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A possible link between the seawater osmium isotope record and weathering of ancient sedimentary organic matter
Chemical Geology, 107 (1993) 255-258 Elsevier Science Publishers B.V., Amsterdam
255
A possible link between the seawater osmium isotope record and weathering of ancient sedimentary organic matter G. Ravizza a and B.K. Esserb "Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, USA bL-396 Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
(Received March 9, 1993: revised and accepted March 30, 1993 )
The Os isotopic composition of leachable Os from a North Pacific pelagic clay sequence (Pegram et al., 1992 ) and bulk sediment samples of metalliferous carbonates deposited near the East Pacific Rise (Ravizza, 1993) display similar patterns of temporal variation (Fig. 1 ). This similarity indicates that the composite record of t87Os/186Os ratio variations reflects changes in the Os isotopic composition of seawater. Temporal changes in the Sr isotopic composition of seawater record changes in the relative rates of continental weathering and oceanic crustal alteration. The Os isotopic 9.0"
composition of seawater should be influenced by similar processes. In both the Rb-Sr and Re-Os systems, the continental crust is characterized by large time-integrated parent/ daughter ratios, compared to the deep Earth, imparting relatively radiogenic Sr and Os signatures to continental material. However, the seawater Sr and Os records are decoupled from one another over the past 27 Ma. From 27 to 15 Ma 87Sr/86Sr increased rapidly while the 18VOs/186Os ratio of seawater remained nearly constant. At 15 Ma the ~87Os/tS6Os ratio of seawater began to increase rapidly but the rate
Zero Age Pacific Sediments
8.5' 8.0 CD CO
©
7.5 7.0
65 6.0 1 5.5
Steady State
~
EPR Metalliferous Carbonates (Ravizza 1993) North Pacilic Pelagic Clay (Pegram eLal. 1992) i i
5.0 0
5
10
i
i
15
20
25
Fig. 1. Temporal variations in the 187Os/186Osratio of seawater inferred from analyses of leachable Os in pelagic clay from LL44-GPC3 and bulk sediment analyses of metalliferous carbonates from DSDP Leg 92. The agreement of these two records over the past 27 Ma indicates that the major features of both records reflect changes in the Os isotopic composition of seawater rather the influence of local diagenesis.
0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
G. RAVIZZAAND B.K.ESSER
256
of increase in the 87Sr/86Sr ratio diminished. Accelerated weathering of ancient sedimentary organic matter may account for this decoupling and the rapid increase in the ~87Os/ ~86Os ratio of seawater over the past 15 Ma (Pegram et al., 1992; Ravizza, 1993). Calculations described here suggest this is a viable hypothesis which has implications for our understanding of changes in the size of the buried organic carbon reservoir during the Neogene. Secular variations in the ~ ~3C of marine carbonates constrain past changes in the relative sizes of the buried organic and inorganic carbon reservoirs. The fi~3C values of bulk carbonate from the South Atlantic display a negative shift of 2%o over the past 15 Ma (Shackleton, 1987 ). While this shift is indicative of net oxidation of organic carbon, carbon isotopic data do not allow enhanced weathering to be distinguished from diminished organic carbon burial. These two means of reducing the size of the buried organic carbon reservoir may have different effects on oceanic nutrient cycling (Delaney and Boyle, 1988). The coincidence of the (~13C shift to more negative values (Fig. 2) and 187Os/186Os shift to more radiogenic values is compatible with an accelerated rate of organic matter weathering. The geochemistry of Re plays a fundamental role in coupling the geochemical cycles of Os and organic matter. The primary sink for Re in the oceans is burial in association with organic-rich sediments, accounting for ~ 50% of the present-day riverine Re flux (Colodner et al., 1993). In this respect the geochemical cycles of Re and organic carbon are linked. Fig. 3 shows that Re and organic carbon concentrations are correlated in both m o d e m and ancient organic-rich sediments. Because decay of 187Re is responsible for the production of ~87Os, organic-rich sediments represent an important crustal reservoir for the production of radiogenic 187Os (here after denoted as ~87Os*). The reduced character of these sediments renders them vulnerable to rapid oxidative weathering. Hence during weathering of organic-rich
