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Iridium in marine organisms
Ceochimica PI Cosmochimica Copyright 0 1988 Pergamon
Acm
Vol. 52, pp.
0016-7037/88/s3.00
1731-1739
+ XC’
Pressplc.Printedin U.S.A.
LETTER
Iridium in marine organisms M. C. WELLS, P. N. BOOTHE and B. J. PRESLEY Department of Oceanography, Texas A&M University, College Station, TX, 77843-3 146, U.S.A. (Received March 1, 1988; accepted in revisedform May ii, 1988) Abstract-Iridium concentrations in several types of marine organisms from the northern Gulf of Mexico averaged 20 parts per trillion (range < 4-80 pptr). Based on this value the Pt/Ir ratio for marine organisms is 10 as compared to the 100 that has been reported for both seawater and aut~gen~c Mn nodules. This low ratio may b-e related to the weaker ability of Ir to form stable chloro-metal complexes in seawater. The Ir inventory in the present-day marine biosphere is five orders of magnitude less than that of Cretaceous/Tertiary (K-T) boundary sediments, making it unlikely that the Cretaceous marine biosphere was a significant source of Ir for K-T boundary sediments. This small, modem inventory also suggests that the recently reported fr enrichment (I 100 ppb) in organic kerogen from K-T boundary sediments probably occurred after deposition as a result of Ir ~ist~bution within the sediments. INTRODUmION
Gulf of Mexico were analyzed for Ir in our laboratory at Texas A&M by radiochemical neutron activation analysis. Samples include primary producers, primary and secondary consumers and detrital feeders. Although not collected specifically for Ir analysis, the samples were originally collected for analysis of other trace metals and had exhibited no contamination in previous studies. They had been stored in closed containers within secondary closed containers. Samples received an integrated thermat neutron exposure of 1.3 x 10” n/cm*. A post-irradiation chemical separation and concentration procedure was then used to maximize analytical sensitivity. Irradiated samples were acid digested (HNOj/HCL04 mixture) and spiked with 20 mg of stable Ir carrier. Experiments have confirmed that isotopic equilib~um between the stable carrier and radioactive “*Ir is achieved with this method. Iridium was separated by a multistep column elution chromatography procedure using Dowex 50 X 8 cation exchange resin. Yields were determined for each sample and quantitation was done using several different primary element standards (GILMORE, 1975; C. J. ORTH,pers. ~rnrn~; WELLS 1987). The detection limit for this method as used was 4 pg/g (pptr) and the limit of quantitation was 11 pptr (CLJRRIE, 1968). Accuracy of the method was evaluated by analyzing materials with known Ir contents and by spike recovery. No bidogical materials with known Ir con~ntmtions could be found. Therefore, USGS standard rocks, which are more difficult to analyze than biological samples, were used. Our values of 0.43 and 1.7 pptr for DTS- 1 and PCC- 1, respectively, are somewhat low compared to the consensus values of 0.6 and 4.8 pptr (GLADNEY et al., 1983), however the difference is only s~tisti~ly significant for PCC-1 (p < 0.05). A Canadian National Research Council trace metal standard (dogfish liver) and station #l I oyster from this study were analyzed by C. J. ORTH (pers. commun.) who obtained values of ~0.6 and 16 pptr, respectively, compared to our values of <4 and 18 pptr. Six spiked samples were analyzed, giving an average recovery of 94%. Precision decreases as Ir values approach the detection limit, but among six typical samples analyzed in duplicate, the highest value for a sample pair averaged 1.6 times the lowest value (range 1.3-2.2). Large analytical variability is dificult to avoid at the extremely low Ir levels found in this study.
