Results of Whole-Rock Organic Geochcmical Analyses of the CRP-3 Drillcore, Victoria Land Basin, Antarctica
Department of Geoscienccs. Universily of Nebraska-Lincoln. Lincoln. N I < 68588-0340 - USA (rketllcrl @unl.cdu)
Abstract - Sediments and rocks recovered in CRP-3 coring operations contain minute amounts of organic matter (average 0.3% T O C ) . T O C contents and C:N
ratios a r e zoned systematically: those rocks encountered at depths greater t h a n 330 meters below sea floor ( m b s f ) contain less organic matter and have higher 'I'0C:N ratios (after correcting for inorganic N) than do shallower rocks. Tlie only two samples that have TOC values greater than 1 % also contain abundant granule l o silt-sized particles of coal. The total sulphur contents of these rocks is very low a n d indicates either that only small amounts of deposited organic matter w e r e labile, or that no source of reactive iron was present during diaeenesis.
INTRODUCTION
The Oligocene and Eocene(?) rocks penetrated by t h e C R P - 3 c o r e (Fig. 1) were deposited in environments that included terrestrial fluvial systems, deltaic systems with iceberg influences, and clear glacial-marine settings. (Cape Roberts Science Team, 2 0 0 0 ; p . 1 8 7 - 1 9 7 ) . T h e s e different depositional environments could lead to differences in the amount, type, and provenance of organic matter preserved within the rocks. Analysis of organic matter present in the CRP-3 core was undertaken as part of the initial c o r e characterisation effort. O w i n g to t h e s m a l l amounts of organic matter in these rocks, this effort was limited to whole-rock measurements of total organic carbon, total nitrogen and total sulfur. The principal o b j e c t i v e of this work is to provide information on the organic matter preserved in these sediments and rocks. This information can be used to determine whether more detailed studies might be fruitful and to make some conclusions regarding the source and composition of organic matter preserved in these sediments and rocks.
METHODS
Eighteen major lithostratigraphical units consisting of 3 7 l i t h o s t r a t i g r a p h i c a l subunits ( L S U s ) w e r e described by the Cape Roberts Science Team (2000;
p . 59-68) (Fig. 2). Seventy o n e samples (Tab. 1 ) representative of the major lithologies penetrated by the CRP-3 hole were collected at c. l 1 m intervals by the CRP curatorial staff at McMurdo Station. Eight of
t h e subunits were not sampled. T h e unsamplecl subunits included LSUs 16.1 - 18.1 (Fig. 2) which are apparently Devonian and outside the scope of t h e Cape Roberts Project, as well as the subunits defined in the dolerite conglomerate and related lithologies present from 789.77-823.1 1 mbsf (Fig. 2). T h e s e latter lithologies are unlikely to contain significant a m o u n t s of organic matter. L S U 1 0 . 1 was not sampled and consists of a mudstone similar to that found in LSU 3.1 and LSU 8.1 (Fig. 2). The samples were shipped unfrozen to Lincoln, Nebraska. After arrival in Lincoln the samples were dried at 35OC for 2 4 hours and g r o u n d and homogenised using an Angstrom mill. All glassware and equipment used in sample preparation was washed in Micro solution and
ROSS SEA
Fig. I - Map of the south west corner of the Ross Sea. showing the locations of the CRP-1. CRP-212a. and CRP-3 drill sites (Taken from Cape Roberts Science Team. 2000).
clay , sand silt gravel 0 - .-:.... . . . . - . ~ . ~ -
* - . m ~
...
-. . .p .... : :
--
:.:.:... -- ::.:::. -L- ...
A . . . - - - - - - - - -- - .p
- - - ~~-
... . ... . . . -~
--.*-.a
1 00-27753- ... 0
. .;....
. .
Fig. 2 - Comparison of values of TOC. TN, TS and T0C:TN with the lithology of CRP-3 core (Cape Roberts Science Team. 2000: p. 7).
rinsed in purified water. This cleaning was followed Hedges & Stern (1984). The results of these analyses by sequential rinses with 1 % hydrochloric acid. are presented in table 1.
methanol, and dichloromethane.