sediments, oxidation of their burden of organic matter should be coupled with the liberation of soluble 187Os*. The older the weathered sediment, the higher will be ratio of t87Os*to oxidized organic matter. A simple model has been constructed to evaluate whether the decrease in the size of the buried organic carbon reservoir inferred from the fi~3C of marine carbonates and the increase in the ~87Os/186Os ratio of seawater can be linked plausibly via weathering of ancient sedimentary organic matter. This model assumes that the increase in the ~87Os/~86Os of seawater which commences at ~ 15 Ma is caused by the addition of 187Os*without perturbing either the size of the seawater Os reservoir or the flux of c o m m o n Os to the oceans. Under these conditions the additional flux of ~8VOs*(denoted as J~ 87- ) which perturbs the steady state can be expressed as a simple function of time: j187" = (dRsw/dt)N 186..~_j186 [Rsw(l) -Rst.st. ] where: is the net flux of ~86Os into oceans. It is estimated from Os burial rates in marine sediments (Esser and Turekian, 1988; Ravizza and Turekian, 1992; Ravizza and McMurtry, 1993 ) to be ~7.4-106 mol Ma -~ 186Os. This parameter is assumed to be constant over the past 25 Ma. - - N 186 is the size of the oceanic reservoir of ~86Os in moles. Using estimates of the Os concentration of seawater (Goldberg and Koide, 1990; Ravizza et al., 1991 ) the reservoir is calculated to be ~ 105 mol 186Os. This parameter is also assumed to be constant over the past 25 Ma. --Rsw(t) = 187Os/186Os ratio of seawater. This value changes with time and has been approximated by two line segments (Fig. 1 ). dR~w/dt is the rate of change of the 187Os/ ~86Os ratio of seawater. The value of dR~w/dt is nearly constant over the past 15 Ma and can be obtained from the slopes of the two line segments shown in Fig. 1.
__ j186
-
-
257
SEAWATEROs ISOTOPERECORDAND WEATHERINGOF ANCIENTSEDIMENTARYORGANICMATTER 3.0 2.5 20
t'n a El. v
1.5 1.0
O
0.5 0.0 -0.5 Data from Shackleton and Hall 1984 -1.0
I
I
i
I
5
10
15
20
Age
25
(Ma)
Fig. 2. Record ofO ~3C variations of bulk carbonate from DSDP Holes 525 and 525A. Shackleton ( 1987 ) interpreted these data as evidence of a net decrease in the size of the buried organic carbon reservoir over the past 15 Ma. - - R s t . s t . is the 1 8 7 O s / 1 8 6 O s ratio of the total Os
input to the oceans at steady state, prior to any perturbation by the increased input of 187Os*. This value ( ~ 6.2) is obtained from the flat portion of the seawater Os curve between 27 and 15 Ma. The size of 187Os~" perturbation integrated over the past 15 Ma is 2.108 mol. Based on Shackleton's estimate of the decrease in the size of the buffed organic carbon reservoir over this same time interval ( 1019 mol C) and assuming c o u p l e d 187Os~t input, the average 187Os~r/organic carbon ratio of organic matter lost from the organic carbon reservoir is 2- 10- l i. If the net loss of organic matter from the organic carbon reservoir was solely the result of a diminished rate of carbon burial, as suggested by Shackleton (1987), the expected I87Os*/organic carbon ratio would be zero and the Os and 813C records should be uncoupled. Using the I87Os*/organic carbon ratio and the slope of the Re-organic carbon reference line from Fig. 3, the mean "'age" of organic matter lost from the buffed organic carbon res-
ervoir can be estimated from the equation: 187OsSr/Corg = ( 1 8 7 R e / C o r g ) 2 t
where 2 is the decay constant of 187Re ( ~ 1.6.10- i1 y r - 1). The resulting estimate is ~ 35 Ma. This "age" is geologically reasonable and suggests that the net oxidation of organic carbon implied by &I3C is better explained by enhanced weathering than by diminished organic matter burial. It is important to note that this "age" does not represent the mean age of all weathered organic matter over this time period but only applies to that small fraction of the buffed organic carbon reservoir oxidized to produce the net reduction in its size over the past 15 Ma. Comparison of Os isotope records with higher temporal resolution to &13C records is required to better evaluate the utility of Os isotopes in elucidating Neogene variations in the carbon cycle. In addition, more complete examination of the present-day geochemical cycles of Re and Os is necessary to diminish the uncertainties associated with the calculations discussed here.