MUCH OF THE RECENTINTERESTin noble metal geochemistry is due to the globally widespread Ir enrichment in Cretaceous/ Tertiary (K-T) boundary clays and its proposed relationship to the large biological extinction which occurred at that time (THIERSTEIN, 1982; ARCHIBALDand CLEMENS,1982; HALLAM, 1984; RWP, 1986). The K-T Ir enrichment is thought to have originated from an extraterrestrial (ALVAREZet al., 1980; ALVAREZ, 1986) or volcanic source (MCLEAN, 1985; OFFICER et al., 1987). The K-T event reduced global marine biomass by approximately 90% (S. GARTNER,pers. commun.) and any biologically associated Ir would have been delivered to underlying sediments. Recent investigations have shown that related platinum group elements (Pt, Pd) are bioaccumulated by marine organisms, and that Ir, which is more hydrophobic than either Pt or Pd, should be even more easily scavenged and highly concentrated in marine organisms. However, an extensive literature survey found little data for Ir in any biological material, and none for marine organisms. Thus the potential biological contribution to the K-T sediment Ir anomaly is unknown. The study reported here provides data on Ir concentrations in selected marine organisms rep~senting all major components of a macro-organisms marine food chain. SAMPLING AND METHODS Only samples from the Gulf of Mexico (GOM) were used in this study. The principal reason for this geographical restriction was the availability of samples collected with sufficient care to permit ultratrace analysis. The data produced by this study should be extrapolatable to the rest of the world oceans because the GOM is a well mixed basin which exchanges water rapidly with the Caribbean and Atlantic Ocean. Furthermore, results from the three year South Texas Outer Continental ShelfBasline Study (BOOTHE and PRESLEY, 1979), and other studies conducted in this laboratory, show that trace element concentrations in GOM organisms are low and comparable to those found in organisms from the Pacific and Atlantic Oceans. Archived samples of marine plankton, nekton, neuston and benthos from nearshore, offshore and estuarine environments in the northern
RESULTS AND DISCUSSION Figure 1 shows the sampling station locations. Table 1 includes sample description, station number, Ir value, station water depth and season. The samples have been divided into three broad types: plankton (including two neuston samples), 1737
M. C. Wells, P. N. Boothe and B. J. Presley
1738
FIG. 1. Sampling sites in the northern Gulf of Mexico. internal organ tissues and whole oyster soft parts. In spite of the big differences in sample type, the results show no statistical difference in the average Ir concentrations among the groups. Iridium concentration in these marine organism samples was found to range from ~4-80 pptr, with an average of 20 pptr for all samples. Depth and season data are limited, but show no correlation with Ir concentrations (Table 1). The mean marine organism Ir concentration of 20 pptr found here is much higher than reported seawater concentrations of 0.0009-0.01 pptr (FRESCO, 1985; HODGE et al., 1986), and thus Ir does seem to be bioconcentrated. However, organism samples sometimes contain included sediment, therefore the lithogenic contribution to Ir concentrations in oyster and plankton samples from this study was estimated by BEWIG’S(1984) method using Fe concentrations in the samples and known suspended matter h/Fe ratios. By this
Table 1.
Iridium in marine organisms from the Gulf of Mexico.
Sample Description
II* (pptrl
Depth (m)
Season**
5 2 3 14 15 13 9 8 7 10 10
71 <4 9 10 31 5 47 23 28 15 20
100 45 >I000 150 2 400 357 45 100 30 10
1 2 2 2 2 2 2 1 I 1
4 1 4 5 2 6
11 15 <4 46 35 54 34 <4 10
67 20 67 100 4s 50
3 3 3 2
(whole soft parts) oystert oyster 11 oyster 11 oyster 16 oyster 16 17 oyster oyster 17 12 oyster 12 oyster
11 <4 <4 <4 <4 7s 4 <4 16
I I 1 I I 1 I 1 I
Plankton: zooplankton zooplankton zooplankton zooplankton zooplankton zooplankton zooplankton ne”st0” neuston phytoplankton phytoplankton Internal Tissues: fish (NOAA)t fish (EPA)? dogfish liver (NRCC)? shrimp hepatopancreas shrimp hepatopancreas shrimp digestive gland shrimp digestive gland squid mantle squid mantle Oysters
Station NO.
