Late Quaternary benthic foraminifera and the Orinoco Plume

Marine Micropaleontology 121 (2015) 85–96

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Late Quaternary benthic foraminifera and the Orinoco Plume Brent Wilson a,⁎, Lee-Ann C. Hayek b a b

Petroleum Geoscience Programme, Department of Chemical Engineering, The University of the West Indies, St. Augustine, Trinidad and Tobago Smithsonian Institution, Mathematics and Statistics, NMNH, MRC-121, Washington, DC, USA

a r t i c l e

i n f o

Article history: Received 14 April 2015 Received in revised form 13 October 2015 Accepted 2 November 2015 Available online 05 November 2015 Keywords: Statistical analysis Younger Dryas Trinidad and Tobago Inter-tropical convergence zone Fundamental niche

a b s t r a c t The bathyal benthic foraminiferal palaeoecology east of Trinidad is currently unknown. The area is oceanographically complex, comprising a pro-delta deep-sea fan building across a transpressional plate boundary. Outflow from the Orinoco River forms a high-productivity surface plume that abuts low-productivity water of the tropical western Atlantic Ocean across a laterally extensive but sharp front. The plume's areal extent differs between the rainy and dry seasons, which are governed by the position of the inter-tropical convergence zone (ITCZ). Upper Quaternary benthic foraminifera are examined in two ~4 m-long piston cores, taken near the eastern edge of the plume. CHIRP profiles show that core BGT086 (water depth 626 m) comprised in situ material throughout, while the lower part of core BGT096 (water depth 700 m) consisted of slumped material. Benthic foraminiferal assemblages in BGT086 were examined quantitatively, while those in BGT096 were examined qualitatively only for comparison. Core BGT086 shows a transition from a low-diversity, Cibicidoides pachyderma-dominated community with subdominant Cassidulina curvata to a later high-diversity, low-dominance Bulimina alazanensis– Osangularielloides rugosa–Epistominella exigua community. The former is indicative of low-productivity, and the latter of high-productivity, surface water. The restriction of E. exigua to the core's upper part may reflect a change from an aseasonal to a seasonal organic flux. The faunal change was detected by both SHE and cluster analyses and an assemblage turnover index (ATI). It may be related to either (a) expansion of the Orinoco plume due to northward migration of the ITCZ approximately 600 yr after the end of the Younger Dryas, (b) eastward progradation of the Orinoco Delta, which would in turn push the front of the Orinoco plume eastward, or (c) both. No other boundary detected was common to all three procedures. A boundary between 124 and 130 cm below the seafloor detected by SHE analysis might equate to the 8000–7500 yr BP meltwater pulse 1C. While some specialist species were restricted to the C. pachyderma or the B. alazanensis–O. rugosa– E. exigua communities, the percentage abundance of some others, concluded to be relative generalists (e.g., Bulimina aculeata and Sphaeroidina bulloides), varied little through the core. The undisturbed section of core BGT096 showed the same general faunal succession as core BGT086. The slumped material in this core was rich in Eponides regularis, which was rare elsewhere. This species shows that the allochthonous sediment was derived from the outer shelf and upper slope (100–400 m water depth) during Late Pleistocene time. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The benthic foraminiferal palaeoecology and ecostratigraphy of the bathyal Orinoco deep-sea fan, in the western tropical Atlantic Ocean, are little known, even though the adjacent island of Trinidad has been a source of oil and natural gas for over a century (Carr-Brown, 2007; Furlonge and Kaiser, 2010) during which foraminiferal biostratigraphy played a major role in hydrocarbon exploration (Saunders and Bolli, 1979). Exploration around Trinidad and Tobago to date has accessed fields in onshore regions or beneath the neritic (typically b150 m water depth) continental shelf, and the few studies of late Pleistocene and Holocene benthic foraminifera associated with this hydrocarbon exploration have focussed on neritic depths proximal to the Orinoco ⁎ Corresponding author. E-mail address: [email protected] (B. Wilson).

http://dx.doi.org/10.1016/j.marmicro.2015.11.004 0377-8398/© 2015 Elsevier B.V. All rights reserved.

Delta (Drooger and Kaasschieter, 1958; Kruit, 1954; van der Zwaan and Jorissen, 1991; Wilson, 2006, 2010). Comparison of geographic maps of the bathyal to abyssal Orinoco fan and surface waters in Callec et al. (2010), Agard and Gobin (2000) and Kempler (2012) shows that the proximal part of the fan lies beneath an offshore hyperpycnal plume with high surface productivity. The impact of this plume on fan foraminiferal assemblages is unknown. Nearby, Gaby and Sen Gupta (1985) examined late Quaternary abyssal (N2000 m water depth) foraminifera in eight piston cores from the Venezuela Basin of the eastern Caribbean Sea. They found distinct upper Pleistocene and Holocene assemblages, the Holocene assemblage being dominated by Planulina wuellerstorfi (Schwager) and the upper Pleistocene assemblage being co-dominated by Massilina sp., Globocassidulina subglobosa (Brady), and Nummoloculina irregularis (d'Orbigny). They found neither Bulimina aculeata d'Orbigny, nor Uvigerina peregrina Cushman. Galluzzo et al. (1990) documented the distribution of benthic

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foraminifera in the Holocene of the Grenada Basin, ~200 km NW of Trinidad, where they found an assemblage dominated by B. aculeata, B. alazanensis Cushman, Osangularia culter (Parker & Jones), Uvigerina auberiana, d'Orbigny, and U. peregrina between 1000 and 2100 m. This they thought was associated with Sub Antarctic Intermediate Water and indicative of low dissolved oxygen levels. Sen Gupta et al. (1991) found only subtle changes in the benthic foraminiferal fauna at the base of the Holocene, as defined using an influx of the planktonic foraminifer Globorotalia menardii, within the Grenada Basin. Wilson and Costelloe (2011) examined benthic foraminifera in the Quaternary of DSDP Hole 148 (13°25.12′ N, 63°43.25′ W, water depth 1232 m) near the eastern edge of the Orinoco plume. Only Sigmoilopsis schlumbergeri (Silvestri), U. auberiana, U. peregrina, and B. aculeata each formed N 5% of the total assemblage from DSDP Hole 148. Species of Uvigerina and Bulimina, which perhaps due to an enhanced organic matter loading are indicative of low dissolved-oxygen content (Kaiho, 1994), respectively formed 22.6% and 14.3% of the total assemblage. Cibicidoides pachyderma (Rzehak) [recorded as C. floridanus (Cushman)], which is indicative of oxic bottom waters (Kaiho, 1994), formed only 2.6% of the total assemblage. Hofker (1983) examined the benthic foraminifera offshore Guyana and Surinam, ~700–1000 km SW of Trinidad, using samples primarily from the continental shelf but including five samples from upper to middle bathyal depths (207–940 m). He concluded that the bathyal fauna, with abundant B. aculeata, B. alazanensis, B. striata mexicana Cushman, C. pachyderma [as C. pseudoungerianus (Cushman)], O. culter, Sphaeroidina bulloides d'Orbigny, and U. peregrina, are indicative of the upwelling of cool water. He noted that, due to the influence of Amazon water, Secchi disc visibility on the continental shelf off Guyana “is nowhere more than 20 m, and often less”. Despite the work by Hofker (1983), Gaby and Sen Gupta (1985), Galluzzo et al. (1990) and Wilson and Costelloe (2011), those wishing to interpret the deeper water palaeoenvironments of the Pleistocene and older Neogene succession of Trinidad have relied on observed foraminiferal distributions in the western Gulf of Mexico (Culver, 1988; Pflum and Frerichs, 1976; Phleger and Parker, 1951; Poag, 1981), perhaps because that area is influenced by Mississippi outflow and Secchi disc visibility there is comparable to that off Trinidad and Guyana (Manheim et al., 1972). Alternatively, they have used the subjective biofacies model developed for the Trinidad Neogene by Batjes (1968). These biofacies, named after the dominant species, are the shallowwater Miliammina, Ammonia and Trochammina biofacies, the more open water Hanzawaia, Buliminella, Eggerella, Uvigerina and Bolivina floridana biofacies, and the predominantly agglutinated, deeper water Glomospira–Alveovalvulina–Cyclammina biofacies. Jones (1998) used both Batjes (1968) system of biofacies and data from the Gulf of Mexico when undertaking palaeoenvironmental interpretations of benthic foraminiferal assemblages in the upper Miocene and Pliocene formations of Trinidad. Hydrocarbon exploration off Trinidad is now moving into deeper water on the continental slope. This necessitates a study of the benthic foraminifera in the upper Quaternary of the Orinoco deep-sea fan. This is an area of considerable geological and oceanographic complexity, however, consisting of a major river delta and fan system (the Orinoco Delta, the adjacent, shallow submarine Orinoco Deltana Platform, and the deeper water Orinoco deep-sea fan) building across the transpressional tectonic plate boundary between the Caribbean and South American plates (Aslan et al., 2003; Callec et al., 2010; Garciacaro et al., 2011), the locus of which currently extends across central Trinidad (Prentice et al., 2010). Phleger (1976) noted that the benthic foraminiferal community at depth varies according to the primary productivity of surface waters, the latter being high in shelf areas affected by outflow from a large river. He found that the benthic foraminiferal fauna in very high production areas, such as beneath the offshore hyperpycnal plume off the eastern Mississippi Delta (see Chin-Leo and Benner, 1992), has low diversity and is dominated by one to three species that collectively constitute N95% of the population. Off the Mississippi the surface productivity is