Measurements of total organic carbon (TOC), total
carbon (TC), total nitrogen (TN), and total sulphur RESULTS AND DISCUSSION (TS) were made on whole-rock samples using the
Carlo Erba-EA 1108 at the University of Nebraska. The sediments and rocks penetrated by the CRP-3 Total organic carbon was measured using the low- drill hole contain very little organic matter: the TOC temperature vapour acidification technique outlined by values measured average 0.3% (Tab. 1). Most of these
Results of Wholc-Rock Organic (;coclwii~iciil An;ilyses of the ("RP-? Drillcore 305 Tub. I - Organic geochcmical data tbr samples obtained fro111 (XI' .1
Notes: 11.q.-not quantified (concentration exceeded limit of detection bill cannot be shown to be greater than zero). Errors listed represent one standard deviation.
s a m p l e s d o not contain s i g n i f i c a n t a m o u n t s of carbonate carbon: only 16 of these samples have total carbon (TC) values that exceed the TOC contents by an amount greater than the combined errors (Tab. 1).
T h e T O C c o n t e n t of these r o c k s chan,. oes at t w o s c a l e s . 1) S a m p l e s of E a r l y O l i g o c e n e rocks encountered at depths more shallow than 330 mbsf contain m o r e organic c a r b o n t h a n d o s a m p l e s obtained from older rocks. The average TOC values f o r rocks above 330 mbsf is 0 . 4 6 % w h e r e a s t h e deeper rocks have average TOC values of 0.26%. The TOC values of the samples obtained at depths above 3 3 0 mbsf a r e c o m p a r a b l e to t h o s e measured i n s a m p l e s obtained from the O l i g o c e n e s e c t i o n penetrated by CRP-212A (Fig. 3). This zoning is also evident in Fig. 2 and can be recognized by the low and uniform TOC values below 330 mbsf: only 3 of the 30 shallow samples have TOC values that are less than 0.255, whereas 30 of the 4 1 deep samples have TOC values that are less than 0.2%. 2) At depths m o r e shallow than 3 3 0 m b s f , r o c k s a s s i g n e d t o highstand systems tracts contain more organic carbon than other rocks. T h e Cape Roberts Science Team described 23 sequences in the top 483 m of the CRP- 3 core and fine-gsained rocks were assigned to the highstand systems tract (HST) (Cape Roberts Science Team, 2000; p. 69). Lithostratigraphical Subunits assigned to HSTs included LSUs 1.3. 3 . 1 , 5.1 and 8.1. Each of these has TOC values that are slightly greater than those found in adjacent rocks (Tab. 1 ; Fig. 2). This relationship is much less well developed
in CRP-3 than the similar relationship described f o r CRP-212A (Kettler and Papastavros, 2000). Although it was possible to identify condensed sections in the CRP-212A core using the diatom abundance data collected by members of the Cape Roberts Science Team (Kettler and Papastavros, 2000), microfossils arc almost conlpletely absent from much of the CRP-3 core. The interpretation that higher TOC values i n rocks assigned to a H S T correspond to periods of relative clastic sediment starvation remains, however.
T h i s relationship is not observed in the d e e p e r portions of the hole (Fig. 2).
The T 0 C : T N ratios measured in these rocks are high and range from 5 to 61. T0C:TN ratios must be interpreted with caution in rocks that contain very little organic c a r b o n . Typically t h e c a u t i o n is warranted because inorganic nitrogen will become an important contributor when TOC values are very low (Stein, 1991). Inorganic nitrogen is apparently a significant c o n t r i b u t o r to t h e N budget of many samples from the CRP-3 core. The relatively coarse- grained rocks in the deep samples would be expected to contain little aquatic organic matter, yet 24 of the 4 1 s a m p l e s o b t a i n e d f r o m d e p t h s greater than 330 mbsf have TOCITN values less than 10. T h e TOCITN values can be corrected for contributions by inorganic nitrogen by regressing the TN values on the T O C data (Leventhal, pers. comm.). Although the shallow and deep samples plot along two different trends in T N vs T O C space (Fig. 4), both yield TN values of c. 0.01% when organic carbon is absent.
l l
l l J CRP 3 Depth>330 mbsf CRP .3 Dcptl~.330 mbsf
^RP-2 Deep (Oligoi ene)
TOC
(wt%)
Fig. 3 - Comparison of TOC values in CRP-3 samples obtained from above 3 3 0 mbsf. below 330 mbsf a n d with the Oligocene section i n CRP-212A.