I
technique sediment-derived Ir in the organisms was found to be negligible. Furthermore, the internal tissue samples could not have been contaminated with sediment. The lr values obtained in this study were used in approximating an average biological concentration factor (Ir in organisms/Ir in seawater). This ratio, although extremely variable, averages 15,000, and can be compared to a ratio of 1000 for platinum in marine phytoplankton estimated from HODGE et al. C1986) data. Table 2 shows Pt, Ir abundances and Pt/Ir ratios for average earth crust material, marine organisms, seawater, and manganese nodules. The Pt/Ir ratio in seawater is much higher than the crustal value because of iridium’s weak complexing abilities and strong tendency to hydrolize relative to platinum (GOLDBERGet al., 1986). Chloro-metal complexes of platinum in seawater are most stable followed by those of Pd and lastly Ir (HOGFELDT, 1982; ELDING, 1972, 1978), explaining why the residence time for Pt in seawater is approximately lo6 years compared to approximately 40,000 years for Ir (GOLDBERG et al., 1986). It has been noted elsewhere (GOLDBERG et al., 1986) that Pt/Ir values for authigenic manganese nodules cluster around 100, the seawater value. suggesting that oxidation and subsequent removal affects the two elements similarly. The Pt/Ir of 10 for marine organisms may be due to preferential uptake of Ir which is more hydrophobic than Pt, but this hypothesis is based on very limited data. More analyses of marine organisms for both Pt and lr are needed. As mentioned, the K-T boundary sediment Ir enrichment is synchronous with a major biological extinction which reduced marine biomass by approximately 90%. Iridium present in the biological reservoir at the time of extinction could have been deposited on the seafloor at that time. Using our mean Ir value (20 pptr) for marine organisms, and an estimate of the present-day marine biomass (dry weight: RYTHER. 1969; EL SAYED, pers. commun.), we estimate the current marine biosphere Ir inventory at 1.8 X 10“’ ng. By comparison there is an estimated 5.3 X IO” ng of Ir in marine K-T boundary sediments (using average K-T boundary Ir concentration of 12 ppb, average boundary thickness ofO.5 cm, average sediment density of 2.7, and 70% of the Earth’s surface as the marine environment; WELLS, 1987). Despite the difficulties in extrapolating from modern to ancient environments, the 300,000 fold difference between Ir in the modem marine biosphere and K-T boundary sediments makes it exceedingly unlikely that a significant portion of the K-T boundary section sediment Ir inventory came from the marine biological reservoir through an instantaneous addition. This conclusion is further supported by the likelihood that
I
I
*dry weight ** I=winter, *=spring. 3=summer. tRcferencc matmists. NOAA=Netional Occamc and Atmosphenc Admmistrakm. EPA=Environmcntal Protection Agency. NRCC=Natiansl Research Councit of Canada. Oyster is hausc standard
Table 2. Platinum and Ir in organisms. and manganese nodules.
seawatci
_______ PI rig/g
lr nlt1.s
organisms
Seawater Mn
PI/II ?
Crustal Marine
_ .__
nodules
IO
(I 2t
0.0?(1
1.5 x lo-4+
I.3 x IO-6i
100
4Y31
4.xt
I (IO
t from Hodge et a!.. 1986.
