Trans-atlantic transport of aerosols: Evidence' from anthropogenic Pb isotope signatures

196 RARE

E A R T H ELEMENT DISTRIBUTIONS

PRODUCTIVITY

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THE

SUBSURFACE BRINE FORMATION BY SEAWATER FREEZING

HIGII

NW INDIAN OCEAN UPWELLINC ZONE.

& H.ELDERFIELD Labs., Earth Sciences, Cambridge,

B. Herut. A. Starimskv,

(Bullard

U.K.)

Rare earth element concentrations have been measured at lhree stations in the NW Indian Ocean. High productivity leads to strong depletion of dissolved oxygen concentrations at mid-depth across much of t h i s region and values o f 15pM or less are observed at a l l t h r e e s t a t i o n s , between the base o f the surface mixed l a y e r and approximately lO00m. 5uboxic c o n d i t i o n s p r e v a i l w i t h i n t h i s oxygen minimum zone, leading to s t r o n g d e n i t r i f i c a t i o n (&N = 5-10~M); dissolved n i t r i t e c o n c e n t r a t i o n maxima are t y p i c a l l y observed close t o the very t o p o f t h i s suboxic zone. Throughout most o f the water column, p r o f i l e s o f dissolved REE c o n c e n t r a t i o n s in the NW Indian Ocean are d i r e c t l y comparable t o those o f the A t / a n t i c and P e c i f i ~ e x h i b i t i n g low c o n c e n t r a t i o n s , c o n s i s t e n t w i t h removal, close to the s u r f a c e and high c o n c e n t r a t i o n s , c o n s i s t e n t w i t h r e g e n e r a t i o n , at depth, However, w i t h i n the oxygen minimum zone, d i s s o l v e d REE p r o f i l e s e x h i b i t pronounced maxima c o i n c i d e n t w i t h enhanced c o n c e n t r a t i o n s o f dissolved Mn; as f o r anoxic basins, i t appears t h a t the distributions of the s t r i c t l y trivalent REE within this portion of the water column are dominated by the redox cycling of Fe and Mn at the oxic/suboxic interface. Unlike the other RE[, dissolved Ce concentrations in the NWIO are extremely law (~5pM) throughout the oxygenated part of the water column but show preferential enrichment, relative to the rest of the series, within the suboxic horizon, consistent with its i n t r i n s i c redox chemistry: Ce3+(aq) . ~ Ce(IV)O 2 In this paper the relative importance of supply of dissolved REE and Mn to the oxygen minimum zone from dissolution of settling particulate material produced in the s u r f a c e ocean and h o r i z o r l l a ] o f f shore advection o f m a t e r i a l released from lhe anoxic pore waters of s h e l f sediments w i l l be addressed.

A. Katz,

A. B e i n and Z. a a r f u n k e l D e o a r t m e n t o~ G e o l o g y , The Hebrew U n l v e r s l t y , Jerusalem. Israel.

C.R.GERMAN

S e a w a t e r f r e e z i n g i s proDoseo as an important process In the f o r m a t i o n of subsurface b r l n e s . New. as well as avaxlable e.oerlmental data show, that both freezlng ane evapora:Ion of s e a w a t e r , f o l l o w e b by smlpSate r e d u c t l o n , d o l o m l ~ e formation or chlorltlzat±on, result in the well known characterxstlcs o f moB: s u b s u r f a c e b r l n e s , i.e.: high sallnity, low (I r a t i o . Hence, these p r o o e r t l e s ~n tmemselves, cannot be used as dlstlnguisnl~g criteria metween t h e two bro~esses. H o w e v e r . t h e ~ r e e z i n g anP e v a b o r a t i o n p a t h s O~ s e a w a t e r a r e c l e a r l y r e s o l v a b l e from each ~ t h e r ~n an N a / C I - B ~ / C I c o o r d i n a t e system, w h i c h i s o~fe~e~ as a new t o o l f o r t h a t muroose. T h i s i s d ~ s p l a y e d by c o m p a r i n g P a t s o¢ e v a p o r a t i v e s u b s u r f a c e b r i n e s from t h e DeaP Sea S~+t anb from M~ss~ss~mp~. w~tm b ~ i n e s o f a y e t u n c l e a r or~g~n f ~ o m t h e Canadian Sh~elb. It ~s sho~n t h a t t h e Canadian b ~ n e s may ~ e l l r e p r e s e n t em~ p r o d u c t s of a l a r g e - s c a l e e v e n t o f s e a w a t e r f r e e z x n g some t l m e a f t e r t h e o n s e t o ; the P l i o c e n e g l a c ~ a t l o n s . •

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TRANS-ATLANTIC TRANSPORTOF AEROSOLS:EVIDENCE' FROM ANTHROPOGENICPb ISOTOPESIGNATURES.

VARIATION OF NITROGEN ISOTOPE F~%CTIONATION DURING DENITRIFICATION AND NITROGEN ISOTOPE BAEAXCE IN THE OCEAN

F.E. GROUSSET, B. HAMELIN, P.E. BISCAYE, and J. PROSPERO. (U.A. CNRS 197, IGBA, Universite Bordeaux I, 351 Coors de la Liberation, 33405 Talence, France).

KON-KEE LIU (Institute of Earth Sciences, Academia Sinica, P.O.Box 23-59, Taipei, Taiwan, ROC) ISAAC R. KAPLAN (Institute of Geophysics and Planetary Physics, UCLA, L.A., CA90024, USA)

Lead isotope ratios are used to distinguish between European and North American sources in aerosols sampled over the Tyrrhenian and Red Seas, the troplcal North East Atlantlc and Barbados since the 1960's. Data are compared with typlcal gasoline- and Industrlal-derived alkylic,wlth pre-Holocene non-contaminated sediments, and with mantle-derived materials. These data indicate that lead compounds In those aerosols are mainly derived from European sources ( 17.25<2°6pb12°apb<18), which are completely different from North American sources (18.5( 2°6Pbl2°4pb< 18.84). Minor contributions by North African sources, however, cannot be ruled out. The occurence of this European, anthropogenlc signature In Cape Verde Basin and Barbados aerosols indicates a southerly or southwesterly particle transfer from Europe to the latitude of the northeasterly Trade Winds, mixing of European wlth African desert-derived particles, and transport across the Atlantic at least as far as Barbados.

Enrichment of 15N in nitrate in the denitrdfying zone was observed in the eastern tropical North and South Pacific (ETNP and ETSP) and the Santa Barbara Basin off southern California. Since denitrification is a multi-step reaction, the isotope fractionation factors for the overall reaction and the single-step reduction of nitrate and n i t r i ~ are examined. The overall factor in favor of N removal varies from 1.008 to 1.032. The single-step factor, ranging from 1.005 to 1.023, decreases by about 10%o for a ten-fold increase in reaction rate. The relationship is similar to that observed in laboratory studies. Because denitrification is the major process for removing fixed nitrogen from the ocean and for fractionating nitrogen isotopes in seawater, constraints on the output fluxes of nitrogen were calculated based on nitrogen isotope balance in the ocean. For the input, we calculated the mean isotopic composition to be i% o heavier than air nitrogen. For the output, we got the following: (i) Less than 23% of the output is attributable to slow denitrification with strong isotope fractionation, such as that in the ETNP; (2) Up to 71% may be accounted for by intense denitrification with moderate fractionation, such as that in the ETSP; (3) 29-77% may be attributed to removal with little or no fractionation, such as burial of org. N or 100% denitrification in the sediments.