highest at intermediate salinity levels (chlorides 15–30 ppt), decreasing into the normal-marine waters of the open Gulf of Mexico. The Orinoco River being the fourth largest in terms of outflow worldwide (Hu et al., 2004), of particular interest to hydrocarbon exploration around Trinidad is the question of how the bathyal benthic foraminiferal community responded to sea-level change and the concomitant migration of the Orinoco Delta and its associated offshore oceanographic features. Furthermore, water and mud from the River Amazon, the world's largest river in terms of outflow (Oki and Kanae, 2006), is carried NW from the Amazon river mouth by the Guyana Current towards Trinidad and eastern Venezuela such that about half the mud forming the Orinoco Delta is of Amazon origin (Aslan et al., 2003). Low salinity Amazon water contributes to the Orinoco plume. During the Pliocene, when glacially-driven sea-level change was muted in comparison to the Pleistocene, sea level rise and coupled sedimentation-driven progradation caused the Orinoco Delta to alternately retreat and advance on the order of 100 km across the continental shelf (Chen et al., 2014), and similar translations are to be expected during the Pleistocene. Knowledge of how the benthic community reacted to these transgressions and regressions (both normal and forced) would aid greatly in interpreting the evolution of the Orinoco fan. This paper examines the benthic foraminiferal community across the Pleistocene–Holocene boundary (Termination I — see Cheng et al., 2009), when sea level rose ~ 120 m (Poag and Valentine, 1976), on the modern-day middle bathyal (500–1000 m) continental slope east of Trinidad. Some comments can be made from the results of this study regarding the relative niche widths of benthic foraminiferal species. The fundamental niche is an n-dimensional hypervolume where each of the n dimensions comprises a physical or biological factor (Hutchinson, 1957; Soberón and Peterson, 2005). The ecological niches of individual benthic foraminiferal species are not yet well understood (Murray, 2001). Factors investigated that determine their fundamental and realised niches (the pre-interaction and post-interaction niches of Morin, 1999) comprise primarily food (organic carbon) and dissolved oxygen (Van der Zwaan et al., 1999), nitrate (Piña-Ochoa et al., 2010) and light (Hallock, 1984). These factors being consumptive resources (Jeffries and Lawton, 1984), these studies overlook the role of species interactions. Nevertheless, it is apparent that the fundamental niche width varies between species, there being distinct generalists (with broad fundamental niches) and comparative specialists. van der Zwaan et al. (1999) suggested that foraminifera generally are not stenotopic to some environmental variables such as temperature and salinity, having adopted a beneficial generalist strategy in which low degree of specialisation prevents rapid extinction. However, not all benthic foraminifera are generalists. Kaiho (1994) distinguished oxic and dysoxic benthic foraminifera, while Sen Gupta and Machain-Castillo (1993) showed that some endobenthic foraminifera are conspicuously dominant in bathyal oxygen minimum zones. Comparing epiphytic and sediment-dwelling foraminiferal communities around Long Island, Matera and Lee (1972) concluded that Elphidium incertum (Williamson) and Ammotium salsum (Cushman and Brönnimann) are generalists, being common in both epiphytic and sediment communities. Steineck and Bergstein (1979) suggested that modern Ammobaculites exiguus Cushman and Brönnimann and Ammonia beccarii (Linnaeus) are opportunistic generalists, occupying a wider range of paralic environments than do other species. This generalist behaviour for A. beccarii may be more apparent than real, however, authors having applied this name to many forms (Hayward et al., 2004). Here we posit the generalist and relative specialist nature of some species recovered from the Orinoco fan. 2. Regional setting The complex slope east of Trinidad is characterised by a sand-rich deep-sea fan building across the active Barbados accretionary prism. The fan is traversed by at least four NE–SW anticlines and more than

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ten NW–SE trending growth faults (Wood, 2000, figure 2). There are three types of mass transport complexes in the region: (1) shelfattached systems fed by shelf-edge deltas with sediment input controlled by sea level change; (2) slope-attached systems where upper slope sediments failed catastrophically due to earthquakes or gashydrate disruptions; and (3) locally detached systems that formed when local seafloor instabilities triggered small collapses (Moscardelli and Wood, 2008). Environmental and palaeoevironmental factors are known to change rapidly eastwards from Trinidad and northern South America, but, with the exception of Wilson and Costelloe (2011), regional interpretations of foraminiferal ecostratigraphy have not yet taken account of this marked west–east gradient. Schmoker et al. (2013) noted that the modern waters of the tropical central Atlantic Ocean vary little over the annual cycle, with high temperatures and low nutrient availability. These waters have the lowest primary productivity worldwide. Palaeoenvironments on the Ceará Rise (~ 1000 km NE of the Amazon River mouth) were relatively stable across the late Pleistocene and Holocene Marine Isotope Stages (MIS) 1–3, and hence across Termination I (which separates MIS 1 from MIS 2; Foster and Sexton, 2014). This was despite an apparent fivefold increase in the upwelling rate of deeper waters in the western tropical Atlantic during the last glacial (MIS 2) (Foster and Sexton, 2014) and a major cosmic-impact event at the onset of the Younger Dryas cooling episode at ≈ 12,800 ± 150 years BP (Kinzie et al., 2014) that effected North America, Europe and the North Atlantic Ocean and Caribbean Sea. Glacial-interglacial sea surface palaeotemperatures in the open western tropical Atlantic differed by only about 2 °C (cf. 4–6 °C in the eastern tropical Atlantic) (Foster and Sexton, 2014, figure 1A) and western tropical Atlantic primary productivity at the ocean surface changed little, averaging ~ 30 g C m−2 yr−1 (Rühlemann et al., 1996). This primary productivity suggests that the Ceará Rise occupied a low productivity region throughout the later Quaternary. Nearer shore, the western tropical Atlantic Ocean along the NE South American margin was, in contrast, more variable during late Quaternary times. It is currently being affected by outflow from the Amazon and Orinoco rivers (Hu et al., 2004) and localised upwelling of nutrientrich water (Agard and Gobin, 2000; Pascual et al., 2009; Wilson and Hayek, 2014b). This engenders a mean primary productivity at the water surface of 1.1–1.2 g C m−2 day−1 along the present-day coastline and around the island of Trinidad (cf. 0.2–0.4 g C m−2 day− 1 on the Ceará Rise; Foster and Sexton, 2014, figure 1B). The nutrient-rich waters of the Amazon and Orinoco Rivers conjoin to form an offshore hyperpycnal lens (the Orinoco plume) that meets the open Atlantic water along an offshore front across which there is little mixing (Del Castillo et al., 1999; López et al., 2013). This is apparently reflected in the offshore distribution of terrestrial pollen derived from the Orinoco River, which is concentrated in a narrow band that parallels the shoreline (Muller, 1959). Currently the eastern edge of the Orinoco plume, which is marked by a sharp boundary between high- and low-productivity surface water, at the October height of the northern South American rainy season lies at ~60°W (Kempler, 2012). It extends approximately N–S, and is thus only subparallel to the SE/NW-trending bathymetric contours in the study area. Wilson (2008a) suggested on the basis of recent benthic ostracod distributions that there occurs within the Orinoco plume off SE Trinidad a secondary front that separates Orinoco water from that derived from the Amazon. This secondary front extended across the shelf perpendicular to the bathymetric contours, compromising the use of ostracod and foraminiferal species as palaeodepth indicators on the continental shelf within the hydrocarbon-rich Columbus Basin (for details of which, see Wood, 2000). Within the Orinoco plume, the Amazon and Orinoco waters around Trinidad induce the development of a rich microphytoplankton and mesozooplankton community indicative of a herbivorous-metazoan food chain and of conditions favourable for an enhanced net carbon flux to the seafloor (Figueroa, 2007) compared to the open tropical Atlantic Ocean (Schmoker et al., 2013).

