Tomoko Komada

Sediment Pore Waters

Abstract Dissolved organic matter (DOM) in marine sediment pore waters plays an important role in sediment carbon and nitrogen remineralization and may also be involved in sediment carbon preservation. In this chapter we examine this topic, focusing on: the composition and reactivity of pore water DOM, with particular reference to its role in sediment organic matter remineralization; the factors that control pore water DOM concentrations over various time and space scales; the role of benthic DOM fluxes in the oceanic cycles of carbon and nitrogen; and role of pore water DOM in sediment carbon preservation. Approaches to modeling pore water DOM are also described.

Fluorescence characteristics of organic matter released from coastal sediments during resuspension

Excitation–emission matrix (EEM) fluorescence spectroscopy was employed to study the chemical nature of organic matter readily released into solution from sediment particles during episodes of resuspension. Surface sediment samples collected from five locations within the Hudson River Estuary and the Inner New York Bight were resuspended in their respective bottom waters for periods ranging from 30 s to 2 h. In most cases, fluorescence characteristics of the sample after resuspension differed from those predicted by conservative mixing of bottom and pore waters. Examination of excitation–emission matrices revealed that resuspension resulted in: (1) more intense humic-like fluorescence (Ex/Em 310/420 nm), and (2) greater fluorescence in the longer wavelength region of the spectrum (Ex/Em 330–388/440–480 nm) relative to the predicted values. Trends in the positions of excitation–emission maxima reported in the literature strongly suggest that fluorophores emitting at longer wavelengths are associated with increasingly degraded and/or aged organic matter. Thus, the data imply that resuspension of estuarine and coastal marine surface sediments releases degraded and/or aged, mineral-bound organic matter from the sediment matrix to the surrounding bottom waters. D 2002 Elsevier Science B.V. All rights reserved.

Emiliania huxleyi increases calcification but not expression of calcification-related genes in long-term exposure to elevated temperature and pCO2.

Increased atmospheric pCO2 is expected to render future oceans warmer and more acidic than they are at present. Calcifying organisms such as coccolithophores that fix and export carbon into the deep sea provide feedbacks to increasing atmospheric pCO2. Acclimation experiments suggest negative effects of warming and acidification on coccolithophore calcification, but the ability of these organisms to adapt to future environmental conditions is not well understood. Here, we tested the combined effect of pCO2 and temperature on the coccolithophore Emiliania huxleyi over more than 700 generations. Cells increased inorganic carbon content and calcification rate under warm and acidified conditions compared with ambient conditions, whereas organic carbon content and primary production did not show any change. In contrast to findings from short-term experiments, our results suggest that long-term acclimation or adaptation could change, or even reverse, negative calcification responses in E. huxleyi and its feedback to the global carbon cycle. Genome-wide profiles of gene expression using RNA-seq revealed that genes thought to be essential for calcification are not those that are most strongly differentially expressed under long-term exposure to future ocean conditions. Rather, differentially expressed genes observed here represent new targets to study responses to ocean acidification and warming.

Resuspension-induced partitioning of organic carbon between solid and solution phases from a river-ocean transition

Ž. Laboratory experiments were conducted to evaluate the net exchange of organic carbon OC between sediments and overlying water during episodes of resuspension. Surface sediment samples collected from six locations within the Hudson River Estuary and the Inner New York Bight were resuspended in their respective bottom waters for periods ranging from 30 Ž. s to 2 h. After resuspension, dissolved organic carbon DOC concentration generally reached levels greater than that predicted by conservative mixing of pore water and bottom water, indicating net release of OC from the sediment particles. The amount of OC released during the 1-h extractions comprised F 0.1% of the total sediment pool, but correlated Ž 2 .Ž . positively R s 0.65, P- 0.052 with the amount of particulate organic carbon POC found in the high-density fraction of the sediment matrix. This suggests that the mineral-bound fraction of sedimentary OC was the major source for the excess Ž. DOC released into solution, and that across various sedimentary environments, only a small but fairly constant fraction of the total sedimentary POC may be poised for rapid transfer to the water column. q 2001 Elsevier Science B.V. All rights reserved.

