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U.S. Geological Survey Open-File Report 00-306: Chapter 13
Porewater Methane Geochemistry of Marion-Dufresne Cores MD99-2205 and -2206
by John W. Pohlman1, John F. Bratton2, and Richard B. Coffin1
Abstract
This study was designed to better constrain mechanisms controlling heterogeneity of methanogenesis in estuarine sediments. Preliminary data for porewater methane analyses are presented for Marion-Dufresne cores MD99-2205 and -2206. The results quantify conclusions of previous qualitative studies that linked methane occurrence in Chesapeake Bay sediments with location of underlying Pleistocene paleochannels. Core MD99-2205, collected in a paleochannel contains significantly more methane than a core collected nearby, MD99-2206. There is also a relatively shallow maximum in methane concentration in MD99-2205. These data suggest that enhanced preservation of organic matter in paleochannel sediment fill is due to a thinner sulfate reduction zone in sediments overlying paleochannels. This, in turn, may result from the contribution of fresh submarine groundwater to paleochannel sediments. Also, in situ generation of methane in sediments with high concentrations appears to be a more likely mechanism to explain the distribution of methane in Chesapeake Bay sediments than the migration of methane from a deeper source.
Introduction
In spite of decades of research, the formation, migration, and distribution of methane in marine sediments and its relation to pore water circulation are still poorly understood. As part of the IMAGES V cruise to Chesapeake Bay in June 1999, cores were collected at sites MD99-2205 and -2206 on the western side of the modern Chesapeake axial channel to examine the formation of biogenic methane in Chesapeake Bay sediments. The paired sites were chosen because they are close to each other (~350 m apart) but one (-2205) has subbottom stratigraphy obscured by gas, while the other (-2206) shows an acoustically transparent section of Holocene sediments overlying earlier Quaternary channel fill. By sampling and measuring methane in sediment pore fluids from these two cores, we were able to test multiple hypotheses describing the vertical geometry, genesis, and migration of methane in Chesapeake Bay sediments. The results of the methane studies are expected to have broad application to shallow-water methane occurrence in other regions as well as marine carbon cycling on a global scale.
Porewater Geochemical Methods
Methanogenesis
Previous geophysical and coring studies (e.g., Reeburgh, 1969; Hill and others, 1992; Hagen and Vogt, 1999) have shown that methane gas is abundant in sediments of the mesohaline Chesapeake Bay and that it occurs primarily in paleochannel fills (see Baucom and others, this volume). Unfortunately, methane-charged sediments cannot be imaged by acoustic methods so the thickness of layers of gas bubbles in sediments or other details of methane occurrence are difficult to determine. By sampling and measuring methane in sediment pore fluids from two adjacent cores, one located in acoustically turbid sediments and the other in acoustically clear sediments, this study was designed to test whether 1) methane is generated at depth in paleochannels from Pleistocene and early Holocene organic matter and migrates to shallower sediments, or 2)methane is produced in situ but is concentrated in paleochannels due to lower sulfate concentrations.
Pore fluid extraction
Cores MD99-2205 and -2206 were collected and processed onboard during the night of June 20-21 for measurement of total methane in pore fluids (gaseous + dissolved) and concentrations of other dissolved constituents. To extract pore fluid with minimal release of methane or sample oxidation, subsamples were collected from the ends of core sections immediately after core sections were cut. A subcoring device was used consisting of a clear polycarbonate tube equipped with a rubber plunger and a plastic cap. The subcorer removed a sediment plug 4 cm in diameter and approximately 15 cm long that was immediately capped and sealed with tape until 5-10 cm of it could be extruded for pressing. Sample spacing was approximately 50 cm in core MD99-2205 ("gas core") and 1.5 m in MD99-2206 ("no-gas core"), although pore fluid was not recovered from all intervals.
Pore fluid was extracted from 50- to 100-cm3 aliquots of wet sediment extruded from a subcorer into the stainless steel cylinder of a Manheim-type sediment squeezer (Manheim and others, 1994). The chamber was then sealed with a Teflon® disk and butyl rubber gasket, and a stainless steel piston was inserted into the top of the cylinder. The chamber was placed in a hydraulic press and compressed, forcing pore fluid, methane gas, and a small amount of air through a filter and conduit and into a 30-ml syringe attached to the squeezing apparatus. The syringe was removed after no more fluid was produced from the squeezer, and fitted with a stainless steel needle. The gas and liquid contents of the syringe were injected through a rubber septum into a pre-evacuated serum vial. All samples were preserved by adding 0.2 ml saturated HgCl2 solution per ml of sample and stored at room temperature. Typically, 3-8 ml of water and <5 ml of gas were recovered from each interval. Duplicate samples were collected from intervals that produced especially large volumes of pore fluid. The error for replicate measurements was less than 7% or 1.1 mM. Samples were transported to the Naval Research Laboratory (NRL) on June 21 and were analyzed within 48 hours.
