Sedimentary Organic Matter Sources in Florida Bay as Revealed Through Molecular
Marker Analysis and Compound Specific Stable Isotope Measurements
Rudolf Jaffé,Yunping Xu, and Ralph Mead
Florida International University, Miami, Fl,USA
Tracing the origin of sedimentary organic matter in near-shore environments can be
difficult due to mixed allochthonous and autochthonous sources. The use of biomassspecific molecular markers, as well as compound specific carbon isotope measurements,
can aid in discerning mixed source assignments. This study was undertaken to assess
organic matter sources in Florida Bay, using specific molecular markers for Thalassia
testudinum and Rhizophora mangle, molecular marker proxies and compound specific
carbon isotope measurements.
Florida Bay is a partially enclosed, sub-tropical estuary with mangrove islands,
Rhizophora mangle, and dense meadows of Thalassia testudinum inter-dispersed
throughout the bay. A transect of surface grab samples was collected from the
Northeastern to Southwestern section of Florida Bay, and the solvent extractable organic
matter fraction was characterized by GC/MS and GC-IR/MS. As mentioned earlier, the
techniques used to trace organic matter sources to the sediments included a proxy
developed by Ficken et al. (2000) along with compound specific stable isotopes and
specific molecular markers for mangrove (-amyrin) and seagrass (C25 ketone)
vegetation.
The proxy used, Paq, takes advantage of the fact emergent and submerged vegetation has
more mid chain length n-alkanes than longer chain n-alkanes. Table 1 shows the Paq
values and compound specific carbon isotope values of n-alkanes obtained for the major
biomass in Florida Bay and Everglades National Park. The Paq values for seagrass and
mangroves are quite different as are the isotope numbers thus allowing for possible
discrimination of these two sources. A more specific way to trace the inputs of seagrass
and mangroves are to use compounds specific for each. Hernandez et al. (2001) found
that C25 ketone (figure 1), was significantly more abundant in seagrass than in any of the
other vegetation analyzed thus areas receiving more seagrass-derived OM would result in
higher relative abundances of the C25 ketone. Similarly another molecular marker was
used to trace mangrove-derived material (see figure1). The presence of -amyrin would
indicate mangrove sources due to this compounds specificity for this environment.
Preliminary results indicate a significant influence of mangrove derived organic matter
on the composition of OM in seagrass beds, particularly in the Northeastern and central
section of the bay, as seen by the presence of -amyrin (figure 2). The compound specific
stable carbon isotope values of the n-alkanes from the same site as in figure 1 also
showed considerable terrestrial inputs of material. Data from the more seagrass
dominated Southwestern section of the Bay presented neither molecular nor isotopic
evidence for significant mangrove derived organic matter inputs. This data would seem to
make sense due to the abundance of mangrove islands in the Northeastern and central
part of Florida Bay and the possible organic matter transport of mangrove material from
the mainland to these sites.
References:
 Ficken, K.J., B.Li, D.L.Swain, G. Eglington, (2000). Organic Geochemistry,
31,pp. 745-749
 Hernandez, M.E., R.Mead, M.C. Peralba, R.Jaffé, (2001). Organic
Geochemistry.32, pp.21-32
Yungping Xu, Florida International University, Miami, Fl 33199
Phone: 305-348-3118, Fax:305-348-4096,yxu003@fiu.edu,Question 4
Plant
C23
C25
C27
C29
C31
Paq
Rhizophora mangle
Cladium jamaicense
Eleocharis cellulosa
Ruppia maritima
Chara sp.
Utricularia sp.
periphyton
Halophila decipiens
Halodule wrightii
Syringodium filiforme
Thalassia testudinum
Sargassum pteropleuron
Caulerpa mexicana
Penicillus capitatus
Sediment
TSPH 9 sed
TSPH 10 sed
TSPH 11 sed
Trout Cove
Bob Allen Key
Russell Key
Rabbit key
-31.22
-31.15
-28.55
-16.64
-24.34
-41.40
-32.25
-17.21
-14.78
-13.25
-14.87
n.m.
n.m.
n.m.
-31.64
-29.11
-31.06
-16.84
-26.30
-41.87
-30.19
-17.24
-15.27
-13.14
-18.64
n.m.
n.m.
n.m.
-32.26
-32.13
-31.77
-17.31
-25.10
-40.45
-30.00
-17.11
-15.56
-12.84
-19.50
n.m.
n.m.
n.m.
-31.92
-32.38
-33.99
-17.52
-25.06
-39.10
-30.10
-19.91
-16.32
-15.81
-19.12
n.m.
n.m.
n.m.
-30.04
-33.66
-33.25
-18.73
N.D.
-40.73
-28.52
N.D.
N.D.
N.D.
-19.12
n.m.
n.m.
n.m.
0.24
0.13
0.10
0.67
0.89
0.85
0.45
0.90
0.99
1.00
0.93
0.88
0.95
0.95
-21.16
-16.21
-12.65
n.m.
n.m.
n.m.
n.m.
-24.10
-21.43
-12.55
n.m.
n.m.
n.m.
n.m.
-25.87
-24.66
-12.06
n.m.
n.m.
n.m.
n.m.
-28.42
-27.84
-18.76
n.m.
n.m.
n.m.
n.m.
-24.82
-22.62
N.D.
n.m.
n.m.
n.m.
n.m.
0.68
0.63
0.92
0.44
0.52
0.58
0.70
Table 1: Compound Specific GC-IRMS and Paq data of vegetation and sediments.
n.m. = not measured
n.d. = not detected
A)
O
B)
HO
Figure 1: A) C25 Ketone molecular marker for seagrass. B) -Amyrin molecular marker
for mangrove input.
-Amyrin
Figure 2: Chromatogram of the alcohol fraction from TSPH 9 showing the mangrove
input molecular marker.