Charlotte A. Brunner

Deep-sea sedimentary record of the late Wisconsin cataclysmic floods from the Columbia River

New results from Ocean Drilling Program Site 1037 and U.S. Geological Survey high-resolution seismic-reflection profiles confirm the great thickness, fast deposition rate, distant source, and convolute path of turbidites that fill the Escanaba Trough, the rift valley of the southernmost segment of the Gorda Ridge. Accelerator mass spectrometry 14 C measurements provide the first direct dating of the Escanaba Trough turbidites, demonstrating an average deposition rate faster than 10 m/k.y. between 32 and 11 ka and as fast as 15 m/k.y. during the oxygen isotope stage 2 lowstand. In the upper 60 m of sediment, the petrology of turbidite sand beds, which are as much as 12 m thick, show that the dominant source for the turbidites is from the Columbia River, which is more than 800 km to the north, rather than from the much closer rivers of northern California. New high-resolution seismic-reflection profiles show that, except for areas of very recent volcanism, the entire Escanaba Trough below 3200 m water depth is floored by the turbidite sequence that was cored in the upper 60 m at Site 1037B. The ages of the upper 120 m of turbidites correspond with the ages of channeled scabland deposits associated with latest Quaternary jokulhlaups from glacial Lake Missoula. The age and source characteristics suggest that these megaturbidite beds in Escanaba Trough are most likely deposits formed by hyperpycnally generated turbidity currents as the largest of the Lake Missoula floods entered the sea.

HYPOXIA HOTSPOTS IN THE MISSISSIPPI BIGHT

Foraminiferal proxies of hypoxia indicate apparent low oxygen to hypoxic conditions in several hotspots in the Mississippi Bight. The foraminiferal hypoxia proxies, the Ammonia to Elphidium (A/E) index and the Pseudononion-Epistominella-Buliminella (PEB) index, were tabulated from three sets of core tops collected in 1951–1956. Additionally, the oxygenation history of a site near the Balize delta was evaluated over the past one hundred years based on A/E and PEB indices and size distributions of pyrite framboids in a gravity core dated by 210Pb geochronology. The results from the 1950’s core-top collections show apparent, recurrent low-oxygen to hypoxic bottom water on the inner shelf at hotspot locales seaward of the Mississippi-Alabama barrier islands and the eastern distributaries of the Balize delta. Specifically, the A/E index exceeds 90% on the inner shelf seaward of Horn and Dauphin islands, both of the Mississippi-Alabama barrier islands, and a center between Pass a Loutre and Main Pass of the Balize delta. In partial support of these results are reports of present-day low oxygen to hypoxic concentrations in bottom waters associated with seasonally high surface chlorophyll a and seasonal strengthening of a brackish-water cap at these locales. In contrast, the PEB index in core tops suggests good oxygenation at mid-shelf depths >30 m. The PEB index, size distributions of framboidal pyrite, and other indicators in a gravity core 44 km northeast of Pass a Loutre indicate no clear change in conditions over the past century, constraining the seaward extent of the hotspot near the Balize delta.

Storm-driven transport of foraminifers from the shelf to the upper slope, southern Middle Atlantic Bight

Storms play an important role in the delivery of benthonic foraminifers to the continental slope, as observed in a study of foraminifer fluxes through the upper slope water column. The authors studied 30 sediment-trap samples with a 13-day average period from the 1988–1989 SEEP II experiment offshore from the Delmarva Peninsula. The traps were suspended at about 125 m water depth on a mooring in 400 m of water. Benthonic and planktonic foraminifers from 10-ml subsamples were measured, identified by taxa and growth stage, and counted. Number fluxes of benthonic foraminifers averaged 155 test/m2/d during periods of relative calm during the spring and summer, when mass fluxes of aluminosilicates were also minimal. In contrast, number fluxes of benthonic foraminifers peaked during a mid-April 1988 storm and ranged from about 300 to 50,000 tests/m2/d from mid-December 1988 to the end of April 1989, when mass fluxes of aluminosilicates also were highly elevated. Highest foraminifer fluxes (29,000 and 50,000 tests/m2/d) coincided with a late February storm. Taxa observed included Bolivina, Nonionella, Trochammina, Rosalina, and other taxa typical of the continental shelf of this region. Number fluxes of planktonic foraminifers peaked during the spring and summer due to production. The peaks from 6000 to 11,000 tests/m2/d were due to peaks in productivity of Globigerinita glutinata in early March and late April, Turborotalita quinqueloba in mid-July, and Globigerinita uvula and Globigerinoides ruber in latest September. Planktonic foraminifer fluxes did not crest during the mid-April or mid-December 1988 storms, but fluxes reached peaks of 38,000 and 41,000 tests/m2/d in late February and early March 1989 when fluxes of benthonic foraminifers and aluminosilicate material also were highest. Storms dominated the delivery of both benthonic and planktonic foraminifers to the slope. The single storm in late February 1989 delivered more foraminifers through the water column to the slope (120 × 104 benthonic and 130 × 104 planktonic tests/m2 in 32 days) than during all the preceding calm days in 1988 (1.9 × 104 benthonic and 72 × 104 planktonic tests/m2 in 217 days). Mid-water advection of benthonic foraminifers from the continental shelf to the slope is an important mechanism of delivery that exceeds by an order of magnitude the numbers of planktonic foraminifers produced in slope waters during periods of relative calm weather.

