Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1986
Description:
A study of the remineralization of organic carbon was conducted in
the organic-rich sediments of Buzzards Bay, MA. Major processes affecting
the carbon chemistry in sediments are reflected by changes in the stable
carbon isotope ratios of dissolved inorganic carbon (ΣCO2) in sediment
pore water. Six cores were collected seasonally over a period of two
years. The following species were measured in the pore waters: ΣCO2,
δ13C-ΣCO2, PO4, ΣH2S, Alk, DOC, and Ca. Measurements of pore
water collected seasonally show large gradients with depth, which are
larger in summer than in winter. The δ13C (PDB) of ΣCO2 varies
from 1.3 o/oo in the bottom water to approximately -10 o/oo at 30 cm.
During all seasons, there was a trend towards more negative values with
depth in the upper 8 cm due to the remineralization of organic matter.
There was a trend toward more positive values below 8 cm, most likely due
to biological irrigation of sediments with bottom water. Below 16-20 cm, a
negative gradient was re-established which indicates a return to
remineralization as the main process affecting pore water chemistry.
Using the ΣCO2 depth profile, it was estimated that 67-85 gC/m2
are oxidized annually and 5 gC/m2-yr are buried. The amount of carbon
oxidized represented remineralization occurring within the sediments. This
estimate indicated that approximately 20% of the annual primary
productivity reached the sediments. The calculated remineralization rates
varied seasonally with the high of 7.5 x 10-9 mol/L-sec observed in
August 84 and the low (0.6 x 10-9) in December 83. The calculated
remineralization rates were dependent on the amount of irrigation in the
sediments; if the irrigation parameter is known to ±20%, then the
remineralization rates are known to this certainty also. The amount of
irrigation in the sediments was estimated using the results of a seasonal
study of 222Rn/22R6a disequilibria at the same study site (Martin,
1985). Estimates of the annual remineralization in the sediments using
solid-phase data indicated that the solid-phase profiles were not at
steady-state concentrations.
The isotopic signature of ΣCO2 was used as an indicator of the
processes affecting ΣCO2 in pore water. During every month, the
oxidation of organic carbon to CO2 provided over half of the carbon added
to the ΣCO2 pool. However, in every month, the δ13C of ΣCO2
added to the pore water in the surface sediments was greater than -15 o/oo,
significantly greater than the δ13C of solid-phase organic carbon in
the sediments (-20.6 o/oo). The δ13C of ΣCO2 added to the pore
water in the sediments deeper than 7 cm was between -20 and -21 o/oo,
similar to the organic carbon in the sediments. Possible explanations of
the 13C-enrichment observed in the surface sediments were:
a) significant dissolution of CaC0, (δ13C = + 1.7 o/oo),
b) the addition of significant amounts of carbonate ion from
bottom water to pore water,
c) an isotopic difference between the carbon oxidized in the
sediments
and that remaining in the sediments.
The effect of CaCO3 dissolution was quantified using measured dissolved
Ca profiles and was not large enough to explain the observed isotopic
enrichment.
An additional source of 13C-enriched carbon was bottom water
carbonate ion. In every month studied, there was a net flux of ΣCO2
from pore water to bottom water. The flux of pore water ΣCO2 to bottom
water ranged from a minimum of 10 x 10-12 mol/cm2-sec in December 83 to
a maximum of 50 x 10-12 mol/cm2-sec in August 84. However, because the
pH of bottom water was about 8 while that of the pore water was less than
or equal to 7, the relative proportion of the different species of
inorganic carbon (H2CO*3, HCO-3, CO2-3 was very different in
bottom water and pore water. Thus, while there was a net flux of ΣCO2
from pore water to bottom water, there was a flux of carbonate ion from
bottom water to pore water. Because bottom water ΣCO2 was more
13C-enriched than pore water ΣCO2, the transfer of bottom water
carbonate ion to pore water was a source of 13C-enriched carbon to the
pore water. If the δ13C of CO2 added to the pore water from the
oxidation of organic carbon was -20.6 o/oo, then the flux of Co2-3 from
bottom water to pore water must have been 10-30% of the total flux of
ΣCO2 from pore water to bottom water. This is consistent with the
amount calculated from the observed gradient in carbonate ion.
Laboratory experiments were conducted to determine whether the
δ13C of CO2 produced from the oxidation of organic carbon
(δ13C-OCox) was different from the δ13C of organic carbon in the
sediments (δ13C-SOC). In the laboratory experiments, mud from the
sampling site was incubated at a constant temperature. Three depths were
studied (0-3, 10-15, and 20-25 cm). For the first study (IE1), sediment
was stirred to homogenize it before packing into centrifuge tubes for
incubation. For the second study (IE2), sediment was introduced directly
into glass incubation tubes by subcoring. The second procedure greatly
reduced disturbance to the sediment. Rates of CO2 production were
calculated from the concentrations of ΣCO2 measured over up to 46 days.
In both studies, the values of Rc in the deeper intervals were
about 10% of the surface values. This was consistent with the field
results, although the rates decreased more rapidly in the field. In all
cases, the remineralization rates during the beginning of IE1 were much
greater than those at the beginning of IE2. The sediment for IE1 was
collected in February 84. The measured value of Rc in the surface
sediment of the laboratory experiment (24 x 10-9 mol/L-sec) was much
greater than the value of Rc observed in the field in another winter
month, December 83 (.62 x 10-9). The sediment for IE2 was collected in
August 85. The measured values of Rc in the surface sediment (6.6-12 x
10-9 mol/L-sec) were consistent with the field values from August 84 (7.5
x 10-9). The ΣCO2 results indicated that IE2 reproduced field
conditions more accurately than IE1 did.
The isotopic results from the experiments strongly suggested that
δ13C-OCox in the surface sediments (-17.8 o/oo ± 1.9 o/oo) was
greater than δ13C-SOC (-20.6 ± 0.2 o/oo). The magnitude of the
observed fractionation was small enough that the observed values of
δ13C-ΣCO2 in the pore waters could be explained by fractionated
oxidation coupled with the diffusion of carbonate ion from bottom water to
pore water. The observed fractionation was most likely due to the multiple
sources of organic carbon to coastal sediments. A study of the natural
levels of radiocarbon In these sediments indicated that the carbon
preserved in the sediments is approximately 30% terrestrial while the rest
is from phytoplankton.
Description:
Financial support was provided by the Education Office of the
Massachusetts Institute of Technology/Woods Hole Oceanographic Institution
Joint Program In Oceanography, by an Andrew W. Mellon Foundation grant to
the Coastal Research Center, WHOI, and by the National Science foundation
under grant NSF OCE83-15412.
Keywords:
Marine sediments
;
Carbon isotopes
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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