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The area covered by the Orinoco plume changed throughout at least the late Pleistocene and Holocene, when the hydrological cycle of the northern South American continent was impacted greatly by latitudinal migrations of the insolation-driven inter-tropical convergence zone (ITCZ) and associated monsoon activity (Arbuszewski et al., 2013). The ITCZ had a mean annual position of 2°S during the Last Glacial Maximum and the Younger Dryas, but shifted to northern South America (10°–12°N) during the early Holocene (Hoffmann et al., 2014). This led to dry Late Pleistocene and wet Holocene palaeoclimates in the northern Venezuela–Trinidad region. These hydrological changes are reflected in the tests of the nearshore planktonic foraminiferal palaeoecommunity. Hoffmann et al. (2014) examined Ba/Ca ratios and δ18 Oseawater in the surface-dwelling planktonic foraminifera Globigerinoides ruber (pink) in a core taken off eastern Trinidad (11°36.53′N, 60°57.86′W; 852 m). They found an abrupt increase in the Ba/Ca ratio in the early Holocene, ~ 600 yr. after the end of the Younger Dryas (YD) cold interval at ca. 10.8 ka. This indicates a major reorganisation of moisture sources in northern South America at the time. In contrast, however, the salinity dependent δ18 Oseawater from the same samples decreased gradually starting at the end of the YD. They suggested that the Ba/Ca ratio documents an abrupt increase in Ba-rich waters of a northern Andean source caused by an insolation-driven shift of the ITCZ. It is not yet known in detail how the plume/open Atlantic front illustrated by Odriozola et al. (2007), figure 3) migrated during the glacial– interglacial cycles of the Pleistocene. This paper addresses this question by examining the benthic foraminiferal palaeoecology and ecostratigraphy in two piston cores taken in the vicinity of the eastern rainy-season boundary of the present day Orinoco plume. 3. Materials and methods A proprietary suite of piston cores was collected from the slope east of Trinidad between 09° 57.8379′–10° 19.7871′N and 059° 56.0245′– 060° 19.2332′W, water depths 168–700 m (Ron Daniel, written communication). Samples from two of these (BGT086, 10.258°N, 59.956°W, water depth 626 m; BGT096, 10.330°N, 59.954°W, water depth 700 m; Fig. 1) were released to the authors for study. According to the time of year, the sites of piston cores BGT086 and BGT096 at present lay either east (dry season) or west (rainy season) of the offshore front. The chronology of these cores is extrapolated from the nearby core M78/1-235-1 (11.607°N, 60.96°W, water depth 852 m) of Hoffmann et al. (2014). Care was taken to avoid the anticlines and faults recorded by Wood (2000) during core selection. Furthermore, the slope-forming clinoforms can show considerable deformation due to the mass transport complexes recorded by Moscardelli and Wood (2008). Although an attempt was made to avoid transported sediment during core selection, some slumping is evident at the base of core BGT096. While high-resolution CHIRP sonar profiles showed that the site of core BGT086 was undeformed, with reflectors paralleling the seafloor throughout and the first marked reflector ~ 200 cm below the seafloor (cmbsf), Site BGT096 showed deformation from ~ 280 cmbsf but undeformed sediment above. Core BGT086 comprised silty clays (colour 5Y 4/1) while BGT096 comprised clay (colour 5Y 3/1). The gross appearance of the slumped material did not differ from that of the overlying in situ sediment. Core BGT086 was analysed quantitatively, but, in view of the soft sediment deformation, BGT096 was primarily analysed qualitatively. Samples had been taken where possible from both cores from 4 cm-thick slices spaced 10 cm apart, although selected samples were retained by the donor for other analyses. The donated samples were washed over a 63 μm mesh to remove silt and clay and the residue dried over a gentle heat before being spread evenly over a picking tray. Specimens were picked from the first three rows in the tray to obtain N 250 benthic foraminifera. For core BGT086, sample BGT086: 380–384 cmbsf yielded 249 benthic foraminifera only in its entirety;

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Fig. 1. The study area. A. The eastern Caribbean Sea highlighting the study area, the rainy-season extent of the Orinoco plume and DSDP Hole 148. B. The study area, showing the locations of Piston Cores BGT086 and BGT096, depth contours in metres, distance in nautical miles.

eight samples from core BGT096 failed to yield 250 specimens. The specimens were sorted into species that were identified using illustrations in Phleger and Parker (1951), Drooger and Kaasschieter (1958), Poag (1981) and Hofker (1983), and the number of specimens in each was counted. The proportional abundances of species were calculated for the total assemblage for each core and per sample and used to suggest overall trends. Further statistical analyses were largely restricted to core BGT086. The PAST palaeontological freeware of Hammer et al. (2001) and Hammer and Harper (2005) was used to determine sample-wise values of the information function H [=−∑pi · ln(pi), in which pi is the proportional abundance of the ith species], species richness S, and the max(pi) index of Berger and Parker (1970) (for further details, see Hayek and Buzas, 2013). PAST was also used to determine rarefied species richness S200 for N = 200 specimens. The sample-wise assemblage turnover index (ATIs) of Hayek and Wilson (2013); Wilson and Hayek (2014a, 2014b) and Wilson et al. (2014) was calculated from the expression ATIs ¼

X jpi2 –pi1 j;

ð1Þ

in which ATIs is the between-sample assemblage turnover, and pi1 and pi2 are the proportional abundances of the ith species, i = 1, …, S, in the lower and upper samples. ATIs gives the proportion or percent of faunal turnover or change between adjacent samples. The mean (x) and standard deviation (σ) of values of ATIs were calculated over all 35 samples from core BGT086. A normal quantile plot test for nonnormality with a critical level of p = 0.05 was used to determine that the values of ATIs were normally distributed, as expected of a statistic that is a sum. A runs test was computed to determine whether values of ATIs were randomly distributed around x. Values of ATIs exceeding (x + σ) indicate the positions of relative major assemblage turnovers (Hayek and Wilson, 2013) and are used to divide the succession into peak-bounded ATIs (PATI) intervals (Wilson and Hayek, 2014b). These intervals were numbered, commencing from PATI-1 for the youngest. PATI-1 and the lowermost PATI are of necessity incomplete, their lower and upper boundaries respectively not being bounded by peaks in ATIs. Hayek and Wilson (2013) presented also two conditioned-onboundary indices (CoBI). These assess which species contributed most

to the ATI at the PATI boundaries, CoBI providing the proportion that each species within an assemblage contributed to the change or turnover specifically across each PATI boundary. For each species at any PATI boundary, CoBI ¼ jpi2 –pi1 j=ATI

ð2Þ

where pij, j = 1, 2 are the ith species proportions on either side of the selected boundary of interest at which the ATI is calculated. There are two forms of CoBI: 1. Partial conditioned-on-boundary index, CoBIp, in which the ATI was calculated between the entire set of samples within the PATI below the ATI peak and the first sample immediately above the peak. In this case the ATI, designated ATIp, was substituted into Eq. (2), as were pi1, the proportional abundance of the ith species in the entire PATI below the peak in ATIs, and pi2, the proportional abundance of that ith species in the first sample above the ATIs peak. The proportional contribution of the change in each species to ATIp is given by the vector of CoBIp values at each ATIs peak. 2. Thorough conditioned-on-boundary index CoBIt, for which the ATI, denoted by ATIt, was calculated between the values in two complete PATIs separated by the peak in ATIs. For this, ATIt is substituted into Eq. (2). The values of pi1 and pi2 in this instance denote the proportional abundance of the ith species across the whole of each of the two PATIs separated by the peak in ATIs. The vector of CoBIt at each ATIs peak shows the proportional contribution of each species' turnover or change to the value of ATIt. SHE analysis for biozone identification (SHEBI — see Buzas and Hayek, 1998) was conducted from the base of the piston core BGT086 upwards as this follows the direction of temporal cause and effect (Wilson, 2008b). Cut-points between abundance biozones (ABs) were determined using a spreadsheet and the ABs numbered from the core base upwards. Cluster analyses on the raw data were conducted using several indices. Each returned the same AB's. For the sake of brevity, further details of this clustering are not provided. The behaviour of cumulative H was investigated across each AB. ATIp, CoBIp, ATIt and CoBIt were investigated across each AB boundary.