Hydrocarbon gas hydrates in sediments of the Mississippi Canyon area, Northern Gulf of Mexico

Abstract The Gulf of Mexico Hydrates Research Consortium has begun installing a seafloor observatory to monitor gas hydrate outcrops and the hydrate stability zone in Mississippi Canyon Area Lease Block 118. Relevant background information concerning the Mississippi Canyon Area and gas hydrate occurrences in the northern Gulf of Mexico is presented. Microbial influences and possible scenarios of hydrate accumulation are considered. The design of the observatory was based on field data recorded in the Mississippi Canyon Area, principally lease block 118 (MC118) and the vicinity of lease block 798 (MC798). Swath bathymetry by autonomous underwater vehicle played a large part, as did seismic imaging within the hydrate stability zone and core sampling. These data and the results of their analyses are discussed in detail. Discussion and interim conclusions are presented.

Effects of Oil from the 2010 Macondo Well Blowout on Marsh Foraminifera of Mississippi and Louisiana, USA

Foraminifera responded to both heavy and light oiling of marshes relative to unoiled control sites by changes to both standing stock and depth of habitation (DOH) in sediment following the 2010 Macondo well blowout. Push cores were taken from the middle marsh at sites classified as unoiled, lightly oiled, and heavily oiled based on concentrations of total polycyclic aromatic hydrocarbons ([TPAH]). Cores were sliced and stained with rose Bengal to detect live specimens of foraminifera. Short-term, sediment-mixing depths were determined using the penetration depths of excess (234)Th, and sedimentary organic carbon and carbonate were measured to distinguish depositional environments. Marsh foraminifera reacted to the highest oil concentration (5,000-18,000 ng/g of TPAH) by reducing standing stock and shortening the DOH compared with the control sites. At a second, less heavily oiled site, foraminifera responded with a shallower DOH, but with a boom in standing stock. Deformed, dead foraminifera occurred in all heavily oiled cores-but not elsewhere. Live foraminifera responded with a population boom at lightly oiled sites with [TPAH] near 1,100 ng/g. Changes in standing stock and DOH with [TPAH] suggest disturbance to the marsh food web, apparently due to oil pollution, and support the use of foraminifera as sentinel species.

Can Fractures in Soft Sediments Host Significant Quantities of Gas Hydrates

The Gulf of Mexico Hydrate Research Consortium has collected several types of data in and around Mississippi Canyon Lease Block 798 (MC798), an area of the northern Gulf of Mexico where fine-grained sediment occurs at the sea floor and where hydrates have been sampled. Swath bathymetry, heat-flow measurements, core samples, and subbottom profiles were collected. Hydrate was grown in the laboratory in sediments subsampled from the cores to demonstrate that the surficial sediments in MC798 are conducive to hydrate formation. Herein, data are presented and results discussed. It is postulated that significant quantities of hydrate could form in fine-grained sediments by filling fracture porosity produced by polygonal faulting. Analyses of cores combined with laboratory experiments indicate that conditions in MC798 are conducive to the formation of polygonal faults. Heat-flow measurements indicate that the hydrate stability zone is about 400 m (1312 ft) thick. Its upper 100 ms or so appears on two-dimensional (2-D) subbottom profiles to be fine grained. Small, near-vertical fractures indicated by features called brooms are common there. Thus, it is possible that a polygonal fault system exists in the upper 100 ms (75 m [246 ft] at 1500 m/s [4921 ft/s]). It is acknowledged that 2-D profiles cannot demonstrate this conclusively. Conclusive proof would require a three-dimensional (3-D) data set with sufficient resolution to demonstrate interconnectivity among the small faults. If polygonal faulting exists, gas and water could circulate through the fractures and be exposed to smectite-rich clays, a situation favorable to hydrate formation. X-ray images of pressure cores have documented hydrate accumulation within small, nearly vertical fractures in fine-grained sediments. Thus, it is possible that polygonal fault systems could host significant accumulations of hydrate in the Gulf of Mexico.