Iridium in marine organisms
only a small fraction of Ir in sinking marine organismal debris would be permanently retained in the sediments. Even if the Cretaceous marine biological Ir reservoir was 100 times @eater than today because of a larger biomass, higher seawater dissolved Ir concentration or other factors, the marine biological contribution to the K-T sediment Ir anomaly would still have been less than 1 percent. Several studies suggest that the K-T boundary events occurred over thousands of years rather than as a geologically “ins~~~us” event (OFFICER and DRAKE, 1985; HALLAM, 1984). In this case the biological Ir produced by thousands of years of marine productivity could have contributed to the K-T sediment Ir anomaly. To evaluate this scenario, the Cretaceous marine productivity rate was assumed to be the same as estimates for the present-day oceans (of 8.1 X 10j6 g dry wt. organic matter per year; MCCARTHY etal. 1986). Also an Ir sediment burial rate ( 14%) similar to that for the bioactive element barium (BROECKER and PING 1982) was used. Assuming the 0.5 cm thick K-T boundary represents 10,000 years of sedimentation (and there is no general a8reement on this contention), then the marine biological contribution to the boundary Ir inventory would be only 4.3 percent under this scenario. Our conclusion that marine organic matter entering the sediment is not highly enriched in Ir is consistent with the SCHMITZ et al. ( 1988) conclusions regarding the source of Ir enrichment in kerogen from K-T boundary clays. They report that up to 50% of the Ir in two K-T boundary clays (StevnsKlint, Denmark and Caravaca, Spain) is concentrated in the organic (kerogen) fraction but consider the Ir to have a lithogenic precursor, and to have subsequently become associated with sedimentary organic matter, such as decaying phytoplankton. Our results show that typical marine biological materials are low in Ir. However, these data cannot exclude all types of organisms as potential Ir accumulators. As part of the present study, National Bureau of Standards SRM 157 1 (Orchard Leaves) were found to contain 40 pptr, and thus some terrestrial plants are also low in Ir. However, VALENTE et al. (1982) have reported 17 to 26 ppb Ir in certain types of hy~~ccumulating terrestrial plants gcawing in mafic soils. These types of plants are reported to be widely distributed now, but the total percent biomass they comprise is unknown. It is curious that the VALENTE et al. (1982) findings have not been cited in recent literature dealing with noble metal geochemistry. Thus, whereas we believe marine organisms can be rejected as a source of Ir to K-T boundary layer sediment, we suggest additional investigations of the Ir levels in terrestrial plants are needed before this potential source can be rejected. Acknowledgements-The authors would like to thank Dr. Carl Orth of Los Alamos National Laboratory for his advice on the analytical aspects of this study. We would also like to thank the Texas A&M University Nuclear Science Center for their assistance with irradiation services, and the Center for Chemical Characterization and Analysis for advice and use of their gamma-ray counting system. Editorial handling: D. M. Shaw
1739
REFERENCES ALVAREZW. ( 1986) Toward a theory of impact crises. Eos 67,649658. ALVAREZL. W., ALVAREZW., ASAROF. and MICHELH. V. (1980) Estraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208, 1095-l 108. ARCHIBALDJ. D. and CLEMENSW. A. (1982) Late Cretaceous extinctions. Amer. Sci. 70, 377-385. BEWIGR. B. (1984) Trace metab in plankton, shrimp and squid Born the Mississippi delta. MS. thesis, FIorida Inst. of TechnoIo8y. -THE P. N. and PRESLEYB. J. (1979) Trace metals in epifauna, zooplankton and macronekton. In Environmental Studies, South Texas Outer Continental Shelf: Biology and Chemistry. Final rept. to Bureau of Land Management, Washington, D.C. (AA550-CT71l), pp. 6-l-6-47. BROECKERW. S. and PENGT.-H. (1982) Tracers in the Sea. Eldigio Press. Columbia University, New York, 21~. @RRIE L. A. (1968) Limits for qualitative detection and qualitative determination: Application to radiochemistry. Anal. Chem. 40, 586-591. ELDINGL. I. (1972) Palladium (II) complexes. I. Stabilities and spectra of Pd(I1) chloro and bromo aqua complexes. Znorg. Chim. Acta 6, 647-65 1. ELDINGL. I. (1978) Stabilities of platinum (II) chloro and bromo compiexes and the kinetics for the anation of t~~quapiatinum (II) ion by halides and thiocyanate. Znorg. Chim. Actu 28, 255262. FRESCOJ. (1985) Iridium in sea-water. Talunta 32, 830-83 1. GILMOREJ. S. (1975)Iridium procedure. In Collected Radiochemical Procedures (ed. H. L. SMITH),4th edn. Los Alamos Report LA1721. GLADNEYE. S., BURNSC. E, and ROELANDTSI. ( 1983) 1982 compilation of elementaf concentrations in eleven United States Geological Survey rock standards. Geostds. Newslet.7, 3-226. GOLDBERGE. D., HODGEV., KAY P., STALLARDM. and KOIDE M. (1986)Some comparative marine chemistries of platinum and iridium. Appl. Geochem. 