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4. Results 4.1. Piston core BGT086 A total of 10,415 benthic foraminifera were obtained from this core (x = 297 per sample, σ = 20.2 specimens; online supplementary data file 1). These were placed in 152 species, of which 31 were either left in open nomenclature or merely compared with previously named species. Thirty species (19.7%) were singletons, while 61

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(40.1%) were represented by b4 specimens. Thus, a large proportion of the species were rare. The total assemblage was dominated by C. pachyderma (26.3%) (= C. pseudoungerianus sensu Hofker, 1983; see Holbourn et al., 2013) with lesser B. aculeata (8.8%), S. bulloides (6.1%), Osangularielloides rugosa (4.8%), S. schlumbergeri (4.0%), G. subglobosa (3.8%), Uvigerina hispidocostata Cushman & Todd (3.7%), Bulimina alazanensis (3.1%) and Cassidulina curvata Phleger & Parker (3.0%). Thus, the nine most abundant species formed ~ 64% of the total assemblage. Qualitative examination of percentage

Fig. 2. The percentage abundances of selected species in Core BGT086. A. Brizalina albatrossi. B. Bulimina aculeata. C. Bulimina alazanensis. D. Cassidulina curvata. E. Cibicidoides pachyderma. F. Epistominella exigua. G. Globocassidulina subglobosa. H. Osangularielloides rugosa. I. Sigmoilopsis schlumbergeri. J. Sphaeroidina bulloides. K. Uvigerina ex. gr. auberiana. L. Uvigerina hispidocostata. The horizontal dashed line indicates the position of the boundary between PATIs-3/4 and abundance biozones 2/3, for explanation of which see Fig. 3. Note differences in horizontal scales.

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abundances of these species (Fig. 2) suggests that they fall into three groups: 1. Species most abundant towards the base of the core and decreasing in abundance abruptly (C. pachyderma, C. curvata; Fig. 2) 2. Species most abundant towards the top of the core and increasing in abundance abruptly (B. alazanesis, O. rugosa, U. hispidocostata) 3. Species increasing gradually in abundance through the core (B. aculeata, S. bulloides). Epistominella exigua (Brady), although never forming N4% of any sample, is abundant only at 194 cmbsf and above and so belongs in Group 2 (Fig. 2F). G. subglobosa (Fig. 2G) also belongs in this group. The between-sample assemblage turnover index ranged from ATIs = 0.27–1.48 (x = 0.61, σ = 0.21) (Fig. 3A). In only three places was ATIs N x + σ. This was used to delimit four PATIs between: 1. 2. 3. 4.

0–194 cmbsf, 20 samples (PATI-1), 194–234 cmbsf, 1 sample (PATI-2), 234–280 cmbsf, 5 samples (PATI-3), 280–384 cmbsf, 9 samples (PATI-4).

The boundary between PATI-1 and PATI-2 encompasses a 20 cm gap from which samples were retained for use elsewhere. The boundary between these PATIs is here drawn at 214 cmbsf, at the midpoint of this gap. A second gap, between 294 and 320 cmbsf, lay within PATI-4. ANOVA and post hoc Tukey–Cramer's Q were used to compare mean ATIs for the three PATIs containing ≥ 2 samples. This showed that mean ATIs did not differ between these PATIs (F = 0.699, p = 0.51, df = 2). Species richness per sample ranged from S = 22–61. Species richness rarefied to 200 specimens per sample (S200) ranged from 17.9– 49.2 (at 220–224 cmbsf and 10–14 cmbsf respectively). S200 is higher towards the top of the piston core (Fig. 3B). This pattern was investigated further across those PATIs with ≥2 samples using ANOVA, post hoc Tukey–Cramer's Q and ln(S200 + 1). ANOVA showed that ln(S200 + 1) differed between at least two PATIs (F = 22.97, p b 0.0001). Tukey– Cramer's Q was not necessary to show that mean S200 did not differ between PATI-3 and PATI-4, which both had a mean of 28.2 species, but this test showed that that mean S200 in PATI-1 was significantly different (mean S200 = 39.3). Since S200 in PATI-2 was less than the minimum in either PATI-3 or 4, S200 did not increase linearly upwards through the core, but changed in a step across PATI-2. The information function showed the same pattern as S200, varying little above 194 cmbsf. This increase in diversity is accompanied by a decrease in dominance, as shown by low levels of max(pi) above 154 cmbsf.

The partial assemblage turnover index across the three PATI boundaries ranged from ATIp = 0.564–1.481 (i.e., the assemblage turnover was 56.4%–148.1%) across the PATI-4/3 and PATI-2/1 boundaries respectively (Table 1). The number of species contributing to ATIp (i.e., for which CoBIp N 0) differed between the three peaks at the PATI boundaries (PATI-4/3 = 77 species, PATI-3/2 = 70 species, PATI-2/ 1 = 53 species). However, only 29 species had a CoBIp N 0.01 (i.e., contributed N 1.0% to ATIp) across any one boundary, while only seven species had a COBI p N 0.05 across any PATI boundary (Brizalina subaenariensis mexicana, B. aculeata, B. marginata, C. curvata, C. pachyderma, O. rugosa, U. hispidocostata). Of these, B. subaenariensis mexicana and B marginata had a CoBIp N 0.05 across one boundary only, reflecting their more common occurrence towards the bottom of the core. Single values of CoBIp N 0.05 across one boundary only for O. rugosa and U. hispidocostata reflected their more common occurrence towards the core top. Only B. aculeata and C. pachyderma presented COBIp N 0.05 across all the PATI boundaries, reflecting a significant negative correlation between the proportional abundances of these species (r = −0.58, p b 0.0001) as B. aculeata replaces C. pachyderma as the dominant species towards the top of the core. The thorough assemblage turnover index across the three PATI boundaries ranged from ATIt = 0.418–1.560 (i.e., the assemblage turnover was 41.8%–156.0%) across the PATI-4/3 and PATI-2/1 boundaries respectively (Table 2). The number of species contributing to ATIt differed between the three PATI boundaries (PATI-4/3 = 87 species, PATI-3/2 = 70 species, PATI-2/1 = 145 species). However, only 24 species had a COBIt N 0.01 across any one boundary, while only five species had a COBIt N 0.05 across any PATI boundary (B. subaenariensis mexicana, B. aculeata, C. curvata, C. pachyderma, U. dirupta). Only C. pachyderma had a COBIt N 0.05 across all three boundaries, while B. aculeata had a COBIt N 0.05 across the uppermost two boundaries. SHE analysis for biozone identification (SHEBI) indicated the presence of four abundance biozones (ABs): 1. 2. 3. 4.

AB1, 330–384 cmbsf, six samples AB2, 220–324 cmbsf, nine samples AB3, 130–194 cmbsf, seven samples AB4, 0–124 cmbsf, thirteen samples.

Of these, the AB2/3 boundary coincided with a 20 cm gap from which samples were retained for use elsewhere. The boundary between these two ABs is drawn at 214 cmbsf, at the midpoint of the gap. A second gap, between 294 and 320 cmbsf, lay within AB2. Although four ABs separated by three AB boundaries were recognised, only one of these boundaries was common to both ATI and SHE analysis (between PATI-

Fig. 3. Statistical measures in Core BGT086. A. Sample-wise assemblage turnover index ATIs. The vertical dashed line indicates a value of mean ATIs plus one standard deviation; horizontal dashed lines mark PATI boundaries. Data points are placed at the midpoint between adjacent samples. B. Rarefied species richness for 200 specimens, S200. Horizontal lines mark the position of abundance biozone boundaries from SHE analysis. C. The information function H. D. Dominance, measured using the max(pi) index.