258
G. R A V I Z Z A A N D B.K. E S S E R
250
200
150 0
• 12
~-
~oo
rr
50
[]
0
0
•
[]
i
J
5
10
Organic
Carbon
%
1
Fig. 3. Below 15% organic carbon, Re and organic carbon are well correlated in both modern sediments (Black Sea, Gulf of California and Walvis Bay) and in Devonian-Mississippian boundary black shales. The reference line drawn through the data is calculated from estimates of the Mo/Co~ ratio of Phanerozoic black shales (Holland, 1984) and the Re/Mo ratio of seawater. Holland has noted a remarkable constancy in the Mo/Corg ratio of black shales. The fact that reference line falls close to the data suggests that Re and Mo behave similarly in anoxic sediments and that this Re/Corg ratio is likely to be representative of organic-rich sediments over a wide range of ages and Corg concentrations.
References Colodner, D., Ravizza, G., Sachs, J., Turekian, K.K., Edmond, J. and Boyle, E., 1993. The geochemical cycle of rhenium: A reconnaissance. Earth Planet. Sci. Lett. (in press). Delaney, M. and Boyle, E., 1988. Tertiary paleoceanic chemical variability: unintended consequences of simple geochemical models. Paleoceanography, 3:137-156 Esser, B.K. and Turekian, K.K., 1988. Accretion rate of extraterrestrial particles determined from the osmium isotope systematics of Pacific pelagic clays and manganese nodules. Geochim. Cosmochim. Acta, 52: 13831388 Goldberg, E. and Koide, M., 1990. Understanding the marine chemistries of platinum group metals. Mar. Chem., 30:249-257 Holland, H.D., 1984. The Chemical Evolution of the Atmosphere and Oceans. Princeton University Press, Princeton, N.J., 582 pp. Pegram, W.J., Krishnaswami, S., Ravizza, G. and Turekian, K.K., 1992. The record of seawater 187Os/186Os
variation through the Cenozoic. Earth Planet. Sci. Lett., 113:569-576 Ravizza, G., 1993. Variations of the ~87Os/~86Os ratio of sea water over the past 27 million years as inferred from metalliferous carbonates. Earth Planet. Sci. Lett. (submitted). Ravizza, G. and McMurtry, G.M., 1993. Osmium isotopic variations in metalliferous sediments from the East Pacfic Rise and the Bauer Basin. Geochim. Cosmochim. Acta (in press). Ravizza, G. and Turekian, K.K., 1992. The osmium isotopic coposition of organic-rich marine sediments. Earth Planet. Sci. Lett., 110: 1-6. Ravizza, G., Turekian, K.K. and Hay, B.J., 1991. The geochemistry of rhenium and osmium in recent sediment from the Black Sea. Geochim. Cosmochim. Acta, 55:3741-3752 Shackleton, N.J., 1987. The carbon isotope record of the Cenozoic: History of organic carbon burial and of oxygen in the ocean and atmosphere. In: J. Brooks and A.J. Fleet (Editors), Marine Petroleum Source Rocks, Vol. 26. Blackwell, London, pp. 423-434.
