Brent Wilson

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

Abstract 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 assemblages 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 bottom-up and top-down SHEBIs. In the San José Calcareous Silt, 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.

Population structures among epiphytal foraminiferal communities, Nevis, West Indies

The taxocene of live epiphytal foraminifera was for one year monitored monthly on six phytal substrates in shallow water (<1 m) in two bays around Nevis, NE Caribbean Sea. Mosquito Bay was subject to a nutrient flux from a leaking septic tank. Long Haul Bay was comparatively undeveloped. SHE Community Structure Investigations (SHECSIs) revealed that the populations on five plants had logarithmic series distributions of species abundances, the slopes of lnS vs. lnE for these five time-series being within −1±0.3. In three time-series, they were within −1±0.05. Cluster analysis of twenty-five sediment samples in shallow water (<3 m) indicates that Nevis is largely surrounded by a single thanatacoenosis, for which SHECSI indicates a logarithmic series population structure. However, it is not possible to reconstruct perfectly the epiphytal population from the sediment thanatacoenosis. The thanatacoenosis included 40% allochthonous Amphistegina gibbosa, Archaias angulatus and Asterigerina carinata, washed in from offshore reefs, and few planorbulinids, although the latter dominates the biocoenosis on seagrass leaves in the backreef.

Late Quaternary benthonic foraminifera in a bathyal core from the Leeward Islands, Lesser Antilles, NE Caribbean Sea

The 3.13 m long bathyal piston core En20-2 from the northern Leeward Islands (17°49.9′N, 63°02.4′W, water depth 680 m) was sampled every 20 cm. The Pleistocene-Holocene boundary occurred at 107 cm, and the base of the core lay above the Atlantic extinction of Pulleniatina obliquiloculata (34 ka BP). The core yielded throughout both (a) a deep-water association (DWA) of middle bathyal benthonic foraminifera and (b) a shallow-water association (SWA; up to 56% of total recovery) comprising back-reef, epiphytal species. The environmental trends suggested by the two groups are compared. Although sea-levels during the Late Pleistocene and Early Holocene rose abruptly, there was no abrupt change in the DWA at the Pleistocene-Holocene boundary, but a gradual change from a higher-productivity fauna (Bulimina spp., Uvigerina spp.) at the base of the core to a lower-productivity fauna (Cassidulina spp., Globocassidulina spp.) at the top. Microhabitat preference changed from shallow-infaunal at the core base to primarily epifaunal at the top. Diversity and species dominance did not differ significantly between the Late Pleistocene and the Holocene. Percentage abundances of species were calculated separately for the DWA and SWA. Seventy significant correlations were found between the percentage abundances of individual DWA and SWA species, the two strongest correlations - both positive - being between (a) Neoconorbina terquemi and Uvigerina laevis, and (b) Amphistegina gibbosa and Cassidulina curvata. The percentage abundance of U. laevis decreased up-section, whereas the percentage abundance of A. gibbosa increased up-section, reflecting a change in the nutrient flux over time that impacted on both the DWA and the SWA. Positive correlations between the DWA low-productivity indicator Globocassidulina subglobosa and the SWA species Asterigerina carinata, Elphidium discoidale, Peneroplis bradyi and P. proteus suggests that these four SWA species comprise a guild.

DETECTING SEASONALITY USING TIME SERIES ANALYSIS: COMPARING FORAMINIFERAL POPULATION DYNAMICS WITH RAINFALL DATA

Foraminiferal population densities vary over time. They are sometimes monitored to assess seasonality, and sometimes to assess the impact of anthropogenic activities. The resulting temporal record, which constitutes a time series, must be carefully analyzed to ensure that the observed fluctuations are truly natural, seasonal phenomena, and not due to anthropogenic causes, before drawing final conclusions. In this paper we use autocorrelation, a mathematical facet of time series analysis (TSA) that unambiguously identifies seasonality. The analyses were conducted using the Statistix 2.0 statistical package, and data normalized by transforming to ln(y) , where y is the population size as a function of time to ensure proper percentage error representation. Using as a data set the monsoonal rainfall patterns on Trinidad, West Indies, we show that autocorrelograms for seasonal time series comprise a sine-like wave that fluctuates around zero. TSA is then used to examine seasonality in the population dynamics of Globigerina bulloides in the Cariaco Basin, Venezuela, Glabratella ornatissima off California, and Quinqueloculina in the Indian River Lagoon, Florida. These examples suggest that TSA can be a useful tool in identifying seasonality effects in foraminiferal population dynamics.

Paleoenvironmental interpretations based on foraminiferal abundance biozones, Mayo Limestone, Trinidad, West Indies, including alpha and beta diversities

Abstract Many carbonate sections are characterized by monotonous successions that mask subtle changes in paleoenvironmental parameters. Foraminiferal populations, however, are more sensitive to environmental parameters and commonly reflect these subtle changes. The accuracy of paleoenvironmental interpretations based on foraminiferal assemblages depends on the precise delineation and interpretation of abundance biozones (ABs). Samples rich in foraminifera were taken from four outcrops (OC1–OC4, oldest to youngest) of the Mayo limestone bioherm (Tamana Formation) that comprised 6–8 limestone-marl alternations each. Bottom-up SHE Analysis for Biozone Identification (SHEBI) indicated the samples to be from seven ABs (AB1–AB7). The percentage of the foraminiferal assemblage as planktonic specimens (%P) ranged from 5%–25% and was generally lower in AB1, AB3, AB5, and AB7 than in intervening biozones. This indicates deposition at shallow- to mid-neritic paleodepths (∼20–55 m) during a series of transgressions and regressions. Per sample (point) diversities, measured using the information function H, ranged from 1.67 to 2.86. Total within-AB diversities (α diversities), were calculated from the final value of H and expressed as the effective number of species SE [ = eH]; these ranged from 7.8 to 18.4. Changes in diversity across AB boundaries (β diversities) were calculated using the percentage change in SE, and ranged from −44% to 137%. Not all β diversities were significant, indicating that paleodepth was not the only control on α diversity. Fluctuations in the percentage abundances of Elphidium spp., Pseudononion atlanticum, and Amphistegina sp. show that changes in the organic matter flux also exerted some control, being lowest in the Amphistegina-rich AB3.