B. Wilson, L.-A.C. Hayek / Marine Micropaleontology 121 (2015) 85–96 Table 1 Partial assemblage turnover index (ATIp) and partial conditioned-on-boundary indices (CoBIp) for PATI boundaries detected in Core BGT086. Only values of CoBIp exceeding 0.01 are shown. Values of CoBIp in bold exceed 0.05. PATI boundary

3/4

2/3

1/2

ATIp CoBIp Brizalina albatrossi Brizalina barbata Brizalina subaenariensis mexicana Brizalina translucens Bulimina aculeata Bulimina alazanensis Bulimina marginata Bulimina striata mexicana Cassidulina curvata Cassidulina laevigata Cassidulina norcrossi australis Cibicidoides bradyi Cibicidoides pachyderma Fissurina marginata Globocassidulina subglobosa Globocassidulina subglobosa subcalifornica Lenticulina norvangilae Martinottiella communis Nummoloculina irregularis Oridorsalis umbonatus Osangularielloides rugosa Pullenia bulloides Quinqueloculina cruziana Sigmoilopsis schlumbergeri Sphaeroidina bulloides Uvigerina dirupta Uvigerina ex gr. auberiana Uvigerina hispidocostata Uvigerina mediterranea

0.564

0.634

1.481

– 0.014 0.074 0.010 0.079 – 0.080 – 0.072 – 0.011 – 0.332 0.014 – – 0.011 0.018 0.011 0.015 – – – 0.013 0.018 0.023 – – –

– – – – 0.058 – 0.048 0.046 0.057 – – – 0.414 – 0.015 – – – 0.010 0.020 0.016 – 0.038 0.032 0.030 – – 0.023 0.020

0.046 – – – 0.061 0.037 0.011 – – 0.013 – 0.013 0.420 – 0.013 0.011 – – – – 0.057 0.015 0.018 0.028 0.032 0.033 0.015 0.068 –

3/4 and AB2/3). Cumulative H increased across AB1, 3 and 4, whereas for AB2 the slope on the equation for H vs. lnN is negative but not significant. The partial assemblage turnover index for AB boundaries from SHEBI ranged from ATIp = 0.79–0.93 (AB1/2 and AB2/3 boundaries Table 2 Thorough assemblage turnover index (ATIt) and thorough conditioned-on-boundary indices (CoBIt) for PATI boundaries detected in Core BGT086. Only values of CoBIt exceeding 0.01 are shown. Values of CoBIt in bold exceed 0.05. PATI boundary

3/4

2/3

1/2

ATIt COBIt Brizalina albatrossi Brizalina barbata Brizalina simplex Brizalina subaenariensis mexicana Brizalina translucens Bulimina aculeata Bulimina alazanensis Bulimina marginata Bulimina striata mexicana Cassidulina curvata Cassidulina laevigata Cassidulina norcrossi australis Cibicidoides pachyderma Globocassidulina subglobosa subcalifornica Globocassidulina subglobosa Nummoloculina irregularis Oridorsalis umbonatus Osangularielloides rugosa Quinqueloculina cruziana Sigmoilopsis schlumbergeri Sphaeroidina bulloides Uvigerina dirupta Uvigerina hispidocostata Uvigerina mediterranea

0.418035

0.633964

1.55967

– 0.016 0.012 0.099 0.014 0.041 – 0.033 0.013 0.081 – 0.014 0.226 – 0.016 0.014 0.013 0.024 – 0.043 0.029 0.058 0.030 0.026

– – – – – 0.058 – 0.048 0.046 0.057 – – 0.414 – 0.015 0.010 0.020 0.016 0.038 0.032 0.030 – 0.023 0.020

0.021 – – – – 0.057 0.031 0.010 – – 0.020 – 0.427 0.030 0.037 – – 0.048 0.017 0.036 0.021 0.030 0.038 –

91

Table 3 Partial assemblage turnover index (ATIp) and partial conditioned-on-boundary indices (CoBIp) for abundance biozone boundaries from SHE analysis for biozone identification detected in Core BGT086. Only values of CoBIp exceeding 0.01 are shown. Values of CoBIp in bold exceed 0.05. Abundance biozone boundary

1/2

2/3

3/4

ATIp CoBIp Brizalina albatrossi Brizalina barbata Brizalina subaenariensis mexicana Brizalina translucens Bulimina aculeata Bulimina marginata Bulimina striata mexicana Cassidulina curvata Cibicidoides pachyderma Dentalina communis Fissurina marginata Globocassidulina subglobosa Lenticulina calcar Lenticulina gibba Lenticulina norvangilae Lenticulina rotulata Nonionella labradorica Nummoloculina irregularis Oridorsalis umbonatus Osangularielloides rugosa Sigmoilopsis schlumbergeri Sphaeroidina bulloides Uvigerina dirupta

0.787

0.934

0.901

0.011 – – – 0.080 – 0.070 0.041 0.569 0.015 0.012 0.011 – – 0.019 – – 0.014 – – – 0.046 –

– – – – 0.048 0.010 0.064 0.095 0.550 – 0.014 – 0.014 – – 0.021 – 0.011 0.023 – – 0.044 0.034

0.011 0.052 0.120 0.034 0.034 0.130 0.090 0.038 0.122 – – – – 0.011 – – 0.030 – – 0.011 0.013 0.013 0.177

respectively; Table 3). Forty-six, 43 and 51 species contributed to ATIp across the AB boundaries 1/2, 2/3 and 3/4 respectively. The mean number of species contributing to ATIp across the AB and PATI boundaries was compared using Student's t-test and the transformation ln(S + 1). The result was statistically significant (mean across AB boundaries = 46.7 species, mean across PATI boundaries = 66.7 species; t = 2.83, p = 0.047, df = 4). The non-parametric test of means likewise was significant. Twenty three species had a COBIp N 0.01 across any one AB boundary, while 13 had a COBIp N 0.01 across one AB boundary only. Six species had a COBIp N 0.05 across any AB boundary (Brizalina barbata, B. aculeata, B. marginata, B. striata mexicana, C. curvata, C. pachyderma). Only B. aculeata presented COBIp N 0.05 across all three AB boundaries, while C. pachyderma had COBIp N 0.05 across the lower two AB boundaries only. This reflects the significant negative correlation between the proportional abundances of B. aculeata and C. pachyderma. The ATIt across the three AB boundaries ranged from 0.48 to 1.01 across the AB1/2 and AB2/3 boundaries respectively (Table 4). The number of species contributing to ATIt differed over the three AB boundaries (AB1/2 = 88 species, AB2/3 = 110 species, AB3/4 = 143 species). The mean number of species contributing to ATIp across the AB and PATI boundaries was compared in the same manner as for ATIt. The result was not statistically different (mean across AB boundaries = 113.7 species, mean across PATI boundaries = 100.7 species; t = 0.58, p = 0.593, df = 4). Thirty species had a COBIt N 0.01 across any one boundary, including E. exigua, which presented COBIt N 0.01 across both the AB2/3 and AB3/4 boundaries. This reflects the occurrence of this species in the upper part of the core. Ten species had a COBIt N 0.05 across any one PATI boundary (B. subaenariensis mexicana, B. aculeata, B. marginata, C. curvata, C. pachyderma, G. subglobosa, O. rugosa, S. schlumbergeri, Uvigerina dirupta and U. hispidocostata). Only C. pachyderma had a COBIt N 0.05 across all three AB boundaries, while B. aculeata had a COBIt N 0.05 across the lowermost two boundaries. 4.2. Piston core BGT096 This piston core yielded 10,644 benthic foraminifera (x = 273 per sample, σ = 61.7 specimens). These were placed in 147 species, of

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Table 4 Thorough assemblage turnover index (ATIt) and thorough conditioned-on-boundary indices (CoBIt) for abundance biozone boundaries from SHE analysis for biozone identification detected in Core BGT086. Only values of CoBIt exceeding 0.01 are shown. Values of CoBIt in bold exceed 0.05. Brizalina albatrossi Brizalina barbata Brizalina subaenariensis mexicana Bulimina aculeata Bulimina alazanensis Bulimina marginata Bulimina striata mexicana Cassidulina curvata Cassidulina laevigata Cibicides sp. Cibicidoides bradyi Cibicidoides mollis Cibicidoides pachyderma Epistominella exigua Eponides regularis Gavelinopsis praegeri Globocassidulina subglobosa subcalifornica Globocassidulina subglobosa Lenticulina calcar Martinottiella pallida Nummoloculina irregularis Oridorsalis umbonatus Osangularielloides rugosa Pseudononion atlanticum Pullenia bulloides Sigmoilopsis schlumbergeri Sphaeroidina bulloides Uvigerina dirupta Uvigerina hispidocostata Uvigerina mediterranea

– 0.016 0.068 0.067 – 0.075 0.024 0.084 – – – – 0.191 – 0.011 – – – 0.014 – 0.021 0.014 0.016 0.010 – 0.028 – 0.120 0.016 0.017