1,227-232. HALLAMA. ( 1984) The causes of mass extinctions. Nature 308,686687. HOEGEV., STALLARDM., KOIDE M. and C~LDBERGE. D. (1986) Determination of platinum and iridium in marine waters, sediments, and organisms. Anal. Chem. 58,6 16-620. HOGFELDTE. ( 1982) Stability constants of metal-ion complexes, Part A: inorganic ligands. ZUPAC Chem. Series No. 21. MCCARTHYJ. J., BREWERP. G. and FELDMANG. (1986) Global ocean flux. Oceanus 29, 16-26. MCLEAND. M. ( 1985) Deccan trap mantle degassing in the terminal Cretamous marine extinctions. Cretaceous Res. 6,235. OFFICER C. B. and DRAKEC. L. (1985) TerminaI Cretaceous environmental events. Science 227, 116 I- 1166. OFFICERC. B., HALLAMA., DRAKEC. L. and DEVINEJ. D. (1987) Late Cretaceous and paroxysmal Cretaceous/Tertiary extinctions. Nature 326, 143-149. RAUP D. M. (1986) Biological extinction in earth history. Science 231, 1528-1533. RYTHERJ. H. (1969) Photosynthesis and fish production in the sea. Science 166,72-76. SCHMITZB., ANDERSSONP. and DAHL J. (1988) Iridium, sulfur isotopes and rare earth elements in the Cretaceous-Tertiary boundary clay at Stevns Khnt, Denmark. Geochim. Cosmochim. Acta 52,229-236. THIERSTEINH. R. (1982) TerminaI Cretaceous ptankton extinctions. Geof. Sot. Amer. Spec, Pap. 198,385-393. VALENTEI. M., MINSIUM. J. and PETERSON P, J. (1982) Neutron activation analysis of noble metals in vegetation. J. Radiounai. Chem. 71, 115-127. WELLSM. C. (1987) Iridium in marine organisms from the Gulf of Mexico, M.S. thesis, Texas A&M University, 84~.
Acm
Vol. 52, pp.
0016-7037/88/s3.00
1731-1739
+ XC’
Pressplc.Printedin U.S.A.
LETTER
Iridium in marine organisms M. C. WELLS, P. N. BOOTHE and B. J. PRESLEY Department of Oceanography, Texas A&M University, College Station, TX, 77843-3 146, U.S.A. (Received March 1, 1988; accepted in revisedform May ii, 1988) Abstract-Iridium concentrations in several types of marine organisms from the northern Gulf of Mexico averaged 20 parts per trillion (range < 4-80 pptr). Based on this value the Pt/Ir ratio for marine organisms is 10 as compared to the 100 that has been reported for both seawater and aut~gen~c Mn nodules. This low ratio may b-e related to the weaker ability of Ir to form stable chloro-metal complexes in seawater. The Ir inventory in the present-day marine biosphere is five orders of magnitude less than that of Cretaceous/Tertiary (K-T) boundary sediments, making it unlikely that the Cretaceous marine biosphere was a significant source of Ir for K-T boundary sediments. This small, modem inventory also suggests that the recently reported fr enrichment (I 100 ppb) in organic kerogen from K-T boundary sediments probably occurred after deposition as a result of Ir ~ist~bution within the sediments. INTRODUmION
Gulf of Mexico were analyzed for Ir in our laboratory at Texas A&M by radiochemical neutron activation analysis. Samples include primary producers, primary and secondary consumers and detrital feeders. Although not collected specifically for Ir analysis, the samples were originally collected for analysis of other trace metals and had exhibited no contamination in previous studies. They had been stored in closed containers within secondary closed containers. Samples received an integrated thermat neutron exposure of 1.3 x 10” n/cm*. A post-irradiation chemical separation and concentration procedure was then used to maximize analytical sensitivity. Irradiated samples were acid digested (HNOj/HCL04 mixture) and spiked with 20 mg of stable Ir carrier. Experiments have confirmed that isotopic equilib~um between the stable carrier and radioactive “*Ir is achieved with this method. Iridium was separated by a multistep column elution chromatography procedure using Dowex 50 X 8 cation exchange resin. Yields were determined for each sample and quantitation was done using several different primary element standards (GILMORE, 1975; C. J. ORTH,pers. ~rnrn~; WELLS 1987). The detection limit for this method as used was 4 pg/g (pptr) and the limit of quantitation was 11 pptr (CLJRRIE, 1968). Accuracy of the method was evaluated by analyzing materials