255
A possible link between the seawater osmium isotope record and weathering of ancient sedimentary organic matter G. Ravizza a and B.K. Esserb "Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, USA bL-396 Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
(Received March 9, 1993: revised and accepted March 30, 1993 )
The Os isotopic composition of leachable Os from a North Pacific pelagic clay sequence (Pegram et al., 1992 ) and bulk sediment samples of metalliferous carbonates deposited near the East Pacific Rise (Ravizza, 1993) display similar patterns of temporal variation (Fig. 1 ). This similarity indicates that the composite record of t87Os/186Os ratio variations reflects changes in the Os isotopic composition of seawater. Temporal changes in the Sr isotopic composition of seawater record changes in the relative rates of continental weathering and oceanic crustal alteration. The Os isotopic 9.0"
composition of seawater should be influenced by similar processes. In both the Rb-Sr and Re-Os systems, the continental crust is characterized by large time-integrated parent/ daughter ratios, compared to the deep Earth, imparting relatively radiogenic Sr and Os signatures to continental material. However, the seawater Sr and Os records are decoupled from one another over the past 27 Ma. From 27 to 15 Ma 87Sr/86Sr increased rapidly while the 18VOs/186Os ratio of seawater remained nearly constant. At 15 Ma the ~87Os/tS6Os ratio of seawater began to increase rapidly but the rate
Zero Age Pacific Sediments
8.5' 8.0 CD CO
©
7.5 7.0
65 6.0 1 5.5
Steady State
~
EPR Metalliferous Carbonates (Ravizza 1993) North Pacilic Pelagic Clay (Pegram eLal. 1992) i i
5.0 0
5
10
i
i
15
20
25
Fig. 1. Temporal variations in the 187Os/186Osratio of seawater inferred from analyses of leachable Os in pelagic clay from LL44-GPC3 and bulk sediment analyses of metalliferous carbonates from DSDP Leg 92. The agreement of these two records over the past 27 Ma indicates that the major features of both records reflect changes in the Os isotopic composition of seawater rather the influence of local diagenesis.
0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
G. RAVIZZAAND B.K.ESSER
256
of increase in the 87Sr/86Sr ratio diminished. Accelerated weathering of ancient sedimentary organic matter may account for this decoupling and the rapid increase in the ~87Os/ ~86Os ratio of seawater over the past 15 Ma (Pegram et al., 1992; Ravizza, 1993). Calculations described here suggest this is a viable hypothesis which has implications for our understanding of changes in the size of the buried organic carbon reservoir during the Neogene. Secular variations in the ~ ~3C of marine carbonates constrain past changes in the relative sizes of the buried organic and inorganic carbon reservoirs. The fi~3C values of bulk carbonate from the South Atlantic display a negative shift of 2%o over the past 15 Ma (Shackleton, 1987 ). While this shift is indicative of net oxidation of organic carbon, carbon isotopic data do not allow enhanced weathering to be distinguished from diminished organic carbon burial. These two means of reducing the size of the buried organic carbon reservoir may have different effects on oceanic nutrient cycling (Delaney and Boyle, 1988). The coincidence of the (~13C shift to more negative values (Fig. 2) and 187Os/186Os shift to more radiogenic values is compatible with an accelerated rate of organic matter weathering. The geochemistry of Re plays a fundamental role in coupling the geochemical cycles of Os and organic matter. The primary sink for Re in the oceans is burial in association with organic-rich sediments, accounting for ~ 50% of the present-day riverine Re flux (Colodner et al., 1993). In this respect the geochemical cycles of Re and organic carbon are linked. Fig. 3 shows that Re and organic carbon concentrations are correlated in both m o d e m and ancient organic-rich sediments. Because decay of 187Re is responsible for the production of ~87Os, organic-rich sediments represent an important crustal reservoir for the production of radiogenic 187Os (here after denoted as ~87Os*). The reduced character of these sediments renders them vulnerable to rapid oxidative weathering. Hence during weathering of organic-rich