0.031 – 0.027 0.055 0.047 0.019 0.026 0.027 0.025 – – 0.012 0.251 0.011 – – 0.029 0.025 – – – – 0.063 – 0.011 0.021 0.020 0.084 0.055 –

0.015 – 0.012 – – – 0.016 0.016 – 0.014 0.012 0.013 0.295 0.016 – 0.021 0.043 0.103 – 0.019 – – 0.021 – – 0.078 – – 0.018 –

which 26 were either left in open nomenclature or merely compared with previously named species. Thirty five species (23.8%) were singletons, while 58 (39.5%) were represented by b 4 specimens. Thus, a large proportion of the species were rare. The total assemblage was dominated by C. pachyderma (25.2%) with lesser B. aculeata (13.0%), Eponides regularis (6.7%), U. hispidocostata (5.9%), C. curvata (4.5%), S. bulloides (4.2%), S. schlumbergeri (3.6%) and Bulimina striata mexicana (3.2%). For distributions, see Fig. 4. These eight most abundant species together formed 66.2% of the total assemblage. Of these, C. pachyderma, S. bulloides, C, curvata, U. hispidocostata and S. schlumbergeri formed N3% of the total assemblage from core BGT086. E. regularis Phleger and Parker formed up to 67% of samples below 350 cmbsf in Core BGT096 but only 0.7% of the total assemblage from Core BGT086. It was closely associated with Cassidulina laevigata d'Orbigny (1.9% of recovery from Core BGT086), which comprised 2.4% of recovery from Core BGT196, but formed up to 12% of recovery from samples below 280 cmbsf. Some species distributions in core BGT096 are comparable to those in core BGT086. B. aculeata ranged though the biofacies from which the slumping below was derived, forming a mean of 7.5% of each sample below 280 cmbsf. In the in situ material above 274 cmbsf this species formed a mean of 15.6% of each sample (Fig. 4). In contrast, B. alazanensis was found primarily above 244 cmbsf, which suggests that this might correlate with 190 cmbsf in core BGT086. C. pachyderma was rare in the slumped interval below 290 cmbsf, increased abruptly in abundance between 290 and 280 cmbsf and thereafter decreased in abundance to the top of the core. It lacked the marked, step-like decrease in abundance found between 150 and 164 cmbsf in core BGT086. S. schlumbergeri was rare in the slumped section but increased gradually in abundance through the in situ material, and U. hispidocostata showed a comparable distribution. E. exigua never formed N 1.4% of any sample, but was mostly concentrated towards the core top, between 20 and 140 cmbsf. COBIt N 0.01 across the AB2/3 and AB3/4 boundaries showed this species in core BGT086 to be abundant in the upper part of that core.

5. Discussion CHIRP profiles showed that core BGT086 contained in situ sediment throughout while core BGT096 contained some slumped material near the core base. Some conjecture can be made regarding the age and origin of the slumped material. The difference in the abundant (N3%) species between the two cores is at least in part due to the slumping below ~ 3 mbsf in Core BGT096, which brought in material from a different biofacies. A zone of carbonate-rich, iron-stained sand of early Holocene age along the shelf edge east of Trinidad (60–70 m water depth) marks the low sea level of the late Quaternary (Carr-Brown, 1972). The Holocene on the shelf is, however, typically only ~0.05–1.6 m thick, most Holocene sediment having been sequestered in the delta itself. This carbonate-rich zone can be followed SE into northern Brazil, where similar sediment is found off the River Amazon at depths of N70 m (Vital, 2014). Off Trinidad it is rich in Amphistegina gibbosa d'Orbigny (Drooger and Kaasschieter, 1958; Wilson, 2010), and it might be expected that slumped material derived from it would contain specimens of this species. No such specimens were found in the present study. This may be because the zone had not developed at the time of deposition of the slumped material in BGT096. The allochthonous material in BGT096 contained abundant E. regularis, which was rare elsewhere in both cores and had evidently brought sediment downslope from another biofacies to the BGT096 site (water depth 700 m). Drooger and Kaasschieter (1958) recorded abundant E. regularis down to 150 m off Trinidad, while Hofker (1983) did not find it in samples from water deeper than 400 m off Surinam and Guyana. Poag (1981) reported that E. regularis is in the Gulf of Mexico “most frequently recorded in moderate to low numbers on [the] outer shelf and upper to middle slope.” Thus the displaced material in BGT096 was derived from a mass transport complex of pre-Holocene age of outer shelf to upper slope origin. It is not clear, however, if it is of type 2 (upper slope-attached systems due to earthquakes or gas-hydrate disruptions) or 3 (locally detached systems formed by local seafloor instabilities) of the classification of mass transport complexes presented by Moscardelli and Wood (2008). When this displaced sediment is excluded, both cores show a transition from an early low-diversity, C. pachyderma-dominated community with subdominant C. curvata to a later high-diversity, low-dominance B. alazanensis–O. rugosa–E. exigua community. Galluzzo et al. (1990) suggested a B. aculeata–B. alazanensis association in the Grenada Basin to be indicative of low dissolved oxygen levels. Wilson and Costelloe (2011) found species of Uvigerina and Bulimina, indicative of a depressed dissolved-oxygen content (Kaiho, 1994), to be abundant below the Orinoco Plume on the Aves Ridge of the central Eastern Caribbean Sea, where C. pachyderma was rare. Large Cibicidoides are indicative of oxic bottom waters (Kaiho, 1994). Wilson (2013) found abundant C. pachyderma in the bathyal upper Quaternary of the Santaren Channel, off the western Bahama Platform, in association with abundant G. subglobosa and Planulina ariminensis, which he concluded to reflect a low flux of organic carbon. This suggests that the change in fauna in both core BGT086 (which was detected by SHE analysis, cluster analysis and ATI) and core BGT096 marked a change from a lower to a higher organic carbon flux, which in turn depressed dissolved oxygen levels. There are two possible causes for the change in the organic flux. In the SE Caribbean region and adjacent tropical Atlantic Ocean, lowproductivity waters occur in the open ocean while the nearshore, hyperpycnal Orinoco plume consists of high-productivity water. The increase in flux at the sampled sites may reflect eastward progradation of the Orinoco Delta and Plataforma Deltana following the end Pleistocene transgression. The transgression would have initially moved the locus of deposition westwards, but subsequent progradation might have brought organic-rich prodelta clays to the site of deposition. Alternatively, the northward shift of the summertime ITCZ starting ~ 600 yr after the end of the Younger Dryas (YD) cold interval at ca. 10.8 ka (Hoffmann et al., 2014) would have increased the volume of rainy

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Fig. 4. The percentage abundances of selected species in Core BGT096. A. Brizalina albatrossi. B. Bulimina aculeata. C. Bulimina alazanensis. D. Cassidulina curvata. E. Cibicidoides pachyderma. F. Epistominella exigua. G. Eponides regularis. H. Globocassidulina subglobosa. I. Osangularielloides rugosa. J. Sigmoilopsis schlumbergeri. K. Sphaeroidina bulloides. L. Uvigerina hispidocostata. The horizontal dashed line indicates the top of the slumped interval interpreted from the CHIRP profile.

season outflow from the Orinoco, which in turn would have enlarged the area covered by the hyperpycnal Orinoco plume. It is probable, however, that these effects operated in concert, the leading edge of the plume being driven eastwards by both the increase in precipitation and delta progradation. The change in community was abrupt at the more southerly Site BGT086, but gradual at Site BGT096. This might reflect the position of the two sites relative to the plume front. One hypothesis can be proposed, although the precise mechanics connected with it have yet to

be investigated. It is possible that the more southerly Site BGT086 was inundated every year by the Orinoco plume following the northward migration of the ITCZ, but that Site BGT096 was initially only infrequently inundated. As the delta prograded and inundation at Site BGT096 became more regular during the later Holocene, it encouraged a gradual transition to a B. alazanensis–O. rugosa–E. exigua community. The Miocene–Pliocene Uvigerina biofacies of Batjes (1968) shows slight similarity to the B. alazanensis–O. rugosa–E. exigua community of this study, containing B. subaenariensis, Bulimina inflata, Cassidulina cf.

