Abstract:

Abundant data on sediment accumulation rates exist for the continental margin in the region of the Mississippi Delta. They were obtained during numerous studies by many institutions and researchers using a range of methods: radioisotope and other tracers, identified events, and biostratigraphy. For several reasons, it is necessary to integrate the data across the region: to test and validate numerical models of sedimentation, and to devise methods for a Big Data integration of similar rates on a global scale. We have collated over 700 records of sediment accumulation rate in the Mississippi region. They were taken over time intervals (viz. integration times, measurement intervals) that range over five orders of magnitude, from 2 days to 1000 years. This range stems from use of diverse analytical methods such as tracers with different half-lives, and also from varying sample lengths and times.

A Sadler Effect relationship is found to apply across the data: intercept 1.71 in log₁₀(cm/yr), slope –0.55, R² 0.66, N 717. It corroborates ad hoc observations in the area by analysts, that short-period rates tend to be higher than long-term rates. Investigations are made on how the Sadler Effect coefficients vary by environment: by water depth, physiographic province, and sediment mud contents. For most environments the coefficients cluster around the general result, though coefficients for prodelta, deep basin, bay, canyon, and upper slope areas are separate enough to be remarked on.

The statistics indicate that measurement interval is responsible for most of the inter-analysis variability. Therefore a transform of the data to a 1-year basis (the Sadler Effect intercept) is proposed, removing measurement-interval effects and providing a general framework for integrating the rates. This is in conformance with a random-walk model for the Sadler Effect of steady, constant deposition plus a spectrum of unsteady nondeposition events. Precise values of the coefficients are different in some environments. After the transform, variations in the rates by spatial and environmental factors are revealed more clearly.

DOI: 10.3390/jmse8080586

Abstract:

Turbidity currents deliver sediment rapidly from the continental shelf to the slope and beyond; and can be triggered by processes such as shelf resuspension during oceanic storms; mass failure of slope deposits due to sediment- and wave-pressure loadings; and localized events that grow into sustained currents via self-amplifying ignition. Because these operate over multiple spatial and temporal scales, ranging from the eddy-scale to continental-scale; coupled numerical models that represent the full transport pathway have proved elusive though individual models have been developed to describe each of these processes. Toward a more holistic tool, a numerical workflow was developed to address pathways for sediment routing from terrestrial and coastal sources, across the continental shelf and ultimately down continental slope canyons of the northern Gulf of Mexico, where offshore infrastructure is susceptible to damage by turbidity currents. Workflow components included: (1) a calibrated simulator for fluvial discharge (Water Balance Model - Sediment; WBMsed); (2) domain grids for seabed sediment textures (dbSEABED); bathymetry, and channelization; (3) a simulator for ocean dynamics and resuspension (the Regional Ocean Modeling System; ROMS); (4) A simulator (HurriSlip) of seafloor failure and flow ignition; and (5) A Reynolds-averaged Navier–Stokes (RANS) turbidity current model (TURBINS). Model simulations explored physical oceanic conditions that might generate turbidity currents, and allowed the workflow to be tested for a year that included two hurricanes. Results showed that extreme storms were especially effective at delivering sediment from coastal source areas to the deep sea, at timescales that ranged from individual wave events (~hours), to the settling lag of fine sediment (~days).

DOI: 10.23784/HN116-11

Abstract:

Severe storms marked by very high wave conditions occur several times per year in the German Bight (North Sea). In places and at certain times the wave-induced flows at the seabed are then powerful enough to significantly displace exposed, heavy UXO such as mines and bombs from the two world wars. To investigate we used kinematic modelling of the objects supported by good-resolution wave and current data. We tested the results against actual observations. Return times on likely object migration are the output. The result provides a more precise and quantitative understanding of UXO object behaviour under the severe storm conditions of the Bight. Subsea infrastructure projects in the region need no longer assume that movements occur everywhere, but have tools to determine where and when different objects in the spectrum of UXO have the potential to migrate and repopulate the operating areas.

DOI: 10.1080/00288306.2018.1523199

Abstract:

This paper provides new maps of the surficial sediment distribution on the continental shelf (0∼150 m water depth) of New Zealand based on a new database – nzSEABED. The maps of percent mud, sand, gravel and carbonate, are compared with previous research to provide a comprehensive update of the surficial sediment distributions on the continental shelf, together with a review of the main environmental (oceanographic and climatic), geomorphological and geological processes and human activities that have influenced sediment deposition. Continental shelves are dynamic regions that are in a constant state of flux from floods, storms, tides, waves, earthquakes and volcanic activity. While some of these events may be captured by individual samples, the compilation of >23,000 samples collected and analysed over 60 years provides a long-term average distribution of sediments on the continental shelf that can inform future research and coastal management.

DOI: 10.1016/j.csr.2007.11.011

Abstract:

We present a methodology for statistical analysis of randomly located marine sediment point data, and apply it to the US continental shelf portions of usSEABED mean grain size records. The usSEABED database, like many modern, large environmental datasets, is heterogeneous and interdisciplinary. We statistically test the database as a source of mean grain size data, and from it provide a first examination of regional seafloor sediment variability across the entire US continental shelf. Data derived from laboratory analyses (“extracted”) and from word-based descriptions (“parsed”) are treated separately, and they are compared statistically and deterministically. Data records are selected for spatial analysis by their location within sample regions: polygonal areas defined in ArcGIS chosen by geography, water depth, and data sufficiency. We derive isotropic, binned semivariograms from the data, and invert these for estimates of noise variance, field variance, and decorrelation distance. The highly erratic nature of the semivariograms is a result both of the random locations of the data and of the high level of data uncertainty (noise). This decorrelates the data covariance matrix for the inversion, and largely prevents robust estimation of the fractal dimension. Our comparison of the extracted and parsed mean grain size data demonstrates important differences between the two. In particular, extracted measurements generally produce finer mean grain sizes, lower noise variance, and lower field variance than parsed values. Such relationships can be used to derive a regionally dependent conversion factor between the two. Our analysis of sample regions on the US continental shelf revealed considerable geographic variability in the estimated statistical parameters of field variance and decorrelation distance. Some regional relationships are evident, and overall there is a tendency for field variance to be higher where the average mean grain size is finer grained. Surprisingly, parsed and extracted noise magnitudes correlate with each other, which may indicate that some portion of the data variability that we identify as “noise” is caused by real grain size variability at very short scales. Our analyses demonstrate that by applying a bias-correction proxy, usSEABED data can be used to generate reliable interpolated maps of regional mean grain size and sediment character.

DOI: 10.1007/s00367-002-0111-0

Abstract:

Sediment colour data are delivered by geologists as Munsell codes (Rock Color Chart) and linguistic descriptions. Using new software suitable for very large data sets, the two types can be brought into conformance and mapped together digitally. The native codes are extracted. For linguistic descriptions chromatic terms are identified with Munsell codes, then mixed in a temporary transform of psychometrically linear CIE colour space. Adjustments are made for dark/light and pale/strong modifiers. The output Munsell codes are statistically validated and mapped using special GIS legends to render them in true colour. The output displays provide a new view of marine sediment facies, comparable to remotely sensed colour imagery.