BENTHIC FORAMINIFERA, THE ORINOCO PLUME AND THE GUIANA CURRENT

Controls on NE South American benthic foraminiferal distributions are little known. The Trinidad and Tobago continental slope is oceanographically complex, being impacted by the Guiana Current to 200 m water depth and the nutrient-rich Orinoco plume to ~50 m but over a wider area. Early micropaleontologists paid scant attention to the plume and current impact.

Piston cores JPC-ASC-C-001 (water depth 428 m), FL Cassra 2A (395 m) and JPC-13A – IB (186 m), sampled every 10 cm, penetrated Holocene sediment along a N-S transect off Tobago. A Current Strength Indicator Index (CSII = proportion Quinqueloculina lamarckiana + Planulina ariminensis) is here used to investigate the area of seafloor impacted by the current, and a Benthic Foraminiferal High Productivity Index (BFHPI = proportion Bolivina + Bulimina + Uvigerina) to determine the area impacted by the plume.

JPC-ASC-C-001 yielded a Cassidulina curvata - Sphaeroidina bulloides community with low BFHPI (mean 0.12) and CSII (mean 0.01). FL Cassra 2A had a Bolivina subaenariensis westermanni – Bulimina marginata community with high BFHPI (mean = 0.52) but low CSII (mean = 0.01). Thus, both cores lie below water with sluggish circulation, but on opposite sides of the plume edge. JPC-13A – IB had a high CSII (mean = 0.16) with abundant Q. lamarckiana, and a high BFHPI (mean = 0.53), with common Uvigerina parvula. This core lay below the plume but in above the current base.

SHE Analysis recalculates the diversity index H, species richness S and evenness E as samples are accumulated and the specimen number N increases. The assemblage in JPC-ASC-C-001 is in balance or stasis, with H constant as S increases and E decreases. The assemblages in FL Cassra 2A and JPC-13A – IB give evidence of thriving, both H and S increasing while E decreases.

Uvigerina spp. and Bolivina spp. ingest fresh phytodetritus, while Bulimina spp. are seasonally phytophagous, ingesting fresh phytodetritus when available but sedimentary organic matter when fresh phytodetritus is absent. The abundant Bulimina spp. in FL Cassra 2A suggests it receives phytodetritus seasonally and is only intermittently below the plume, while the abundant Uvigerina spp. at JPC-13A – IB indicate a constant phytodetrital input. This reflects seasonal plume expansion and contraction.