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laevigata and some Lenticulina spp. However, Batjes (1968) did not record O. rugosa or E. exigua, nor did he record any biofacies comparable to the C. pachyderma community recorded from the present study. Wilson (2008a) using ostracods found a front between Orinoco and Amazon water within the Orinoco plume that was not parallel to bathymetric contours. It seems probable that the front at the plume's leading edge similarly does not parallel bathymetric contours. If this is so, then the change in fauna at Sites BGT086 and BGT096 cannot be used as a palaeobathymetric marker. This would require some revision of the manner in which micropalaentologists in the SE Caribbean region use benthic foraminifera to suggest palaeodepths, the change in the fauna not reflecting changes in palaeodepth alone, but incorporating also the migration of the Orinoco plume. The change from the C. pachyderma community to the B. alazanensis– O. rugosa–E. exigua community is at Site BGT086 detected by both the sample-wise assemblage turnover index (ATIs), SHE analysis for biozone identification (SHEBI) and various cluster analyses. However, these techniques detected other boundaries that require some explanation. The arrival of the ITCZ post-dated the end of the Younger Dryas (YD, 11,500 yr BP) by 600 years. Hoffmann et al. (2014) found that depositional rates on the Orinoco fan range from 13 to 42 cm yr−1. This would equate to 7.8–25.2 cm of deposition at Site BGT086 between the end of the YD and the later arrival of the ITCZ, here postulated to be between 194 and 220 cmbsf. SHEBI did not detect a further AB boundary until 324–330 cmbsf, over 1 m below the change in community. ATIs, however, detected a turnover boundary between 224 and 230 cmbsf, and it is possible that this equates to the end of the YD. The position of the beginning of the Younger Dryas might be determined using nanodiamonds produced by the cosmic-impact event at the onset of the Younger Dryas (≈12,800 ± 150 years BP; Kinzie et al., 2014). However, this was outside the scope of the present project. The causes of the other PATI and AB boundaries in core BGT086, which indicate points of marked faunal turnover, have to be determined, but it is possible that these relate to sea level change. Fairbanks (1989) showed on the basis of downed coral reefs off Barbados, ~200 km N of Trinidad, that sea level was 121 ± 5 m below present during the last glacial maximum. Sea level rise probably began at 23.0 ± 0.6 ka, coincident with melting of the southern Laurentide ice-sheet, which is marked by retreat from last glacial maximum moraines in Wisconsin, USA (Ullman et al., 2015). The sunken coral reefs around Barbados show that deglacial sea level rise was marked by three intervals of especially rapid rise called meltwater pulses (MWP — Blanchon, 2011). Between ~17,100 and 14,000 yr BP, sea level increased by 20 m. This first phase of deglaciation was followed by a rise of 24 m in b 1000 years (MWP-1A — see fig. 7.1 in Murray-Wallace and Woodroffe, 2014) that corresponds to the latter part of Termination 1a of deep sea records. The rate of sea level rise subsequently slowed, being low at the beginning of the Younger Dryas. The later part of the Younger Dryas is marked by increasing rates of sea level rise, however, that culminated in MWP-1B between 11,500–11,000 yr BP. This is closely tied to Termination 1B in the deep-sea, during which sea level rose by ~28 m. This event has already been suggested to equate to the ATI peak at 224–230 cmbsf. Following a period of comparatively low sea level rise, the rate again increased between 8000 and 7500 yr BP (MWP-1C). This might equate to the AB boundary at 124–130 cm detected by SHEBI. Seasonality of organic matter flux may have played a role in the development of the B. alazanensis–O. rugosa–E. exigua community. E. exigua is characteristic of cold (b 3 °C) and well oxygenated (N3.5 ml/l) water with a low carbon flux to the seafloor (b3 g C m−2 yr−1) (Murgese and De Deckker, 2005). It is also indicative of periodic (seasonal) delivery of phytodetritus to the seafloor (Smart et al., 1994). Being an opportunist that lives within the phytodetrital layer, it is rarely found in the underlying sediment (Murray, 2006). The appearance of E. exigua in the upper part of cores BGT086 and BGT096 may therefore reflect the development of a seasonal phytodetrital rain following the arrival of the Orinoco plume. The nutrient-rich plume at present expands and contracts between the

northern South American rainy and dry seasons (Odriozola et al., 2007, figure 3). Thus, the plume's eastern boundary at times lies east of the core sites, and at other times west of it. This migration of water with a higher primary productivity would induce the seasonality in the phytodetrital flux. The onset of this seasonality is in core BGT086 marked by the first consistent occurrence of E. exigua at 190–194 cmbsf, at the base of PATI-1 and the base of AB3. This is the only boundary common to both ATI and SHEBI. In core BGT096 E. exigua occurs mostly between 20 and 140 cmbsf. That Batjes (1968) did not record E. exigua in his Miocene–Pliocene Uvigerina biofacies assemblage might reflect a lack of seasonality in the outflow from the Orinoco then, the mean position of the ITCZ being considerably north of Trinidad at that time (Van Vliet-Lanoe, 2007). Alternatively, E. exigua might have formed such a small proportion of the Uvigerina biofacial assemblage that Batjes (1968) did not think it worthwhile mentioning in his qualitative study. This seems possible, Wilson and Hayek (2015) having found E. exigua in the Middle Miocene Globorotalia fohsi robusta planktonic foraminiferal Zone (= Zone N12) of the Cipero Formation of Central Trinidad, where it formed up to 2.5% of each sample. The mean ATIs (0.61) in the bathyal core BGT086 did not differ significantly from that (0.58) in the upper Quaternary of the abyssal ODP 1261A of the Demerara Rise (tobserved = 0.544, tcritical = 1.991, df = 77, p = 0.588) (see Wilson and Hayek, 2014b), but was significantly less than that (0.71) for the upper Quaternary in the abyssal ODP Hole 994C of Blake Ridge (tobserved = 2.136, tcritical = 1.985, df = 97, p = 0.035) recorded by Hayek and Wilson (2013). It has yet to be determined, however, if this indicates there is little variation in the mean values of ATIs between bathyal and abyssal sites off northern South America. Graphs of percentage abundances in core BGT086 suggest that the fundamental niche width varies between species, as noted also by Wilson and Hayek (2015). Species that only gradually increase in abundance through the core (B. aculeata, S. bulloides) will have wider fundamental niches than those more abundant towards the core base (C. pachyderma, C. curvata) or top (B. alazanesis, O. rugosa, U. hispidocostata). Galluzzo et al. (1990) suggested that both B. aculeata and B. alazanensis in the Holocene of the Grenada Basin were part of a low-oxygen assemblage. However, B. alazanensis showed more specialist behaviour in the cores BGT086 and BGT096 off eastern Trinidad than did B. aculeata. This is apparently at odds with the suggestion by Ujiié (1990) that B. aculeata was off Japan “characteristically dominant during glacial intervals”. However, Ujiié's (1990) frequency polygon shows that B. aculeata was abundant across Termination II in both glacial and interglacial times. Reiss and Hottinger's (1984) observations on the depth distributions of species of Amphistegina in the neritic Red Sea show that congeneric species occupy different niches. The present study shows that congeneric bathyal species such as B. aculeata and B. alazanensis may also occupy fundamental niches of markedly different dimensions. Some evidence suggests that the amount of flux alone cannot be invoked as the sole factor determining the change in fauna through core BGT086. S. bulloides occupies primarily areas with an organic carbon flux of 4–9 g m− 2 yr− 1, while B. striata mexicana and C. pachyderma (as C. pseudoungerianus), both shown by CoBIt for abundance biozone boundaries to be more abundant towards the core base, occupy primarily areas with a flux of 3–8 g m− 2 yr− 1 and 5–12 g m− 2 yr− 1 (Altenbach et al., 1999). C. laevigata, concentrated towards the core top, primarily occupies areas with an organic carbon flux of 6–20 g m − 2 yr − 1, while Uvigerina ex gr. auberiana, showing a similar distribution in the core, prefers areas with a flux of 4–9 g m − 2 yr − 1. These flux rates show considerable overlap. It thus is possible that seasonality of input plays a role in the fundamental niche width of species other than E. exigua, although other factors such as the degree of degradation of the organic matter or nitrate availability might also play a part.

