CHAPTER 19 . Palaeoenvironmental Reconstructions
429
depth) about 100 years ago. This isotopic trend is consistent with an increased influx
of C4-derived carbon resulting from clearing of forests in the catchment and replacement with grasses and sugarcane. Alternately, the shift in vegetation could result from
aboriginal burning or a decrease in rainfall to the basin, both of which would also
change the ecosystem. Overall, the shift in isotopic signal in the past century is twice
as large as any other changes over the past 8000 years.
Organic carbon isotope signatures can provide palaeoenvironmental information
over long geologic time frames. Schoell et al. (1994) measured 8 13 C values for C27
steranes and C35 hopanes in the Naples Beach section of the Monterey (California)
formation. The Miocene Monterey formation has received considerable attention from
petroleum geochemists, since it is an important source bed for the oil fields on the
California continental margin. Steranes are fossil biomarkers derived from sterols
biosynthesized by many phytoplankton that live in surface waters (0-30 m) in the
contemporary ocean and presumably a similar depth range during the Miocene. The
hopanes are derived from bacteriohopane polyols biosynthesized exclusively by
prokaryotes, including cyanobacteria, the bulk of which generally reside deeper in the
photic zone (e.g. 70-100 m). Convergence of 8 13 C values of sedimentary steranes and
hopanes over the same time period would imply that both phytoplankton and
cyanobacteria took up the same dissolved inorganic carbon (DIC) pool with a single
8 13 C DI c> and thus the water column was mixed. Divergence of sterane and hopane
8 13 C values would result if the water column had been stratified and there were two
DIC pools having different 8 13 CDIC values. The variation in sterane and hopane
8 13 C values over the -22 million years of the Naples Beach section of the Monterey
formation is shown in Fig. 19.5. Isotope values for the steranes were relatively constant
( - -25 to -26%0), while there was a wider range (-31%0 increasing to -26%0) for the
hopanes. There was a large difference between sterane and hopane 813C values (~13CH_S)
of -6%0 early in the record during a cold climatic period (see the Antarctic ice sheet
and 8 18 0 records in Fig. 19.5) vs. roughly equal values later in the record during a
warmer climate. These data suggest that during the colder early Miocene, the water
column was more stratified and there was a greater separation between zones of productivity of phytoplankton (shallow photic zone) and cyanobacteria (deep photic
zone). However, later the water column was relatively well-mixed.
19.3
Depositional Environment - Anoxygenic Photosynthesis
Preservation of organic matter in sediments is a prerequisite for the development of a
record that can be studied. It was traditionally accepted that anoxic depositional environments favour preservation of organic matter (Demaison and Moore 1980), and
there has been considerable research both supporting and refuting this hypothesis
(e.g. Emerson and Hedges 1988; Calvert and Pederson 1992; Lee 1992; Paropkari et al.
1992; Keil and Cowie 1999). Since anoxia and enhanced organic matter preservation
might lead to formation of petroleum source rocks, recognition of any biomarkers that
might aid in discerning anoxic palaeoenvironments would be of great interest to petroleum geochemistry and organic geochemistry alike.
Anoxic environments that might be prototypes for petroleum source beds have been
intensely studied. One such environment is the Black Sea, where circulation in the
429
depth) about 100 years ago. This isotopic trend is consistent with an increased influx
of C4-derived carbon resulting from clearing of forests in the catchment and replacement with grasses and sugarcane. Alternately, the shift in vegetation could result from
aboriginal burning or a decrease in rainfall to the basin, both of which would also
change the ecosystem. Overall, the shift in isotopic signal in the past century is twice
as large as any other changes over the past 8000 years.
Organic carbon isotope signatures can provide palaeoenvironmental information
over long geologic time frames. Schoell et al. (1994) measured 8 13 C values for C27
steranes and C35 hopanes in the Naples Beach section of the Monterey (California)
formation. The Miocene Monterey formation has received considerable attention from
petroleum geochemists, since it is an important source bed for the oil fields on the
California continental margin. Steranes are fossil biomarkers derived from sterols
biosynthesized by many phytoplankton that live in surface waters (0-30 m) in the
contemporary ocean and presumably a similar depth range during the Miocene. The
hopanes are derived from bacteriohopane polyols biosynthesized exclusively by
prokaryotes, including cyanobacteria, the bulk of which generally reside deeper in the
photic zone (e.g. 70-100 m). Convergence of 8 13 C values of sedimentary steranes and
hopanes over the same time period would imply that both phytoplankton and
cyanobacteria took up the same dissolved inorganic carbon (DIC) pool with a single
8 13 C DI c> and thus the water column was mixed. Divergence of sterane and hopane
8 13 C values would result if the water column had been stratified and there were two
DIC pools having different 8 13 CDIC values. The variation in sterane and hopane
8 13 C values over the -22 million years of the Naples Beach section of the Monterey
formation is shown in Fig. 19.5. Isotope values for the steranes were relatively constant
( - -25 to -26%0), while there was a wider range (-31%0 increasing to -26%0) for the
hopanes. There was a large difference between sterane and hopane 813C values (~13CH_S)
of -6%0 early in the record during a cold climatic period (see the Antarctic ice sheet
and 8 18 0 records in Fig. 19.5) vs. roughly equal values later in the record during a
warmer climate. These data suggest that during the colder early Miocene, the water
column was more stratified and there was a greater separation between zones of productivity of phytoplankton (shallow photic zone) and cyanobacteria (deep photic
zone). However, later the water column was relatively well-mixed.
19.3
Depositional Environment - Anoxygenic Photosynthesis
Preservation of organic matter in sediments is a prerequisite for the development of a
record that can be studied. It was traditionally accepted that anoxic depositional environments favour preservation of organic matter (Demaison and Moore 1980), and
there has been considerable research both supporting and refuting this hypothesis
(e.g. Emerson and Hedges 1988; Calvert and Pederson 1992; Lee 1992; Paropkari et al.
1992; Keil and Cowie 1999). Since anoxia and enhanced organic matter preservation
might lead to formation of petroleum source rocks, recognition of any biomarkers that
might aid in discerning anoxic palaeoenvironments would be of great interest to petroleum geochemistry and organic geochemistry alike.
Anoxic environments that might be prototypes for petroleum source beds have been
intensely studied. One such environment is the Black Sea, where circulation in the