SEASONAL SEDIMENTATION AND STABLE-ISOTOPE RECORDS OF PTEROPODS OFF CAP BLANC

Sediment trap studies conducted off Cap Blanc (Mauretania) during 1989 -1990 showed large seasonal flux variations of pteropods. Limacina inf lata clearly dominated pteropod flux in terms of numbers and mass. Vel igers of this species had a prominent peak of about 10,600 ind m-2 day -1 in boreal summer during the period of highest organic carbon flux. The flux of adult individuals (> 1 mm) peaked about 3 weeks earlier (3 12 ind m-2 day-1). Aragonite produced by this size fraction of L. infl ata then amounted to about 75% of total carbonate. Clio pyramidata, th e second most abundant species showed less tendency toward seasonal va riation, and peaked in spring (43 ind m-2 day-1). Although many pterop ods undergo vertical migrations of several hundred meters, the oxygen isotope compositions of L. inflata and C. pyramidata shells appear to reflect water temperatures of the species' shallower limits (50-120 m) . Since most larger pteropods have life spans of approximately one yea r, shells derived from sedimentary pteropod layers may record past mea n annual surface water temperatures. A few specimen of C. pyramidata w ith shell lengths from 8.5 to 13 mm were collected in August, 1989, an d March, 1990, and were cut into several segments and analyzed for sta ble isotope composition. Although growth is discontinous, C. pyramidat a appears to record summer to winter temperature fluctuations in its u pper migration range (around 120 m). Thus, larger specimens taken from pteropod-rich sedimentary layers could provide past seasonal variatio ns in surface or subsurface water temperatures. There appears to be a substantial enrichment of C-13 in precipitated aragonite in L. inflata (> 250 mum size) relative to bicarbonate. We suggest that the deltaC- 13 values in pteropod shells are primarily controlled by the isotope r atio of total dissolved CO2 and by seasonal temperature variations. Th e stable carbon isotope values of L. inflata covary with the oxygen is otope values, and thus correlate negatively with water temperature (de ltaC-13 = 3.41 -0.10T). Our data confirm the negative temperature depe ndency reported in the literature.A four-year seasonal sedimentation r ecord for the Cap Blanc site (G. Wefer and G. Fischer, unpubl. data) s hows only one distinct pteropod sedimentation event in August, 1989. T his indicates strong interannual variability of pteropod production an d sedimentation. The distinct maximum in August, 1989, is probably rel ated to episodically occurring unusually high surface water temperatur es. Similar conditions might have been present during the sedimentatio n of pteropod-rich layers described from the deglaciation period and H olocene maximum in the Atlantic Ocean.