Ratios of T0C:TON (total organic nitrogen) can then be calculated assuming a constant inorganic N value of 0.01% (Tab. 1). Whereas the average TOCITN and TOCITON values for the deep samples are 14 and 42, respectively, they are 18 and 36, respectively, for the shallow samples. Although the high T 0 C : T N a n d TOCJTON ratios obtained in this work should b e i n t e r p r e t e d c a u t i o u s l y , t w o mutually c o n s i s t e n t i n t e r p r e t a t i o n s a r e p o s s i b l e . 1 ) O r g a n i c m a t t e r produced in the water column experienced extensive remineralization and only the most resistant fraction of the aquatic organic matter was preserved. 2) A significant fraction of the preserved organic matter comprises detrital coal. The second interpretation is confirmed easily by visual inspection of the samples:
t h e s a m p l e ( C R P - 3 - 6 2 8 . 1 9 - 6 2 8 . 2 1 ) that h a s t h e highest TOC value (2.75%) also contains abundant granule-sized (2-4 mm) clasts of coal.
Interpretation of the S data is complicated by the very low TOC values of these rocks. When plotted in T S vs T O C space (Fig. 5 ) , the CRP-3 samples p l o t a r o u n d t h e "normal marine" trend a n d i n c l u d e a number of samples with very low TS:TOC ratios.
Although very low T S : T O C ratios a r e considered evidence that sediments were deposited in fresh water (Berner and Raiswell, 1984), that conclusion is not appropriate for t h e s e samples f o r two reasons. 1) T S : T O C ratios d i s c r i m i n a t e b e t w e e n m a r i n e a n d freshwater sedimentary rocks poorly when the T O C values of are less than 1 % (Berner and Raiswell, 1984). 2) The organic matter deposited at the CRP-3 site comprises a mixture of aquatic organic matter a n d coal. B e c a u s e c o a l i s r e s i s t a n t t o b a c t e r i a l degradation, the low TS:TOC ratio of CRP-3-628.19-
6 2 8 . 2 1 need not he evidence of a f r e s h w a t e r clcpositional cm /n'onmmt. '
I'hcrc arc ;I Sew samples that have TS:TOC ratios that exceed 1 ;ind would be considered to b e iinom:iloiis (Fig. 5 ) . Only o n e sample (CRP-3-286.77- 286.79) has a '1'S:TOC ratio that is clearly indicative of epigenctic addition ol' sulphur (Leventhal. 1995) a n d that s a m p l e also contains pyrite v e i n l e t s . Although six other samples have TS:TOC ratios that exceed unity (Fig. 5 ) i t is unlikely that these samples have experienced epigenetic addition of sulfur. A n u m b e r of these samples o c c u r near t h e tops o f stratigraphic sequences i n rocks that were assigned t o the HST. Others occur at the tops of small groups of samples in which the TS content increases upwards (Fig. 2). Relating these latter high TS:TOC ratios t o positions within the sequences is difficult because the C a p e Roberts Science Team was not able to define sequences in this portion of the core during the initial c o r e d e s c r i p t i o n . Why should T S : T O C r a t i o s b e r e l a t e d to stratigraphic p o s i t i o n ? T h e tops o f stratigraphic sequences were apparently marked by decreased rates of clastic sedimentation and greater relative inputs of biogenous material. Aquatic organic matter would have been far a better substrate than detrital coal for sulfate-reducing bacteria. Although the ratio of aquatic organic matter to coal detritus delivered to the sediment would have been maximized din-ing interglacial episodes ( H S T ) , this ratio - a s reflected by T 0 C : T N ratios or T O C values- would not necessarily be preserved. T h e aquatic organic matter could be almost completely mineralized during early diagenesis whereas the refractory coal detritus would remain.
T h e sulphur data are also important because of suggestions that modiolid mussels found in sub units 1 . 1 a n d 1 3 . 1 a r e indicative of p e c u l i a r b o t t o m conditions that might be characterized by significant H,S production (Cape Roberts Science Team, 2000: p.
