Planktonic foraminifera as indicators of oceanographic complexity on the southern Caribbean Sea continental shelf

Abstract

The southern Caribbean Sea over the northern South American continental shelf is oceanographically complex. It is subject to (1) widespread coastal upwelling, (2) wakes and eddies in the NW-flowing Guiana and Caribbean Currents to the lee of islands, and (3) the hypopycnal Orinoco River plume. Upwelled water is dispersed NW along high-productivity filaments. Offshore northern Trinidad, an intra-plume front separates higher-productivity Orinoco-derived water to the west from lower-productivity Amazon-derived water. This paper examines planktonic foraminiferal assemblages in surface sediment samples (a) below upwelling foci in the Gulf of Venezuela (GoV). North of Lake Maracaibo, and (b) at an upwelling focus in the North Coast Marine Area (NCMA) offshore northern Trinidad, near but west of the intra-plume front. The samples were collected for previous studies and only certain size fractions, which differed between the study areas, were available for current study.Samples from the GoV (>250 μm fraction; taken using an Alpine corer or a Petersen dredge) were assigned a priori to groups underlying upwelled and non-upwelled water. Discriminant analysis after cluster analysis confirmed these assignments. Total recovery was dominated by Neogloboquadrina dutertrei with lesser Globigerinoides ruberpink and the upwelling indicator Globigerina bulloides. All four non-upwelled samples, co-dominated by N. dutertrei and Gs. ruberpink, reflect seasonal freshwater outflow from Lake Maracaibo. Five upwelled-area samples were dominated by G. bulloides with subdominant N. dutertrei or Globigerinoides ruber Morphotype B. The remaining two upwelled samples (dominant N. dutertrei, subdominant Gs. ruber Morphotype A) apparently reflect the transition between upwelling and non-upwelling areas. The mean proportions of assemblages as Trilobatus sacculifer Morphotype A did not differ markedly between the upwelled and non-upwelled GoV clusters. This morphotype was absent from the Gulf proper but present to the north.NCMA total recovery (>150 μm fraction; samples taken using a van Veen grab) was dominated by Gs. ruber Morphotype B. Cluster analysis here indicated three clusters: Cluster 1, proximal to the intra-plume front; Cluster 2, distal to the intra-plume front; and Cluster 3, scattered throughout the NCMA. Upwelling was pervasive, canonical variate analysis (CVA) verifying that G. bulloides did not differentiate the clusters. Instead, these clusters were differentiated by Gs. ruber Morphotype B only, the proportional abundance of which was highest in Cluster I and lowest in Cluster 2. This pattern indicates that the influence of the Orinoco plume on assemblage composition is stronger than that of upwelling, Cluster 1 reflecting higher productivity near the front.