Carbon cycling in Santa Barbara Basin sediments: A modeling study

The primary input of organic matter to almost all marine sediments comes from deposition at the sediment surface. However, in many continental margin settings, reduced carbon can also be added to sediments from below—for example, from “deep” geologic hydrocarbon reservoirs derived from ancient source rocks or from the decomposition of deeply buried gas hydrate deposits. To examine the impact of these two differing reduced carbon inputs on sediment biogeochemistry, a modified reaction-transport model for anoxic marine sediments is described here and applied to data from sediment cores in Santa Barbara Basin to a depth of 4.6 m. Excellent model fits yield results consistent with previous studies of Santa Barbara Basin and other continental margin sediments. These results indicate that authigenic carbonate precipitation in these sediments is not centered around the sulfatemethane transition zone (SMTZ), as is seen in many other sedimentary environments but occurs at shallower depths in the sediments and over a relatively broad depth range. Sulfate profiles are linear between the surface sediments (upper ∼20 cm) and the top of the SMTZ (∼105 cm) giving the appearance of refractory particulate organic carbon (POC) burial and conservative sulfate behavior in this intermediate region. However, model results show that linear profiles may also occur when high rates of sulfate reduction occur near the sediment surface (as organoclastic sulfate reduction [oSR]) and in the SMTZ (largely as anaerobic oxidation of methane) with low, but nonzero, rates of oSR inbetween. At the same time, linearity in the sulfate profile may also be related to downward pore-water advection by compaction and sedimentation plus a decrease with depth in sulfate diffusivity because of decreasing porosity. These model-determined rates of oSR and methanogenesis also result in a rate of POC loss that declines near-continuously in a logarithmic fashion over the entire sediment column studied. The results presented further here indicate the importance of a deep methane flux from below on sediment biogeochemistry in the shallower sediments, although the exact source of this methane flux is difficult to ascertain with the existing data.

Composition of Dissolved Organic Matter in Pore Waters of Anoxic Marine Sediments Analyzed by 1H Nuclear Magnetic Resonance Spectroscopy

Marine sediments are globally significant sources of dissolved organic matter (DOM) to the oceans, but the biogeochemical role of pore-water DOM in the benthic and marine carbon cycles remains unclear due to a lack of understanding about its molecular composition. To help fill this knowledge gap, we used 1H nuclear magnetic resonance (NMR) spectroscopy to examine depth variability in the composition of pore-water DOM in anoxic sediments of Santa Barbara Basin, California Borderland. Proton detected spectra were acquired on whole samples without pre-concentration to avoid preclusion of any DOM components from the analytical window. Broad unresolved resonance (operationally assigned to carboxyl-rich alicyclic molecules, or CRAM) dominated all spectra. Most of the relatively well-resolved peaks (attributed to biomolecules or their derivatives) appeared at chemical shifts similar to those previously reported for marine DOM in the literature, but at different relative intensities. DOM composition changed significantly within the top 50 cm of the sediment column, where the relative intensity of CRAM increased, and the relative intensity of resolved resonances decreased. The composition of CRAM itself also changed throughout the entire length of the 4.5-m profile, as CRAM protons became increasingly aliphatic at the expense of functionalized protons. Given that pore-water DOM is generated from sedimentary organic matter that includes pre-aged and degraded material, and that DOM could theoretically be subjected to microbial reworking in the pore waters for centuries to millennia, these data suggest that marine sediments may be sources of CRAM that are compositionally unique from CRAM generated in the upper ocean.

Methane dynamics in Santa Barbara Basin (USA) sediments as examined with a reaction-transport model

Here we describe a new reaction-transport model that quantitatively examines δ13C profiles of porewater methane and dissolved inorganic carbon (DIC) (δCCH4 and δCDIC) in the anoxic sediments of the Santa Barbara Basin (California Borderland region). Best-fit solutions of the model to these data suggest that CO2 reduction is the predominant form of methanogenesis in these sediments. These solutions also accurately reproduce the isotope depth profiles, including a broad minimum in the δCDIC profile and a much sharper (angular) minimum in the δCCH4 profile, both of which appear near the base of the transition zone in the sediments between sulfate reduction and methanogenesis (referred to here as the sulfate-methane transition zone, or SMTZ). Such minima in pore-water profiles of δCCH4 near the base of the SMTZ have been seen in a number of other marine sediments across a range of depth and timescales. We show here that this minimum in the δCCH4 profile in Santa Barbara Basin sediments results from the balance between (1) anaerobic oxidation of methane (AOM), which leads to an increase in δCCH4 with decreasing depth in the sediment column through and above the SMTZ; (2) methanogenesis, which produces 13C-depleted methane, both in and below the SMTZ; and (3) an upward flux of CH4 from depth that is relatively enriched in 13C as compared with the methane in these pore waters. Possible sources of this deep methane include the following: geologic hydrocarbon reservoirs derived from ancient source rocks; decomposition of buried gas hydrates; and biogenic (or perhaps thermogenic) methane produced hundreds of meters below the seafloor stimulated by increasing temperatures associated with the sediment geothermal gradient. Although we are unable to resolve these possible sources of deep methane, we believe that the significance of an upward methane flux as an explanation for minima in δCCH4 pore-water profiles may not be limited to Santa Barbara Basin sediments but may be common in many continental margin sediments.