Seasonal sedimentation and stable isotope records of pteropods off Cap Blanc

Abstract

Sediment trap studies conducted off Cap Blanc (Mauretania) during 1989–1990 showed large seasonal flux variations of pteropods. Limacina inflata clearly dominated pteropod flux in terms of numbers and mass. Veligers 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 (312 ind m −2 day −1). Aragonite produced by this size fraction of L. inflata then amounted to about 75% of total carbonate. Clio pyramidata, the second most abundant species showed less tendency toward seasonal variation, and peaked in spring (43 ind m −2 day −1). Although many pteropods 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 year, shells derived from sedimentary pteropod layers may record past mean annual surface water temperatures. A few specimen of C. pyramidata with shell lengths from 8.5 to 13 mm were collected in August, 1989, and March, 1990, and were cut into several segments and analyzed for stable isotope composition. Although growth is discontinous, C. pyramidata appears to record summer to winter temperature fluctuations in its upper migration range (around 120 m). Thus, larger specimens taken from pteropod-rich sedimentary layers could provide past seasonal variations in surface or subsurface water temperatures. There appears to be a substantial enrichment of 13C in precipitated aragonite in L. inflata (> 250 μm size) relative to bicarbonate. We suggest that the δ 13C values in pteropod shells are primarily controlled by the isotope ratio of total dissolved CO 2 and by seasonal temperature variations. The stable carbon isotope values of L. inflata covary with the oxygen isotope values, and thus correlate negatively with water temperature ( δ 13C = 3.41 − 0.10 T). Our data confirm the negative temperature dependency reported in the literature. A four-year seasonal sedimentation record for the Cap Blanc site (G. Wefer and G. Fischer, unpubl. data) shows only one distinct pteropod sedimentation event in August, 1989. This indicates strong interannual variability of pteropod production and sedimentation. The distinct maximum in August, 1989, is probably related to episodically occurring unusually high surface water temperatures. Similar conditions might have been present during the sedimentation of pteropod-rich layers described from the deglaciation period and Holocene maximum in the Atlantic Ocean.