Geochimico er Cosmochimica Acta Vol. 55. pp. 3321-3331 Copyright 8 1991 Pergamon Press plc. Printed in U.S.A. oOl6-7037/91/$3.00 + .LM Barium in planktonic foraminifera DAVID W. LEA* ,+and EDWARD A. BOYLE Department of Earth, Atmospheric, and Planetary Sciences, MassachusettsInstitute of Technology, Cambridge, MA 02 139, USA zyxwvutsrqpo (Received May 10, 1990; acceptedin revised form August 15, 199 1) Abstract-Reconstructions of Ba distributions in ancient oceanic surface waters could provide new insight into paleoceanographic change. Calcite shells of planktonic foraminifera potentially provide a means of reconstructing such paleo-Ba distributions if lattice-bound Ba can be determined on shells recovered from deep-sea cores. Planktonic foraminifera shells from a series of cores were purified of non-lattice- bound Ba associated with organic or sedimentary phases by a combination of physical agitation, oxidative- reductive steps, acid leaches, and a novel alkaline-DTPA step to dissolve bar&e. A sequential dissolution of a large sample of cleaned shells of the planktonic foraminifer Globigerinoides conglobatus indicates homogeneous distribution of Ba in the shell material. Comparison of shells from sediments, sediment traps, and plankton tows indicates no significant differences in the Ba content of the purified shells. Cleaned samples of the planktonic foraminifera Globigerinoides sacculifea, G. ruber, G. conglobatus, Orbulina spp., and Globoquudrina dutertrei from the equatorial Pacific, North Atlantic, and Mediterranean Sea have Ba/Ca ratios between 0.6 and 1.0 rmol/mol(O.8 to 1.4 ppm). Variation in foraminiferal Ba contents between the three basins is consistent with the trend in surface seawater Ba. The calculated distribution coefficient for Ba incorporation in these five species based on these data is 0.19 * 0.05. Several species of the non-spinose planktonic foraminifera Globorotalia have Ba/Ca ratios ranging from 2 to 13 pmol/mol; these high Ba contents might be explained by differences in the way these foraminifera precipitate their shells. A temporal record of Ba/Ca in samples of Globigerinoides and O rbulina from a core in the northwest Atlantic suggests that the Ba concentration of surface waters at this site has not changed by more than 20% over the last 14 kyr. INTRODUCIION THE INCORPORATION OF trace elements into marine carbon- ates has proved to be a useful method for probing paleo- chemical changes in ancient oceans. Since these tracers are characterized by unique marine geochemical cycles, each element can provide novel insight into oceanic change. The observation that water masses are often characterized by contrasting trace element signatures is one of the keys to the utility of these paleo-metal probes. Reconstructions of past trace metal distributions accomplished by using an array of sediment cores yield direct insight into changes in oceanic circulation parameters. Foraminiferal Cd has received the most attention in this regard (BOYLE, 1981, 1988; BOYLE and KEIGWIN, 1982, 1985, 1987); the effectiveness of the Cd method rests on the similarity of Cd distributions to that of the limiting nutrient phosphorous. A recent study demonstrated that a new pa- leochemical tracer, Ba in benthic foraminifera, can be used to reconstruct Ba in past bottom waters (LEA and zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA BOYLE, 1989). Like Cd, Ba is also a nutrient-Like tracer, but its oceanic distribution contrasts with that of Cd due to differences in uptake and regeneration of Ba in the water column. The distribution of barium in the oceans reflects Ba’s in- volvement in the biological cycle of uptake in surface waters and regeneration in deep waters (BISHOP, 1988; CHAN et al., 1977; CHOW and GOLDBERG, 1960). Barium is depleted in * Presentaddress: Department of Geological Sciences, University of California, Santa Barbara, CA 93 106, USA. ’ Author to whom correspondence should be addressed. tropical and sub-tropical surface waters; removal of dissolved Ba from surface waters appears to be controlled by precipi- tation of barite (BaSO,) in decaying marine particulate matter (BISHOP, 1988; CHOW and GOLDBERG, 1960; DEHAIRS et al., 1980). The barite in the sinking particulates dissolves deep in the water column and/or in the sediments, creating deep water maxima throughout the world’s oceans (CHAN et al., 1977). The concentration of Ba in oceanic surface waters (ex- cluding high latitudes) is quite uniform, about 34 nmol/kg in the Pacific and 41 nmol/ kg in the Atlantic ( CHAN et al., 1977; OSTLUND et al., 1987). Higher Ba values in Atlantic surface waters are probably due to the large river inputs that drain into the Atlantic. Because the residence time of Ba in surface waters is about 10 times longer than that typical for more bio-active elements like Cd or P, Ba anomalies due to high Ba river inputs persist within ocean basins. The magnitude of depletion of Ba in surface waters (as low as 20% of the highest deep water values) is much smaller than is observed for elements like P, zyxwvutsrqponmlkjihgfed NOs , Si, and Cd, which are highly depleted in surface waters. Unlike these bio-limiting elements, the concentration of Ba in surface waters must in part reflect the mean Ba concentration of the ocean; simple models suggest that surface Ba rises with increasing mean ocean Ba (LEA, 1990). Oceanic Ba is primarily supplied by riverine input of Ba, although Ba from deep ocean hydro- thermal vents might account for up to 20% of the Ba input to the oceans ( VON DAMM et al., 1985). Although Ba is a large ion relative to Ca ( 1.35 vs. 1 .OO A in 6-fold coordination; SHANNON, 1976), it can substitute for Ca in calcite (KITANO et al., 1971). The Ba content of 3321