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

Foraminiferal communities are not static, but change in response to environmental perturbations. Given sufficient time, the change will be recorded in the total (live+dead) seafloor assemblage, from which valuable information regarding environmental trends can be obtained by re-sampling assemblages at the decadal scale. The seafloor assemblage in the 5 km × 6 km Ibis Field, off southeast Trinidad, first surveyed in 1953, was re-examined in 2005. The fauna had changed markedly between the surveys. Overall increases in the proportional abundances of Uvigerina subperegrina, Ammonia pauciloculata/Rolhausenia rolhauseni and Pseudononion atlanticum indicate an increase in nutrient supply that apparently killed off Cibicidoides pseudoungerianus and Miliolinella subrotunda, and reduced the relative abundance of Hanzawaia concentrica, but did not affect the relative abundance of Cancris sagrai. As shown by similar 1953 and 2005 planktonic/benthonic foraminiferal ratios, the increased nutrient supply impacted on both surface and bottom waters. Of the six most abundant species in 2005, five showed the same general biogeographical distributions within the field in 1953 and 2005. However, whereas the proportional abundance of Uvigerina subperegrina in 1953 increased southwards, in 2005 it increased northwards. Trinidad cannot be the source for the nutrient enrichment: the island lies down-current from the Ibis Field. Sources must therefore be sought up-current and to the southeast, in the Amazon, Essequibo and Orinoco river basins, or along the South American shoreline. It is speculated that the nutrient enrichment may be a consequence of increased phytoplankton primary production associated with nitrogen-rich run-off from South American sugarcane plantations, or from flushing of organic carbon from poorly regulated sewage systems or shrimp farms in South America.

Abundance biozone boundary types and characteristics determined using beta diversity: An example using Pleistocene benthonic foraminifera in DSDP Hole 148, eastern Caribbean Sea

Abstract There is not yet a precise classification of or terminology for ecostratigraphic boundaries. SHE analysis for biozone identification (SHEBI) objectively places boundaries between abundance biozones (ABs). Alpha diversity ( = SE  =  eH, where H is the information function) measures the community diversity within an AB in units of effective species, whereas the dimensionless β diversity ( =  SE2/SE1, where SE1 and SE2 are the α diversities of successive ABs) quantifies the difference in α diversity between successive ABs. Three categories of AB boundary are recognized depending on the value of β. In Type –1β, the α diversity of the younger AB is significantly less than that of the older. In Type 0β, SE2 ≈ SE1, while in Type +1β, the α diversity of the younger AB is significantly greater than in the older. Benthonic foraminifera were sampled from the ∼124-m-thick Pleistocene strata of DSDP Hole 148 (eastern Caribbean Sea). SHEBI indicated 18 ABs; of the 17 AB boundaries, seven were Type –1β, two were Type 0β, and eight, Type +1β. The direction of inflection of the graph of lnE versus lnN did not indicate AB boundary type. Although the Pleistocene was characterized by repeated alternations between glacial and interglacial conditions, there was no regular alternation of Type –1β and +1β AB boundaries. Complementarity (i.e., species level distinctiveness of successive ABs) was measured using a percentage similarity index, CP. Differing complimentarities show that boundaries between ABs varied with respect to permeability to species, while beta diversities and CP were uncorrelated.

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

Thirty two seafloor samples were taken over an area of 3600 km2 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 byBairdoppilata dorsoangulata andBradleya ex gr.dictyon, with lesserRadimella ovata, Loxoconcha sp. 2sensuBreman, 1982 andCosta variabilicostata 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 biofacies immediately under the front yielded particularly high proportions ofB. ex. gr.dictyon andB. dorsoangulata, but relatively fewLoxoconcha 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.

Determining carrying capacity from foraminiferal time-series

The carrying capacity K is the equilibrium population density of a species that an area can support while adequately meeting the needs of every individual. Although widely used in ecology, it has yet to be applied rigorously to living foraminifera. K is readily determined from time-series of population densities. Given that Nt+1 = Nt + RNt, in which Nt is the population densities at time t, Nt+1 is the density at a subsequent time t+1 and R is the per capita rate of change in population density, then linear regression gives Rt = Rm − sNt, in which Rt is the per capita rate of increase at time t, the constant Rm is the maximum possible individual rate of increase, and the negative slope s represents the strength of intraspecific interactions. Setting Rt = 0, so that Nt = K and Rm – sK = 0, gives K = Rm/s, which is applicable in aseasonal environments. There are two carrying capacities in seasonal environments, depending on whether the season is favourable (Kmax) or unfavourable (Kmin). Values of Kmax and Kmin are estimated for Nonion depressulus in the Exe estuary, UK (25 monthly samples), Quinqueloculina spp. in the Indian River Lagoon, USA (60 monthly samples) and Haynesina germanica in Cowpen Marsh, UK (25 fortnightly samples). The most precise estimate was for H. germanica, but it was unclear if this was due to the high rate of sampling or the large number of replicates used to erect this time-series.

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

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.