Because of time constraints and porewater yields that were lower than expected, additional aliquots of unoxidized sediment samples were extruded from subcorers and collected directly in Whirl-Pak® bags for later analysis. The bags were compressed to remove air and sealed. Sealed bags were covered with moist peat moss shortly after collection to exclude oxygen. These samples were taken to a shore-based laboratory at the Maryland Geological Survey (MGS) in Baltimore, Maryland, and stored in a refrigerator until they could be centrifuged to separate porewater for additional analyses.
Methane analysis
Total methane in pore fluid from cores MD99-2205 and -2206 was collected on board as described above, and measured in a shore-based laboratory at NRL, Washington, D.C., by a modification of the headspace equilibration technique described by McAuliffe (1971). Both gaseous and aqueous pore fluid phases were collected in syringes and injected together into serum vials. During and after collection, dissolved methane from the samples equilibrated with methane in the headspace of the vial. Thus, in order to determine the total concentration of methane in the core pore fluid, it was necessary to measure the quantity of methane in both the sample headspace and the sample porewater (Eq. 1).
(1) [CH4]Sample = [CH4]Headspace + [CH4]Dissolved
The headspace volume in the vial was collected and measured in a syringe by displacing the gas from the vial with nitrogen-sparged water until all of the headspace gas entered the syringe. The plunger of the syringe was allowed to equilibrate to atmospheric pressure before the volume was measured. The sample was injected through a manual valve with a sample loop, allowed to equilibrate to atmospheric pressure, and carried to the flame ionization detector of a Shimadzu GC-14A gas chromatograph equipped with a Hayesep-Q 80/100 packed column (Alltech). The column temperature was set at 40° C to prevent thermal degradation of the methane during separation. The headspace concentrations of CH4 were quantified in units of ppm on a volume/volume basis (ppmv) by referencing the peak areas to those generated from CH4 standard gases (Scott Specialty Gas, Plumbsteadville, PA).
Following the analysis of the sample headspace, the dissolved methane remaining in the porewater sample was measured by the headspace equilibration technique described by McAuliffe (1971). The sample was drawn into a syringe that was subsequently filled with an equal volume of nitrogen gas. Methane was extracted from the solution by vigorously shaking the syringe for 2 minutes. Multiple nitrogen extractions were performed (usually two) until at least 99% of the methane dissolved in the sample was removed. The quantity of methane measured in the original sample headspace analysis was added to the quantity of dissolved methane measured by the headspace equilibration technique to give the total methane extracted from the sample. The pore water methane concentration was calculated using the ideal gas law.
Results and Discussion
As shown in Figure 13.1, the core collected in the paleochannel (MD99-2205) contains significantly more methane than the adjacent core (MD99-2206). Pore-fluid concentrations of methane in MD99-2205 ranged from 304 to 491 mM in the upper 300 cm of the sediment core. At the base of the Holocene deposits, concentrations dropped to 1 mM and ranged from 0.6 to 8.4 mM to the maximum depth sampled (650 cm). The elevated methane concentrations suggest that fresh groundwater is migrating into the channel fill precluding sulfate from the organic-rich Holocene deposits. Sulfate reduction is reduced to a thin zone at the sediment-water interface and fermentative methanogenesis is the dominant diagenetic pathway for organic matter degradation in the deeper Holocene deposits. The pore water methane peak measured at 150 cm suggests in situ generation of methane, rather than migration of methane from a deeper source (i.e., fluvial peats in bases of paleochannels). Porewater chemistry data (e.g., chlorinity, sulfate concentration, total organic carbon) needed to confirm this interpretation are not yet available.
Acknowledgements
We thank the captain, crew, and members of the scientific party of the R/V Marion-Dufresne, particularly Elisabeth Michel and Yvon Balut, for assistance with core collection and processing.We are also grateful to Peter Vogt from NRL, Jim Hill from MGS, and Scott Schubert from Inchcape Shipping for assistance with pre-cruise planning and equipment logistics.
References Cited
Hagen, R.A., and Vogt, P.R., 1999, Seasonal variability of shallow biogenic gas in Chesapeake Bay: Marine Geology, v. 158, no. 1-4, p. 75-88.
Hill, J.M., Halka, J.P., Conkwright, R., Koczot, K., and Colman, S., 1992, Distribution and effects of shallow gas on bulk estuarine sediment properties: Continental Shelf Research, v. 12, p. 1219-1229.
McAuliffe, C., 1971, Gas chromatographic determination of solutes by multiple phase equilibrium: Chemtech, v. 1, p. 46-51.
Reeburgh, W.S., 1969, Observations of gases in Chesapeake Bay sediments: Limnology and Oceanography, v. 14, no. 3, p. 368-375.
U.S. Department of Interior, U.S. Geological Survey
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