CHAPTER 5 . Organic Matter Preservation: Lipid Behavior from Plankton to Sediments
137
These scaled profiles of relative abundances demonstrate (a) rapid losses of planktonic material in surface waters, (b) elevated proportions of heterotrophic biomarkers
in the mid-water column and sediments, and (c) preservation of selected components
of bacteria, plankton, and vascular plants in sediments. This diagenetic selection is
largely responsible for the marked compositional differences between plankton and
subsurface sediments (Fig. 5.8). Planktonic biomarkers exhibit markedly different types
of behavior. Polyunsaturated compounds such as 22:6w3 and short-chain chain
alcohols such as hexadecanol are subject to extensive decomposition early durin~ transit through the water column, 24-methylcholesta-5,24(28)-dien-3,B-ol (28Ll S ,24 28» is
relatively better preserved, while 24-ethylcholest-5-en-3,B-ol, 4a,23,24-trimethylcholest22E-en-3,B-ol and alkenones are remarkably well preserved in sediments. It is uncertain as to why 24-ethylcholest-5-en-3,B-ol and 4a,23,24-trimethylcholest-22E-en-3,B01 are so well preserved, but the stability of alkenones has been suggested to result
from their unusual double bond configurations (Rechka and Maxwell 1988a,b). The
mid-water maximum for cholest-5-en-3,B-ol is indicative of a mid-water zooplankton
source (Wakeham et al. 1984b). Cis-vaccenic acid and iso- and anteiso-15:0 acids are
present in the water column in minor abundances, possibly due to their production
by bacteria in zooplankton guts or on particles; their enhanced abundances in sediments could indicate preferential preservation in sediments as well as production in
situ. Bisnorhopane and squalene appear to be produced largely in sediments via microfaunal activity. Long -chain hydrocarbons, fatty alcohols and fatty acids derived from
vascular plants are relatively refractory constituents of water column particles that
become magnified in sediments as most of the surface-derived marine material is respired. The stability of vascular plant biomarkers may be enhanced by relatively high
molecular weights and a protective waxy particle matrix (Wakeham et al. 1984b).
5.6
Summary
Interpreting past oceanic environments from the sediment record must be undertaken
with caution since the lipid assemblages preserved in sediments may significantly
under-represent processes occurring in the overlying water column. Diagenesis in the
water column and surface sediments degrades most organic matter. Furthermore, the
tiny residue of organic matter that is buried in sediments may present a highly biased
view of organic matter inputs. Sedimentary lipid distributions, in general, poorly reflect the activities and abundances of most planktonic organisms, while at the same
time overrepresent the importance of other organic matter sources, including some
phytoplankton, terrigenous vascular plants, and bacteria. The factors resonsible for these
biases require further investigation in oceanic water columns and diagenetically active
sediments before sedimentary lipid records may be applied to their full potential.
Acknowledgements
Work described here resulted from collaborative projects, and I thank C. Lee, J. Hedges,
P. Hernes, and M. Peterson for their assistance. These projects have been supported by
the U. S. National Science Foundation.
137
These scaled profiles of relative abundances demonstrate (a) rapid losses of planktonic material in surface waters, (b) elevated proportions of heterotrophic biomarkers
in the mid-water column and sediments, and (c) preservation of selected components
of bacteria, plankton, and vascular plants in sediments. This diagenetic selection is
largely responsible for the marked compositional differences between plankton and
subsurface sediments (Fig. 5.8). Planktonic biomarkers exhibit markedly different types
of behavior. Polyunsaturated compounds such as 22:6w3 and short-chain chain
alcohols such as hexadecanol are subject to extensive decomposition early durin~ transit through the water column, 24-methylcholesta-5,24(28)-dien-3,B-ol (28Ll S ,24 28» is
relatively better preserved, while 24-ethylcholest-5-en-3,B-ol, 4a,23,24-trimethylcholest22E-en-3,B-ol and alkenones are remarkably well preserved in sediments. It is uncertain as to why 24-ethylcholest-5-en-3,B-ol and 4a,23,24-trimethylcholest-22E-en-3,B01 are so well preserved, but the stability of alkenones has been suggested to result
from their unusual double bond configurations (Rechka and Maxwell 1988a,b). The
mid-water maximum for cholest-5-en-3,B-ol is indicative of a mid-water zooplankton
source (Wakeham et al. 1984b). Cis-vaccenic acid and iso- and anteiso-15:0 acids are
present in the water column in minor abundances, possibly due to their production
by bacteria in zooplankton guts or on particles; their enhanced abundances in sediments could indicate preferential preservation in sediments as well as production in
situ. Bisnorhopane and squalene appear to be produced largely in sediments via microfaunal activity. Long -chain hydrocarbons, fatty alcohols and fatty acids derived from
vascular plants are relatively refractory constituents of water column particles that
become magnified in sediments as most of the surface-derived marine material is respired. The stability of vascular plant biomarkers may be enhanced by relatively high
molecular weights and a protective waxy particle matrix (Wakeham et al. 1984b).
5.6
Summary
Interpreting past oceanic environments from the sediment record must be undertaken
with caution since the lipid assemblages preserved in sediments may significantly
under-represent processes occurring in the overlying water column. Diagenesis in the
water column and surface sediments degrades most organic matter. Furthermore, the
tiny residue of organic matter that is buried in sediments may present a highly biased
view of organic matter inputs. Sedimentary lipid distributions, in general, poorly reflect the activities and abundances of most planktonic organisms, while at the same
time overrepresent the importance of other organic matter sources, including some
phytoplankton, terrigenous vascular plants, and bacteria. The factors resonsible for these
biases require further investigation in oceanic water columns and diagenetically active
sediments before sedimentary lipid records may be applied to their full potential.
Acknowledgements
Work described here resulted from collaborative projects, and I thank C. Lee, J. Hedges,
P. Hernes, and M. Peterson for their assistance. These projects have been supported by
the U. S. National Science Foundation.