1 6 9 - 1 7 0 ) . T h e T S : T O C ratios o b s e r v e d i n t h e s e
Fig. 4 - Plot of T N versus TOC for CRP-3 samples. Lines are the result of least squares regressions of the shallow ( d e p t k 3 3 0 mbsf) and deep (depth>330 mbsf) samples.
308 R.M. Kcttlcr
Total Organic Carbon (wt%)
F/,(;. 5 - Plot of TS versus TOC for CRP-3 samples with "normal marine" trends and unit TS:TOC ratio shown for reference.
subunits (Tab. 1) d o not support such a n interpretation. The TS:TOC ratio in sample 8.13-8.15 is 0.45; a value that is wholly compatible with normal marine rocks containing less than 1% TOC. Although S is detectable in LSU 13.1, 11 of the 12 samples from this sub-unit have less than 0.1% S and 8 have TOC values less than 0.1%. Even the three samples that have TOC values greater than 0.5% have very low sulphur contents. A conventional interpretation of these data is that very little labile organic matter was deposited with these sediments and, therefore, very little H2S was produced. It is unlikely, therefore, that the modiolid mussel communities reflect an H2S-rich environment unless the samples obtained in this study are not representative of the sedimentary section in which the mussels occur or the iron in the sediments was resistant to sulfidation.
These rocks probably contain little solvent-soluble organic matter: the very low TOC values require that little solvent soluble organic matter be present. It is unlikely that Rock-Eval pyrolysis of these or other C R P - 3 s a m p l e s would yield useful data b e c a u s e Rock-Eval pyrolysis data are very difficult to interpret when the samples analyzed have very low TOC and bitumen contents (Peters, 1986). M o l e c u l a r characterisation of solvent-soluble organic matter would also be unlikely to yield valuable information.
Previous extraction of rocks with similarly low TOC values h a s yielded masses of bitumen t h a t a r e typically less than 0.001 g (Kettler, 1998; Kettler and Papastavros, 2000). The molecular composition of the small amounts of extractable organic matter in these rocks c o u l d have been affected significantly b y contamination at the drill site, during core processing, o r by microbial alteration d u r i n g shipping. A n y conclusions drawn from molecular analyses would likely be equivocal; thus. isolation and molecular characterisation of the bitumen was not undertaken.
CONCLUSIONS
The scdimentiiry rocks recovered by the C K P .1 core contain small amounts of orgcinic inallixr.
Although the Oligoccnc section above 3 3 0 nlhsl' contains amounts of organic matter similar to (lie Oligocene section i n the CRP-212A core. most of tl1c samples obtained from below 330 mbsf contain less than 0.2% TOC. Interpretation of T 0 C : T N ratios is difficult in rocks that have such low TOC values. The T 0 C : T N ratios do increase with increasing TOO' values. This relationship is evidence that inorganic N contributes significantly to TN values in samples that contain little TOC. a n d that much of the preserved organic matter in these rocks comprises detrital coal.
T h e TS:TOC ratios are consistent with a m a r i n c system in which much of the deposited o r g a n i c carbon was refractory. There is evidence i n one sample of epigenetic addition of sulfur (confirmed by the presence of pyrite veinlets). Although s i x other samples have TS:TOC ratios that exceed unity, i t is likely that these elevated TS:TOC ratios result from near complete consumption of aquatic organic matter by sulfate reducing bacteria. The sulphur data d o not support the hypothesis that t h e d e p o s i t i o n a l environments of LSUs 1.1 and 13.1 were H$-rich.
ACKNOWLEDGEMENTS - The Cape Roberts Project was made possible by the resources and close collaboration o f the Antarctic programmes of Italy, New Zealand, United States of America, Germany, Australia, and Great Britain.
with field operations organised by Antarctica New Zealand.
Reviews by A. Bruker-Wein, M.G. Laird, J.A. Leventhal.
and P.A. Meyers improved this manuscript. The author is grateful f o r t h e opportunity to participate in t h e C a p e Roberts Project. This work was supported by NSF grant OPP-9527070,
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