with known Ir contents and by spike recovery. No bidogical materials with known Ir con~ntmtions could be found. Therefore, USGS standard rocks, which are more difficult to analyze than biological samples, were used. Our values of 0.43 and 1.7 pptr for DTS- 1 and PCC- 1, respectively, are somewhat low compared to the consensus values of 0.6 and 4.8 pptr (GLADNEY et al., 1983), however the difference is only s~tisti~ly significant for PCC-1 (p < 0.05). A Canadian National Research Council trace metal standard (dogfish liver) and station #l I oyster from this study were analyzed by C. J. ORTH (pers. commun.) who obtained values of ~0.6 and 16 pptr, respectively, compared to our values of <4 and 18 pptr. Six spiked samples were analyzed, giving an average recovery of 94%. Precision decreases as Ir values approach the detection limit, but among six typical samples analyzed in duplicate, the highest value for a sample pair averaged 1.6 times the lowest value (range 1.3-2.2). Large analytical variability is dificult to avoid at the extremely low Ir levels found in this study.
MUCH OF THE RECENTINTERESTin noble metal geochemistry is due to the globally widespread Ir enrichment in Cretaceous/ Tertiary (K-T) boundary clays and its proposed relationship to the large biological extinction which occurred at that time (THIERSTEIN, 1982; ARCHIBALDand CLEMENS,1982; HALLAM, 1984; RWP, 1986). The K-T Ir enrichment is thought to have originated from an extraterrestrial (ALVAREZet al., 1980; ALVAREZ, 1986) or volcanic source (MCLEAN, 1985; OFFICER et al., 1987). The K-T event reduced global marine biomass by approximately 90% (S. GARTNER,pers. commun.) and any biologically associated Ir would have been delivered to underlying sediments. Recent investigations have shown that related platinum group elements (Pt, Pd) are bioaccumulated by marine organisms, and that Ir, which is more hydrophobic than either Pt or Pd, should be even more easily scavenged and highly concentrated in marine organisms. However, an extensive literature survey found little data for Ir in any biological material, and none for marine organisms. Thus the potential biological contribution to the K-T sediment Ir anomaly is unknown. The study reported here provides data on Ir concentrations in selected marine organisms rep~senting all major components of a macro-organisms marine food chain. SAMPLING AND METHODS Only samples from the Gulf of Mexico (GOM) were used in this study. The principal reason for this geographical restriction was the availability of samples collected with sufficient care to permit ultratrace analysis. The data produced by this study should be extrapolatable to the rest of the world oceans because the GOM is a well mixed basin which exchanges water rapidly with the Caribbean and Atlantic Ocean. Furthermore, results from the three year South Texas Outer Continental ShelfBasline Study (BOOTHE and PRESLEY, 1979), and other studies conducted in this laboratory, show that trace element concentrations in GOM organisms are low and comparable to those found in organisms from the Pacific and Atlantic Oceans. Archived samples of marine plankton, nekton, neuston and benthos from nearshore, offshore and estuarine environments in the northern
RESULTS AND DISCUSSION Figure 1 shows the sampling station locations. Table 1 includes sample description, station number, Ir value, station water depth and season. The samples have been divided into three broad types: plankton (including two neuston samples), 1737
M. C. Wells, P. N. Boothe and B. J. Presley
1738
FIG. 1. Sampling sites in the northern Gulf of Mexico. internal organ tissues and whole oyster soft parts. In spite of the big differences in sample type, the results show no statistical difference in the average Ir concentrations among the groups. Iridium concentration in these marine organism samples was found to range from ~4-80 pptr, with an average of 20 pptr for all samples. Depth and season data are limited, but show no correlation with Ir concentrations (Table 1). The mean marine organism Ir concentration of 20 pptr found here is much higher than reported seawater concentrations of 0.0009-0.01 pptr (FRESCO, 1985; HODGE et al., 1986), and thus Ir does seem to be bioconcentrated. However, organism samples sometimes contain included sediment, therefore the lithogenic contribution to Ir concentrations in oyster and plankton samples from this study was estimated by BEWIG’S(1984) method using Fe concentrations in the samples and known suspended matter h/Fe ratios. By this
Table 1.
