Trouble in Paradise? A comparison of 1953 and 2005 benthonic foraminiferal seafloor assemblages at the Ibis Field, offshore eastern Trinidad, West Indies

2008

PALAIOS

SHE analysis for biozone identification (SHEBI) is a new technique that identifies abundance biozones (ABs) by accumulating species abundance data along a time series, recalculating species richness (S), the information function (H), and the equitability index (E) with the addition of each sample. Boundaries are drawn between ABs where SHE analysis detects a change in either (1) the population structure or (2) the assemblage's species composition, species having joined or left the community in quantities that affect the evenness of the distribution of species abundances. This study uses foraminiferal data from two Miocene sections on Trinidad (Cipero Formation, Catapsydrax stainforthi Zone; San José Calcareous Silt, Globorotalia acostaensis Zone) to test if top-downward or bottom-upward analyses indicate the same AB boundaries. The results differ. In the Cipero there are no matches between AB boundaries determined by bottomup and top-down SHEBIs. In the San José CalcareousSilt, only some boundaries match. It is recommended that SHEBI proceed in the order in which the samples in the ABs were deposited, treating the ABs as successive ecostratigraphic units. Intriguingly, no matter whether ABs were determined by bottom-up and top-down SHEBIs, SHE community structure investigations indicated logarithmic series population structures. A graphical technique for determining the species responsible for the change in ABs reveals that a peak in the per sample percentage abundance of Nuttallides umbonifera in the Cipero Formation is offset by one sample from the AB boundaries it engendered, marking a temporary decrease in nutrient flux.

Section: Cipero Formationmentioning

confidence: 99%

Using SHEBI (SHE Analysis For Biozone Identification): To Proceed From The Top Down Or The Bottom Up? A Discussion Using Two Miocene Foraminiferal Successions From Trinidad, West Indies

2008

PALAIOS

“…Second, WILSON (2006d) concluded that the rate of organic matter input to the Ibis Field has increased markedly over the past few decades. Mixing across fronts, however, is limited.…”

Section: Discussionmentioning

confidence: 99%

“…Finding marked increases in the proportional abundances of Uvigerina subperegrina, Pseudononion atlanticum and Rolhausenia rolhauseni, he concluded that the Ibis Field has over recent decades been subjected to an increased organic matter loading. WILSON (2006d) nevertheless concluded that the cause was to be sought in northern South America, perhaps being associated with shrimp farms, unregulated sewer systems, or run-off from sugar plantations. SEN GUPTA eta[.…”

Section: Hydrography Of the Eastern Caribbean And Its Implications Fomentioning

confidence: 99%

“…DROOGER & KAASSCHIETER (1958) surveyed the foraminifera on the continental shelf between the Essequibo River and north-east Venezuela, presenting 36 maps that documented species' distributions in 1953. WILSON (2006d) documented that changes in the foraminiferal community between 1953 and 2005 at the Ibis Field Table 4. It is unclear if this is due to the front impinging on the seafloor -although this seems unlikely given the boundary layer depths noted elsewhere -or to a change in the nature of the phytodetrital food supply across the front.…”

Section: Indications Of a Shelfal River-induced Frontmentioning

confidence: 99%

See 1 more Smart Citation

Distributions of ostracod (Crustacea) biofacies on the continental shelf off south-east Trinidad, western central Atlantic Ocean, suggest the location of an offshore river-induced front within the Orinoco Plume

Senckenbergiana lethaea

2008

Self Cite

Thirty two seafloor samples were taken over an area of 3600 km 2 off south-east Trinidad (water depths 38-102 m), north-east of the Orinoco Delta. A total of 7097 ostracod carapaces and valves from 37 species were picked from them. Total recovery was co-dominated by Bairdoppitata dorsoangutata and Brad#ya ex gr. dictyon, with lesser Radimella ovata, Loxoconcha sp. 2 s e n s u BREMAN, 1982 and Costa variabilicastata recticostata. Cluster analysis revealed four biofacies approximately perpendicular to the bathymetric contours. This arrangement is contrary to the usual pattern, in which biofacies parallel contours, and is suggested to reflect the occurrence of a shelfal, river-induced front entrained within the hypopycnal Orinoco Plume. The front marks where outflow from the Essequibo and Orinoco rivers abuts against surface water rich in outflow from the Amazon. The bioiCacies immediately under the front yielded particularly high proportions of B. ex. gr. dic~yon and B. dorsoangulata, but relatively few Loxoconcha sp. 2. Application of SHE Analysis for Biozone Identification (SHEBI) indicated that within the biofacies there occur abundance biozone boundaries at -55m, 75 m and 100 m that separate areas with differing population structures. SHE Analysis for Community Structure Investigation (SHESCI) indicated that, although the species proportions within most abundance biozones have a logarithmic series distribution, the rate of increase in species richness as specimens were accumulated was lowest in an abundance biozone below the front.

“…biofacies. Wilson (2006b) surveyed the 2005 benthonic foraminiferal fauna from the Ibis Field, about 50 km due west of the Savonette Field. Given the distance between Drooger & Kaasschieter's (1958) transects and the Savonette region, however, these percentages must be regarded as approximate only.…”

Section: Previous Workmentioning

confidence: 99%

The significance of iron-stained foraminifera off SE Trinidad, West Indies, western central Atlantic Ocean

2010

Geol. Mag.

Self Cite

Eleven samples of seafloor sediment were taken from water depths of 78-90 m within the Savonette Field, off SE Trinidad, western Atlantic Ocean. This surface sediment is relict, having been deposited during an early Holocene transgression. The samples yielded much iron-stained quartz and a rich assemblage of dead foraminifera, of which 75 % of planktonic foraminifera were stained with iron, as was 66.5 % of the calcareous benthonic foraminiferal assemblage. The fauna, both iron-stained and unstained, was dominated by Cibicidoides ex. gr. pseudoungerianus, and is concluded, despite the proximity of the Orinoco Delta, to be equivalent to a relict Cibicidoides biofacies in carbonate-rich areas of the Gulf of Mexico. Staining was by limonite and hematite. Differing percentages of calcareous benthonic species had been stained with iron, ANOVA revealing three groups of species within which the mean percentage of iron-stained specimens per sample did not differ: (a) Globocassidulina subglobosa and Hanzawaia concentrica; (b) Amphistegina gibbosa, Cassidulina norcrossi australis Cibicioides ex. gr. pseudoungerianus, C. io, Elphidium translucens and Quinqueloculina lamarckiana; and (c) Eponides antillarum and E. repandus. It is concluded that species differ in their susceptibility to iron staining, and that planktonic foraminifera are more susceptible than most benthonic species. Although waters off northern South America are turbid and the photic zone only ∼ 25 m deep, the relict assemblage contained 8.4 % algal symbiont-bearing foraminifera (especially A. gibbosa and E. translucens) that would be limited to the photic zone. These are thought to have lived at a time early in the Holocene transgression when sequestration of sediment within the Orinoco delta rendered the water sufficiently clear for viable populations of symbiont-bearing foraminifera. Should iron-stained foraminifera prove to be restricted to transgressive systems tracts, this would make them a useful sequence stratigraphic tool.