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Deshmukh (1986, p. 181) pondered whether communities, as objective realities formed from discrete collections of species populations, occurred such that certain groups of species populations, which differ from adjacent communities, would recur in similar environments. Whittaker (1975) suggested that there are two end-member distributions of populations of different species along an environmental gradient. In the first, communities are clearly defined by groups of populations having more or less coincident distributions. In the second, each population is independently distributed along the gradient such that no clearly defined communities are formed. In core BGT086, there are three groups of species: (1) those common towards the core base (C. pachyderma, C. curvata); (2) those abundant towards the core top (B. alazanesis, O. rugosa, U. hispidocostata); and (3) those present throughout (B. aculeata, S. bulloides). Groups (1) and (2) conform to Whittaker's (1975) suggested discrete communities, while Group (3) is reminiscent of his suggested independent populations. Such a distribution is not unprecedented. Randall (1970) examined the distribution of plants along a beach transect in Barbados. He found some groups of species to have similar distributions while others were independent. Those species showing greater independence, which are comparable to the third group of species recorded here, are those deemed to be generalists. 6. Conclusions Outflow from the Orinoco River currently forms a hyperpycnal, lowsalinity plume with high primary productivity that abuts sharply against low-productivity water of the western tropical Atlantic Ocean. This forms an extensive offshore front that currently lies east of Trinidad and crosses the Columbus Basin. However, the precise position of the front varies between the northern South American rainy and dry seasons, which change the volume of Orinoco outflow. Bathyal benthic foraminifera suggest that the position of the front varied through at least the late Quaternary, when the Orinoco delta was displaced westward during the transgressions accompanying glacial terminations, and then prograded eastward during highstands and ensuing regressions. The progradation after the end-Pleistocene was accompanied by a northward shift in the mean position of the intercontinental convergence zone (ITCZ), which would have increased Orinoco rainy-season outflow and the areal extent of the Orinoco Plume. This northward shift occurred about 600 yr after the end of the Younger Dryas. Transgression induced the development of a low-productivity fauna dominated by C. pachyderma with lesser C. curvata. In core BGT086 this is abruptly replaced by a high-productivity B. alazanensis–O. rugosa– E. exigua assemblage, the change in fauna being detected by both sample-wise assemblage turnover index (ATIs), SHE analysis and cluster analysis. The presence of E. exigua in the latter assemblage but not the former one indicates a change from a constant to a pulsed organic matter flux. The transition between the two assemblages is here ascribed to the arrival of the Orinoco plume following the abrupt migration of the ITCZ, coupled with eastward progradation of the Orinoco delta. ATIs detected a boundary in core BGT086 between PATI-3/4 that may equate to the end of the Younger Dryas and reflect the change in sea level at that time. The causes of other peaks in ATIs and of abundance biozone boundaries from SHE Analysis have yet to be determined and it is not yet clear if these other events were regional ones suitable for correlation between cores. While some species changed in proportional abundance markedly between the C. pachyderma–C. curvata and B. alazanensis– O. rugosa–E. exigua assemblages, others (B. aculeata, S. bulloides) did not. This is concluded to reflect differences in the species' fundamental niche widths. This indicates that the cogeneric B. alazanensis and B. aculeata have different niche widths, the former being a specialist species and the latter a generalist. Those using foraminifera to reconstruct the evolution of the Orinoco deep-sea fan will need to take into account not only the impact of the offshore estuarine front on faunal assemblages, but also the discordant relationships between the front at

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the sea surface and palaeobathymetric contours. Those working off other major rivers might find similar phenomena. Acknowledgements Thanks are due to BG (Trinidad and Tobago) for sample donation and to Ron Daniel for aid in interpreting the CHIRP profiles. BW would like to thank the Ministry of Energy and Energy Affairs of Trinidad and Tobago for financial support for this project, and Andrew Jupiter in particular for helping arrange this support. Constructive reviews by Ian Boomer and Flavia Fiorini are gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.marmicro.2015.11.004. References Agard, J.B.R., Gobin, J.F., 2000. The Lesser Antilles, Trinidad and Tobago. In: Sheppard, C. (Ed.)Seas at the Millennium: An Environmental Evaluation. Elsevier Science Limited, pp. 627–641. Altenbach, A.V., Pflaumann, U., Schiebel, R., Thies, A., Timm, S., Trauth, M., 1999. Scaling percentages and distributional patterns of benthic foraminifera with flux rates of organic carbon. J. Foraminifer. Res. 29, 173–185. Arbuszewski, J.A., Cléroux, C., Bradtmiller, L., Mix, A., 2013. Meridional shifts of the Atlantic intertropical convergence zone since the Last Glacial Maximum. Nat. Geosci. 6, 959–962. Aslan, A., White, W.A., Warne, A.G., Guevara, E.H., 2003. Holocene evolution of the western Orinoco Delta, Venezuela. Geol. Soc. Am. Bull. 115, 479–498. Batjes, D.A.J., 1968. Palaeoecology of foraminiferal assemblages in the late Miocene cruse and forest formations of Trinidad. Antilles, Fourth Caribbean Geological Conference, 1965, Trinidad, pp. 141–156. Berger, W.H., Parker, F.L., 1970. Diversity of planktonic foraminifera in deep-sea sediments. Science 168, 1345–1347. Blanchon, P., 2011. Meltwater pulses. In: Hopley, D. (Ed.)Encyclopaedia of Modern Coral Reefs. Springer, pp. 683–690. Buzas, M.A., Hayek, L.-A.C., 1998. SHE analysis for biofacies identification. J. Foraminifer. Res. 28, 233–239. Callec, Y., Deville, E., Desaubliaux, G., Griboulard, R., Huyghe, P., Mascle, A., Mascle, G., Noble, M., Padron de Carillo, C., Schmitz, J., 2010. The Orinoco turbidite system: tectonic controls on sea-floor morphology and sedimentation. AAPG Bull. 94, 869–887. Carr-Brown, B., 1972. The Holocene/Pleistocene contact in the offshore area east of Galeota Point, Trinidad, West Indies. VI Conferencia Geologica Del Caribe, Margarita, Venezuela, pp. 381–397. Carr-Brown, B., 2007. The contribution of Trinidad micropaleontology to global E&P. 100 Years of Petroleum in Trinidad and Tobago: Celebrating a Century of Commercial Oil Production. Government of Trinidad and Tobago, Port of Spain, Trinidad, pp. 158–167. Chen, S., Steel, R.J., Dixon, J.F., Osman, A., 2014. Facies and architecture of a tidedominated segment of the Late Pliocene Orinoco Delta (Morne L'Enfer Formation) SW Trinidad. Mar. Pet. Geol. 57, 208–232. Cheng, H., Edwards, R.L., Broecker, W.S., Denton, G.H., Kong, X., Wang, Y., Zhang, R., Wang, X., 2009. Ice age terminations. Science 326, 248–252. Chin-Leo, G., Benner, R., 1992. Enhanced bacterioplankton production and respiration at intermediate salinities in the Mississippi River plume. Mar. Ecol. Prog. Ser. 87, 87–103. Culver, S.J., 1988. New foraminiferal depth zonation of the northwestern Gulf of Mexico. PALAIOS 3, 69–85. Del Castillo, C.E., Coble, P.G., Morell, J.M., López, J.M., Corredor, J.E., 1999. Analysis of the optical properties of the Orinoco River plume by absorption and fluorescence spectroscopy. Mar. Chem. 66, 35–51. Deshmukh, I., 1986. Ecology and Tropical Biology. Blackwell Scientific Publications, Palo Alto, California (387 pp.). Drooger, C.W., Kaasschieter, J.P., 1958. Foraminifera of the Orinoco–Trinidad–Paria Shelf. Report of the Orinoco Shelf Expedition, Verhandlungen Koninklijk Nederland Akademie Wetenschappelijke 4, pp. 1–108. Fairbanks, R.G., 1989. A 17,000 year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation. Nature 342, 637–642. Figueroa, D.F., 2007. Variation of Planktonic Community Structure Along the Orinoco River Plume MSc Thesis Department of Marine Sciences. University of Puerto Rico, Mayaguez, p. 29. Foster, G.L., Sexton, P.F., 2014. Enhanced carbon dioxide outgassing from the eastern equatorial Atlantic during the last glacial. Geology 42, 1003–1006. Furlonge, H., Kaiser, M., 2010. Overview of natural gas sector developments in Trinidad and Tobago. Int. J. Energy Sect. Manage. 4, 535–554. Gaby, M.L., Sen Gupta, B.K., 1985. Late Quaternary benthic foraminifera of the Venezuela Basin. Mar. Geol. 68, 125–144. Galluzzo, J.J., Sen Gupta, B.K., Pujos, M., 1990. Holocene deep-sea foraminifera of the Grenada Basin. J. Foraminifer. Res. 20, 195–211.

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