Iridium in marine organisms from the Gulf of Mexico.
Sample Description
II* (pptrl
Depth (m)
Season**
5 2 3 14 15 13 9 8 7 10 10
71 <4 9 10 31 5 47 23 28 15 20
100 45 >I000 150 2 400 357 45 100 30 10
1 2 2 2 2 2 2 1 I 1
4 1 4 5 2 6
11 15 <4 46 35 54 34 <4 10
67 20 67 100 4s 50
3 3 3 2
(whole soft parts) oystert oyster 11 oyster 11 oyster 16 oyster 16 17 oyster oyster 17 12 oyster 12 oyster
11 <4 <4 <4 <4 7s 4 <4 16
I I 1 I I 1 I 1 I
Plankton: zooplankton zooplankton zooplankton zooplankton zooplankton zooplankton zooplankton ne”st0” neuston phytoplankton phytoplankton Internal Tissues: fish (NOAA)t fish (EPA)? dogfish liver (NRCC)? shrimp hepatopancreas shrimp hepatopancreas shrimp digestive gland shrimp digestive gland squid mantle squid mantle Oysters
Station NO.
I
technique sediment-derived Ir in the organisms was found to be negligible. Furthermore, the internal tissue samples could not have been contaminated with sediment. The lr values obtained in this study were used in approximating an average biological concentration factor (Ir in organisms/Ir in seawater). This ratio, although extremely variable, averages 15,000, and can be compared to a ratio of 1000 for platinum in marine phytoplankton estimated from HODGE et al. C1986) data. Table 2 shows Pt, Ir abundances and Pt/Ir ratios for average earth crust material, marine organisms, seawater, and manganese nodules. The Pt/Ir ratio in seawater is much higher than the crustal value because of iridium’s weak complexing abilities and strong tendency to hydrolize relative to platinum (GOLDBERGet al., 1986). Chloro-metal complexes of platinum in seawater are most stable followed by those of Pd and lastly Ir (HOGFELDT, 1982; ELDING, 1972, 1978), explaining why the residence time for Pt in seawater is approximately lo6 years compared to approximately 40,000 years for Ir (GOLDBERG et al., 1986). It has been noted elsewhere (GOLDBERG et al., 1986) that Pt/Ir values for authigenic manganese nodules cluster around 100, the seawater value. suggesting that oxidation and subsequent removal affects the two elements similarly. The Pt/Ir of 10 for marine organisms may be due to preferential uptake of Ir which is more hydrophobic than Pt, but this hypothesis is based on very limited data. More analyses of marine organisms for both Pt and lr are needed. As mentioned, the K-T boundary sediment Ir enrichment is synchronous with a major biological extinction which reduced marine biomass by approximately 90%. Iridium present in the biological reservoir at the time of extinction could have been deposited on the seafloor at that time. Using our mean Ir value (20 pptr) for marine organisms, and an estimate of the present-day marine biomass (dry weight: RYTHER. 1969; EL SAYED, pers. commun.), we estimate the current marine biosphere Ir inventory at 1.8 X 10“’ ng. By comparison there is an estimated 5.3 X IO” ng of Ir in marine K-T boundary sediments (using average K-T boundary Ir concentration of 12 ppb, average boundary thickness ofO.5 cm, average sediment density of 2.7, and 70% of the Earth’s surface as the marine environment; WELLS, 1987). Despite the difficulties in extrapolating from modern to ancient environments, the 300,000 fold difference between Ir in the modem marine biosphere and K-T boundary sediments makes it exceedingly unlikely that a significant portion of the K-T boundary section sediment Ir inventory came from the marine biological reservoir through an instantaneous addition. This conclusion is further supported by the likelihood that
I
I
*dry weight ** I=winter, *=spring. 3=summer. tRcferencc matmists. NOAA=Netional Occamc and Atmosphenc Admmistrakm. EPA=Environmcntal Protection Agency. NRCC=Natiansl Research Councit of Canada. Oyster is hausc standard
Table 2. Platinum and Ir in organisms. and manganese nodules.