sediments, oxidation of their burden of organic matter should be coupled with the liberation of soluble 187Os*. The older the weathered sediment, the higher will be ratio of t87Os*to oxidized organic matter. A simple model has been constructed to evaluate whether the decrease in the size of the buried organic carbon reservoir inferred from the fi~3C of marine carbonates and the increase in the ~87Os/186Os ratio of seawater can be linked plausibly via weathering of ancient sedimentary organic matter. This model assumes that the increase in the ~87Os/~86Os of seawater which commences at ~ 15 Ma is caused by the addition of 187Os*without perturbing either the size of the seawater Os reservoir or the flux of c o m m o n Os to the oceans. Under these conditions the additional flux of ~8VOs*(denoted as J~ 87- ) which perturbs the steady state can be expressed as a simple function of time: j187" = (dRsw/dt)N 186..~_j186 [Rsw(l) -Rst.st. ] where: is the net flux of ~86Os into oceans. It is estimated from Os burial rates in marine sediments (Esser and Turekian, 1988; Ravizza and Turekian, 1992; Ravizza and McMurtry, 1993 ) to be ~7.4-106 mol Ma -~ 186Os. This parameter is assumed to be constant over the past 25 Ma. - - N 186 is the size of the oceanic reservoir of ~86Os in moles. Using estimates of the Os concentration of seawater (Goldberg and Koide, 1990; Ravizza et al., 1991 ) the reservoir is calculated to be ~ 105 mol 186Os. This parameter is also assumed to be constant over the past 25 Ma. --Rsw(t) = 187Os/186Os ratio of seawater. This value changes with time and has been approximated by two line segments (Fig. 1 ). dR~w/dt is the rate of change of the 187Os/ ~86Os ratio of seawater. The value of dR~w/dt is nearly constant over the past 15 Ma and can be obtained from the slopes of the two line segments shown in Fig. 1.
__ j186
-
-
257
SEAWATEROs ISOTOPERECORDAND WEATHERINGOF ANCIENTSEDIMENTARYORGANICMATTER 3.0 2.5 20
t'n a El. v
1.5 1.0
O
0.5 0.0 -0.5 Data from Shackleton and Hall 1984 -1.0
I
I
i
I
5
10
15
20
Age
25
(Ma)
Fig. 2. Record ofO ~3C variations of bulk carbonate from DSDP Holes 525 and 525A. Shackleton ( 1987 ) interpreted these data as evidence of a net decrease in the size of the buried organic carbon reservoir over the past 15 Ma. - - R s t . s t . is the 1 8 7 O s / 1 8 6 O s ratio of the total Os
input to the oceans at steady state, prior to any perturbation by the increased input of 187Os*. This value ( ~ 6.2) is obtained from the flat portion of the seawater Os curve between 27 and 15 Ma. The size of 187Os~" perturbation integrated over the past 15 Ma is 2.108 mol. Based on Shackleton's estimate of the decrease in the size of the buffed organic carbon reservoir over this same time interval ( 1019 mol C) and assuming c o u p l e d 187Os~t input, the average 187Os~r/organic carbon ratio of organic matter lost from the organic carbon reservoir is 2- 10- l i. If the net loss of organic matter from the organic carbon reservoir was solely the result of a diminished rate of carbon burial, as suggested by Shackleton (1987), the expected I87Os*/organic carbon ratio would be zero and the Os and 813C records should be uncoupled. Using the I87Os*/organic carbon ratio and the slope of the Re-organic carbon reference line from Fig. 3, the mean "'age" of organic matter lost from the buffed organic carbon res-
ervoir can be estimated from the equation: 187OsSr/Corg = ( 1 8 7 R e / C o r g ) 2 t
where 2 is the decay constant of 187Re ( ~ 1.6.10- i1 y r - 1). The resulting estimate is ~ 35 Ma. This "age" is geologically reasonable and suggests that the net oxidation of organic carbon implied by &I3C is better explained by enhanced weathering than by diminished organic matter burial. It is important to note that this "age" does not represent the mean age of all weathered organic matter over this time period but only applies to that small fraction of the buffed organic carbon reservoir oxidized to produce the net reduction in its size over the past 15 Ma. Comparison of Os isotope records with higher temporal resolution to &13C records is required to better evaluate the utility of Os isotopes in elucidating Neogene variations in the carbon cycle. In addition, more complete examination of the present-day geochemical cycles of Re and Os is necessary to diminish the uncertainties associated with the calculations discussed here.
