Data and modeling procedures:

 

The purpose of this study was to determine the depth-dependence of oxic organic matter decomposition within the sediment column at a wide range of sites in the deep sea. The measurements used to constrain decomposition rates were pore water profiles of (1) NO₃^-, with samples obtained by shipboard whole-core squeezing (Bender et al., 1987; Martin et al., 1991) or sectioning and centrifugation or by in situ whole-core squeezing (Martin and Sayles, 1996; Sayles et al., 2001), and in situ pore water sampling (Sayles, 1981); and (2) O₂, with profiles obtained by shipboard whole-core squeezing (Martin et al., 1991) or by in situ microelectrode profiling. The latter measurements were made by Hales and Emerson, 1997.

The regions sampled, and the measurements used from each region, are summarized in the table below:

Pore water profiles analyzed for this study
Site Information			Profiles per region			Near-interface (~0-5 cm) data Number of profiles
Region	Latitude & Longitude	Water depth	# of sites	# of profiles	Sites with Profiles > 15 cm deep	ship wcs NO3	ship wcs O2	in situ wcs NO3	in situ mep O2 (Hales & Emerson, 1997)
Equatorial Pacific	6°S - 11°N, 133°-136°W	4230-5050	10	18	0	10	8
Ceara Rise	5°-6°N, 43°-44°W	3280-4680	7	11	7	6		2	3
Cape Verde Plateau	13°-18°N, 21°-23°W	3100-4760	4	11	3	3	4	3
Sierra Leone Rise	5°-10°N, 22°-25°W	3590-5120	6	11	6	5		4
AESOPS Transect	66°-57°S, 170°W	2740-4970	7	12	6	2	2	5

One table of results has been constructed for each of the studied regions.

In order to derive organic matter oxidation rates from the pore water profiles, the profiles were fit with a diagenetic model taking the form of a steady-state differential equation,

 

 

φ is porosity, D_sed is the diffusion coefficient for NO₃^- or O₂, corrected for tortuosity using measured porosity and resistivity profiles, C is the pore water NO₃^- or O₂ concentration, w is the sedimentation rate, and P is the production rate of NO₃^- or consumption rate of O₂, which has the form,

 

 

j₀ and j₂ are reported in μmol/cm³_pw/y; j₁ and j₃ are reported in 1/cm_sed. The solution method and fitting procedures used to obtain P(x) from pore water data are described in Sayles et al., 2001.

For input into the diagenetic equation, porosity profiles were fit with the exponential function,

 

 

Resistivity and porosity data were used to determine sedimentary diffusion coefficients by, first, obtaining ν from

 

 

in which F is the formation factor, the ratio of sediment resistivity to overlying water resistivity, and, second, obtaining D_sed by

 

 

The rationale for this procedure is described in Martin et al., 1991.

 

 

Explanation of data tables:

 

Station Data:

Site: cruise and site designation. There is one entry for each modeled profile. Therefore, there may be more than one entry per site.

Latitude and Longitude: self-explanatory.

Water depth: given in meters.

 

Input Data:

Samples used: Describes the profiles (i.e., the types of samples and number of profiles) that were combined to make the profiles for fitting with the diagenetic model. When specific samples were omitted from the final profile, those samples are identified. Near-surface samples from sectioning / centrifugation were generally omitted because of sampling artifacts. The maximum depth of the omitted samples varied, depending on comparisons between results of centrifugation and other sampling techniques. Often, individual samples from in situ pore water profiles were omitted when they fell far off the trend of the remaining data in the profile. Such departures from the trend are usually due to contamination by bottom water. The following shorthand for sample types is used:

• iswcs = in situ whole-core squeezer
• sec = sectioning / centrifugation
• ispw = in situ, harpoon-type pore water samples
• lwcs = shipboard whole-core squeezer
• mep = in situ microelectrode profiles

n data: number of data used for the fit.

Depth range: shows the range of depths below the sediment-water interface that are included in the model fit. For Cape Verde Plateau profiles, x* is the depth of the oxic decomposition / denitrification boundary.

Colw: the solute concentration at x = 0 cm.

p0, p1, p2: Parameters used to describe the porosity profile at each site, as described above.

ν: The parameter used to convert seawater diffusion coefficients into sedimentary diffusion coefficients, as described above.

 

Best-fit parameters:

j0, j1, j2, and j3 in P(x), as described above.

 

Calculated fluxes:

Flux (x=0): The flux across the sediment-water interface that is calculated from the best-fit model profile. Units are μmol/cm²_sed/y.

Pno3 or Co2: The production rate of NO₃^-, or consumption rate of O₂, by oxic organic matter decomposition. In the case of NO₃^-, this number may be greater than Flux (x=0). When this occurs, there is denitrification below the oxic zone, with a flux of NO₃^- downwards. Then, Pno3 is the sum of this downward flux and Flux (x=0).

Corg ox - R: The rate of oxic decomposition of organic matter that is implied by Pno3 or Co2, assuming Redfield stoichiometry (Redfield et al., 1963), i.e., Corg ox = Pno3 * 106/16 and Corg ox = Co2 * 106/138.

Corg ox - AS: The rate of oxic decomposition of organic matter that is implied by Pno3 or Co2, assuming the decomposition stoichiometry of Anderson and Sarmiento, 1994, i.e., Corg ox = Pno3 * 7.3 and Corg ox = Co2 * 0.69.

 

Data Sources:

Equatorial Pacific: Martin et al., 1991

Ceara Rise: Martin and Sayles, 1996. O₂ microelectrodc profiles from Hales and Emerson, 1997

Cape Verde Plateau: Sayles and Martin, unpublished data

Sierra Leone Rise: Sayles and Martin, unpublished data

AESOPS transect: Sayles et al., 2001