Thomas Whelan

Acoustical penetration and shear strength in gas‐charged sediment

Abstract Methane concentrations and sediment shear strengths were measured in three foundation borings taken from areas of variable acoustical penetration in the Mississippi river delta front. Acoustically impenetrable or “turbid”; zones were associated with sedimentary methane concentrations above about 30 ml/liter, measured at atmospheric pressure. Sediments in the high‐gas, acoustically turbid zones demonstrated a smaller percentage increase in shear strength with depth than in zones of low gas concentration. The results indicate that a 3.5‐kHz system used for sub‐bottom profiles is unable to determine the thickness of gas‐charged sediments.

Methane-derived carbonate cements in barrier and beach sands of a subtropical delta complex

Abstract Highly negative δC 13 values, −18 to − 40%., for carbonate cements found in Recent barrier and beach sands of the Mississippi River Delta complex strongly suggest that considerable carbon is furnished to the CaCO 3 cements by either chemical or biological oxidation of CH 4 . These cemented sands are commonly found on beaches of the Chandeleur barrier island chain and other sites along the Louisiana coast where Holocene sands are rapidly transgressing over highly organic marsh deposits. Generation of CH 4 from underlying anoxic marsh sediments, followed by vertical migration and oxidation to CO 2 in the porous overlying sand, appears to be the unique set of conditions regulating this process of carbonate cementation.

Methane, carbon dioxide and dissolved sulfate from interstitial water of coastal marsh sediments

The depth distributions of dissolved CH4, total CO2, Cl− and SO42− in interstitial waters are reported from three subenvironments in the coastal marsh sediments of south Louisiana. Dissolved CH4 ranged from 8.0 × 10−3 to 7.2 ml/l, with each core demonstrating a slight increase in concentration with depth. The highest concentrations of methane were observed in the northern subenvironment of the marsh, which was low in salinity. Extraction under vacuum of methane from whole sediment samples indicated that a significant amount of gas is present as bubbles. Total CO2 ranges from 110 to 410 ml/l. Carbon dioxide, extracted under vacuum, yielded δ13CPDB values of −18.9 and −22.5‰ for the southern and northern areas, respectively. The ∑CO2 concentration in two areas was greater than or equal to that predicted by the depletion of dissolved SO42− from microbial reduction processes. In one core, however, the ∑CO2 concentration was less than could be accounted for by the SO42− deficit. In general, where SO42− reduction was active or where the dissolved SO42− concentration was low, the concentration of CH4 increased. Relatively small vertical gradients of dissolved CH4 and ∑CO2, the low hydrostatic head above the sediment column and the high degree of unconsolidation suggest that gas is migrating upward through the sediment column. Coastal marshes may be an important source of methane to the atmosphere and waters of coastal environments.