seawatci
_______ PI rig/g
lr nlt1.s
organisms
Seawater Mn
PI/II ?
Crustal Marine
_ .__
nodules
IO
(I 2t
0.0?(1
1.5 x lo-4+
I.3 x IO-6i
100
4Y31
4.xt
I (IO
t from Hodge et a!.. 1986.
Iridium in marine organisms
only a small fraction of Ir in sinking marine organismal debris would be permanently retained in the sediments. Even if the Cretaceous marine biological Ir reservoir was 100 times @eater than today because of a larger biomass, higher seawater dissolved Ir concentration or other factors, the marine biological contribution to the K-T sediment Ir anomaly would still have been less than 1 percent. Several studies suggest that the K-T boundary events occurred over thousands of years rather than as a geologically “ins~~~us” event (OFFICER and DRAKE, 1985; HALLAM, 1984). In this case the biological Ir produced by thousands of years of marine productivity could have contributed to the K-T sediment Ir anomaly. To evaluate this scenario, the Cretaceous marine productivity rate was assumed to be the same as estimates for the present-day oceans (of 8.1 X 10j6 g dry wt. organic matter per year; MCCARTHY etal. 1986). Also an Ir sediment burial rate ( 14%) similar to that for the bioactive element barium (BROECKER and PING 1982) was used. Assuming the 0.5 cm thick K-T boundary represents 10,000 years of sedimentation (and there is no general a8reement on this contention), then the marine biological contribution to the boundary Ir inventory would be only 4.3 percent under this scenario. Our conclusion that marine organic matter entering the sediment is not highly enriched in Ir is consistent with the SCHMITZ et al. ( 1988) conclusions regarding the source of Ir enrichment in kerogen from K-T boundary clays. They report that up to 50% of the Ir in two K-T boundary clays (StevnsKlint, Denmark and Caravaca, Spain) is concentrated in the organic (kerogen) fraction but consider the Ir to have a lithogenic precursor, and to have subsequently become associated with sedimentary organic matter, such as decaying phytoplankton. Our results show that typical marine biological materials are low in Ir. However, these data cannot exclude all types of organisms as potential Ir accumulators. As part of the present study, National Bureau of Standards SRM 157 1 (Orchard Leaves) were found to contain 40 pptr, and thus some terrestrial plants are also low in Ir. However, VALENTE et al. (1982) have reported 17 to 26 ppb Ir in certain types of hy~~ccumulating terrestrial plants gcawing in mafic soils. These types of plants are reported to be widely distributed now, but the total percent biomass they comprise is unknown. It is curious that the VALENTE et al. (1982) findings have not been cited in recent literature dealing with noble metal geochemistry. Thus, whereas we believe marine organisms can be rejected as a source of Ir to K-T boundary layer sediment, we suggest additional investigations of the Ir levels in terrestrial plants are needed before this potential source can be rejected. Acknowledgements-The authors would like to thank Dr. Carl Orth of Los Alamos National Laboratory for his advice on the analytical aspects of this study. We would also like to thank the Texas A&M University Nuclear Science Center for their assistance with irradiation services, and the Center for Chemical Characterization and Analysis for advice and use of their gamma-ray counting system. Editorial handling: D. M. Shaw
1739
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