258
G. R A V I Z Z A A N D B.K. E S S E R
250
200
150 0
• 12
~-
~oo
rr
50
[]
0
0
•
[]
i
J
5
10
Organic
Carbon
%
1
Fig. 3. Below 15% organic carbon, Re and organic carbon are well correlated in both modern sediments (Black Sea, Gulf of California and Walvis Bay) and in Devonian-Mississippian boundary black shales. The reference line drawn through the data is calculated from estimates of the Mo/Co~ ratio of Phanerozoic black shales (Holland, 1984) and the Re/Mo ratio of seawater. Holland has noted a remarkable constancy in the Mo/Corg ratio of black shales. The fact that reference line falls close to the data suggests that Re and Mo behave similarly in anoxic sediments and that this Re/Corg ratio is likely to be representative of organic-rich sediments over a wide range of ages and Corg concentrations.
References Colodner, D., Ravizza, G., Sachs, J., Turekian, K.K., Edmond, J. and Boyle, E., 1993. The geochemical cycle of rhenium: A reconnaissance. Earth Planet. Sci. Lett. (in press). Delaney, M. and Boyle, E., 1988. Tertiary paleoceanic chemical variability: unintended consequences of simple geochemical models. Paleoceanography, 3:137-156 Esser, B.K. and Turekian, K.K., 1988. Accretion rate of extraterrestrial particles determined from the osmium isotope systematics of Pacific pelagic clays and manganese nodules. Geochim. Cosmochim. Acta, 52: 13831388 Goldberg, E. and Koide, M., 1990. Understanding the marine chemistries of platinum group metals. Mar. Chem., 30:249-257 Holland, H.D., 1984. The Chemical Evolution of the Atmosphere and Oceans. Princeton University Press, Princeton, N.J., 582 pp. Pegram, W.J., Krishnaswami, S., Ravizza, G. and Turekian, K.K., 1992. The record of seawater 187Os/186Os
variation through the Cenozoic. Earth Planet. Sci. Lett., 113:569-576 Ravizza, G., 1993. Variations of the ~87Os/~86Os ratio of sea water over the past 27 million years as inferred from metalliferous carbonates. Earth Planet. Sci. Lett. (submitted). Ravizza, G. and McMurtry, G.M., 1993. Osmium isotopic variations in metalliferous sediments from the East Pacfic Rise and the Bauer Basin. Geochim. Cosmochim. Acta (in press). Ravizza, G. and Turekian, K.K., 1992. The osmium isotopic coposition of organic-rich marine sediments. Earth Planet. Sci. Lett., 110: 1-6. Ravizza, G., Turekian, K.K. and Hay, B.J., 1991. The geochemistry of rhenium and osmium in recent sediment from the Black Sea. Geochim. Cosmochim. Acta, 55:3741-3752 Shackleton, N.J., 1987. The carbon isotope record of the Cenozoic: History of organic carbon burial and of oxygen in the ocean and atmosphere. In: J. Brooks and A.J. Fleet (Editors), Marine Petroleum Source Rocks, Vol. 26. Blackwell, London, pp. 423-434.