Holocene sedimentation at Cape Sable, south Florida

Abstract A regionally distinct mosaic of sedimentary environments including beaches and beach ridges which formed under relatively high-energy conditions through low energy, tide-dominated environments to quiescent inland lakes and ponds, exists at Cape Sable, south Florida. Environments of deposition from the sandy capes inland are: (1) shell beaches and beach ridges; (2) black mangrove mudflats; (3) ponded mudflats; (4) exposed mudflats; (5) Lake Ingraham; (6) coastal levees and supratidal plain; and (7) ephemeral ponds. With exception of the beaches and beach ridges, sediments of the other environments are dominantly calcareous muds and silts and represent a depositional history related to frequency of tidal inundation and storms. Of the three capes exposed roughly from northwest to southeast at Cape Sable, radiocarbon dating shows that Northwest Cape formed in its present position or possibly migrated to it 1980 ± 100 years B.P., while Middle Cape and East Cape data from 1610 ± 100 years B.P. and 1230 ± 95 years B.P. Relict shoreline features, coastal levees, on the supratidal plain indicate an episode of coastal progradation which started 2280 ± 100 years B.P. and apparently ended about 1560 ± 80 years B.P. The entire column of Holocene carbonate sediments in the cape area rests on a thin, intermittent basal peat which dates 4950 ± 120 years B.P. Cores through the supratidal plain reveal a basal carbonate-mud sequence much like subtidal sediments of modern Florida Bay. This unit has a typical marine carbonate-mineral suite, numerous shallow-marine molluscs, and an abundance of Thalassia root burrows. An intermediate intertidal unit characterized by marine brackish carbonate sediments with algal laminations is overlain by massive aragonitic supratidal silts and silty clays. The entire sequence is capped by a dark, highly organic zone representative of the coastal levee and intervening algal-flat sediments. Trace quantities of dolomite and substantial amounts of low-Mg calcite found throughout the cores are considered to be of detrital origin. Localized surface concentrations of dolomite associated with the coastal levees may have a mixed in-situ and detrital origin. Investigation of the extractable organic matter in the intertidal facies of cores through the three prominent coastal levees indicates that diagenesis of organic matter has systematically occurred from the youngest to the oldest levee. Fatty alcohols and total hydrocarbons increase with time while fatty acids decrease. Partial decarboxylation and chemical reduction of fatty acids can occur within the time interval of 750 years between the oldest and youngest coastal levees. Geochemistry of interstitial water from the supratidal-plain cores indicated a general increase in chlorinity from the youngest to the oldest coastal levee. Chlorinity levels drop at the base of all three cores analyzed which may be related to fresher groundwater influence from the Miami Limestone. Ratios of calcium, magnesium, and strontium to chloride are remarkably constant throughout the cores. Higher relative strontium concentrations in the core from the intermediate coastal levee may indicate a subtle dissolution of aragonite. In general, evidence of diagenesis in these carbonate sediments is not readily apparent.

Carbon isotope composition of diagenetic carbonate nodules from freshwater swamp sediments

ABSTRACT The carbon isotope composition of freshwater diagenetic carbonate nodules ranges from -9.5 (vs. NBS 20) in well-drained environments to -19.1 in poorly-drained environments of a freshwater swamp. The 13C values of carbonate nodules are not a function of depth of burial. Depositional environment determines the isotope composition. The data from two cores taken in different sedimentary sequences suggest that in poorly-drained sections transformations of isotopically light organic matter yield carbon, which is incorporated into the diagenetic carbonate. In well-drained sections, renewal of fresh water, in which carbon species are dissolved, provides the source of carbon for diagenetic carbonate nodules.

Long-term chemical effects of petroleum in south Louisiana wetlands—I. Organic carbon in sediments and waters☆

Abstract The chemical effects of chronic petroleum input into a shallow water marsh were examined by measuring hydrocarbon levels and dissolved organic carbon content of sediments associated with two active oil fields in south Louisiana. Annual levels of total organic carbon in the surface waters of the oil fields were higher by 1 mg C/l. in the salt marsh and 5 mg C/l. in the fresh marsh than the respective controlsites. Average dissolved organic carbon concentrations in the interstitial waters of cores taken within the oil field environments were 105% higher than the control in the salt marsh and 43% higher than the control in the fresh marsh. Significantly lower ratios of C17 to pristane occurred in both oil field sediments; however, average odd-even predominance values were not indicative of petroleum contaminated sediments. The results indicate that microbial processes are responsible for dissolution of petroleum into dissolved organic carbon and that dissolved organic carbon concentrations may be a more significant measure of chronic petroleum input than hydrocarbon distribution.

Methane and Carbon Dioxide in Coastal Marsh Sediments

In order to better understand processes of diagenetic gas production in organic-rich sediments, CH4 and CO2 were analyzed, using two extraction techniques, in several environments of the South Louisiana marshlands. Investigation of twelve surface cores and two 100-cm cores indicated that total methane ranged in concentration from 0.12 to 12.2 µl/gm dry sediment, and dissolved methane ranged from 0.13 to 7.4 µl/ml interstitial water. Total CO2 in the interstitial water ranged from 224 to 410 µl/ml IW. Carbon isotope ratios were determined on CO2 released from sediment under vacuum. Dissolved organic carbon in the interstitial water of surface sediments ranged from 15.0 to 38.0 mg C/l. The environmental variation and mechanism of methane production are discussed.