426
S. Wakeham
significant amount of terrigenous OC is deposited in off-shore sediments. In part, this
difference in interpretation arises from the fact that organic matter delivered by rivers to continental shelves may not necessarily have a uniform carbon isotopic composition, and this composition may change over time as the ecosystem in the catchment
basin changes, for example in response to climate change. In the case of the Gulf of
Mexico, a continental end-member value of -26%0 assumes input primarily from C 3
vascular plants and neglects C4 plants (Golli et al. 1998). However, material carried by
the Mississippi River into the Gulf of Mexico includes contributions by both C 3 (trees
and shrubs) and C4 (grasses) plants on the North American continent. Different soil
provenances display different 8l3C values, ranging from --19 to -230/00 for C 4 grasslands to -26%0 for C3 forests (Fig. 19.2; Onstad et al. 2000). Particulate matter delivered to the Gulf is relatively enriched in l3C (8 13 C -19 to - 24%0). Thus, 8l3c values of
-20 to - 220/00 for OC in Gulf of Mexico sediments (Golli et al. 1998) might in fact result from significant inputs of terrigenous C4-derived OC rather than strictly marine
OC. If so, then the terrigenous contribution to these sediments would be higher than
previously believed, possibly greater than 50% (Golli et al. 1998).
Part of the uncertainty in interpreting isotopic data for bulk organic carbon arises
from the inclusion of organic materials originating from a variety of photosynthetic
sources that individually imprint different 8l3C signals depending on their carbon
source, degree of photosynthetic isotope fractionation, and differing response to environmental factors. Analyses of individual biomarker distributions in conjunction
with bulk isotopic compositions help resolve these complications (Prahl et al. 1994;
Fig. 19.2. Stable carbon isotope value for riverine suspended particulate matter in the United States.
The stippled areas are C4 grasslands. From Onstadt et al. (2000)
S. Wakeham
significant amount of terrigenous OC is deposited in off-shore sediments. In part, this
difference in interpretation arises from the fact that organic matter delivered by rivers to continental shelves may not necessarily have a uniform carbon isotopic composition, and this composition may change over time as the ecosystem in the catchment
basin changes, for example in response to climate change. In the case of the Gulf of
Mexico, a continental end-member value of -26%0 assumes input primarily from C 3
vascular plants and neglects C4 plants (Golli et al. 1998). However, material carried by
the Mississippi River into the Gulf of Mexico includes contributions by both C 3 (trees
and shrubs) and C4 (grasses) plants on the North American continent. Different soil
provenances display different 8l3C values, ranging from --19 to -230/00 for C 4 grasslands to -26%0 for C3 forests (Fig. 19.2; Onstad et al. 2000). Particulate matter delivered to the Gulf is relatively enriched in l3C (8 13 C -19 to - 24%0). Thus, 8l3c values of
-20 to - 220/00 for OC in Gulf of Mexico sediments (Golli et al. 1998) might in fact result from significant inputs of terrigenous C4-derived OC rather than strictly marine
OC. If so, then the terrigenous contribution to these sediments would be higher than
previously believed, possibly greater than 50% (Golli et al. 1998).
Part of the uncertainty in interpreting isotopic data for bulk organic carbon arises
from the inclusion of organic materials originating from a variety of photosynthetic
sources that individually imprint different 8l3C signals depending on their carbon
source, degree of photosynthetic isotope fractionation, and differing response to environmental factors. Analyses of individual biomarker distributions in conjunction
with bulk isotopic compositions help resolve these complications (Prahl et al. 1994;
Fig. 19.2. Stable carbon isotope value for riverine suspended particulate matter in the United States.
The stippled areas are C4 grasslands. From Onstadt et al. (2000)