Paul Loubere

Deep‐Sea Benthic Foraminiferal Assemblage Response to a Surface Ocean Productivity Gradient: A Test

A transect of samples from the eastern equatorial Pacific, where surface ocean productivity is the only changing environmental variable, was used to test the response of benthic foraminiferal assemblages to an open-ocean productivity gradient. The transect runs along the East Pacific Rise from the highly productive waters near the equator to the oligotrophic South Pacific subtropical gyre. Surface sediment benthic assemblages show progressive change along the transect with strong compositional shifts corresponding to the rapid productivity gradient near the equator and to the position of the southern convergence, near 18°–19°S. The relationship of the assemblages to surface ocean productivity was tested by statistical regression of quantitative estimates of productivity on the assemblage principal component scores calculated for each sampling location. The regression yielded an r-squared of 0.87, demonstrating a clear relationship between surface ocean productivity and deep-sea benthic foraminiferal assemblage composition. The species most important to this relationship display a consistent response to oceanic productivity for long stretches of geologic time. This conservative behavior and the results of the regression analysis indicate that benthic foraminiferal assemblages can be used to estimate surface ocean productivity for the geologic past.

The surface ocean productivity and bottom water oxygen signals in deep water benthic foraminiferal assemblages

Abstract A previous study of the relative abundances of benthic foraminifera in the deep Pacific revealed that surface ocean productivity and bottom water oxygen signals were strongly imbedded in the foraminiferal data. The present report examines the nature of the species associations that carry the productivity and bottom water oxygen concentration signals. Principal components analysis is used to define species associations independently of the environmental variables. Principal components analysis of the correlation and the covariance matrices of species percents, and of taxon accumulation rate indices, was undertaken. The correlation analysis gives equal weight to all taxa while the covariance analysis weights results to the more abundant species. The accumulation rate index analysis reduces the distorting effects of calculating percents, and matrix closure, on the taxon abundance patterns. Deep water benthic foraminiferal assemblages from the Eastern Pacific Ocean show species distribution patterns dominated by a response to surface ocean productivity. Principal components analysis of either relative abundances or accumulation rate indices reveals a consistent set of species bearing the productivity signal. These include species normally found at depth (e.g. hispid Uvigerina) and taxa more commonly found on continental margins which are associated with infaunal microhabitats. The principal components analysis of both relative abundance and accumulation rate indices also shows that there are no taxa which are exclusively responsive to changing bottom water oxygen concentration. Rather, this factor acts in concert with other environmental variables (productivity in this study) to control benthic assemblage abundance patterns. Using relative abundance data convolves the oxygen concentration signal with the productivity response of the benthic assemblage. However, experimentation with absolute abundance indices suggests that the oxygen signal can be extracted from the assemblage data. This result is consistent with the previous analysis (Loubere, 1994) which yielded an r2 of 0.96 for regression of assemblages against oxygen.

The surface ocean productivity response of deeper water benthic foraminifera in the Atlantic Ocean

We test the relationship of deep sea benthic foraminiferal assemblage composition to the surface ocean productivity gradient in the low latitude Atlantic Ocean using 81 surface sediment samples from a water depth range between 2800 and 3500 m. The samples are selected so that the surface ocean productivity gradient, controlling the flux of organic carbon to the seabed, will be the most important environmental variable. The first two principal components of the assemblage data account for 73% of data variance and are clearly linked to the productivity gradient across the Atlantic. These components show that under higher productivity the assemblages contain a higher abundance of Uvigerina peregrina, Melonis barleeanum, Globobulimina spp. and other taxa with probable infaunal microhabitats. Alabaminella weddellensis, a species linked to episodic phytoplankton debris falls, is also important in these assemblages. As productivity decreases there is a regular shift in assemblage composition so that low productivity assemblages are dominated by Globocassidulina subglobosa and several Cassidulina species along with Epistominella exigua. We hypothesize that these taxa are epifaunal to very shallow infaunal since nearly all organic carbon oxidation occurs near the sediment-water interface in low productivity settings. Discriminant function analysis of the foraminiferal assemblages, with groups selected on the basis of surface ocean productivity, shows clear separation among five productivity levels we used. This analysis demonstrates that productivity variations have a strong influence on assemblage composition. Finally, we used two groups of samples from the Rio-Grande Rise representing water depths from 2007 to 2340 m and 2739 to 3454 m to test for effects produced by changing water depth. All these samples are from a low productivity region and represent nearly identical environmental conditions. Although the low productivity nature of all the Rio-Grande Rise samples is obvious, there are assemblage differences between our depth groups. We cannot account for the assemblage differences with changes in organic carbon flux, dissolution effects or other physical/chemical properties of the ocean. Thus there are as yet unidentified factors related to water depth which cause some assemblage variation in the low productivity setting we investigated.

Quantitative estimation of surface ocean productivity and bottom water oxygen concentration using benthic foraminifera

An electronic supplement of this material may be obtained on adiskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN toAGUs FTP account using ANONYMOUS as the usemame andGUEST as the password. Go to the right directory by typing CDAPEND. Type LS to see what files are available. Type GET and thename of the file to get it. Finally, type EXIT to leave the system.)(Paper 94PA01624, Quantitative estimation of surface oceanproductivity and bottom water concentration using benthicforaminifera, by P. Loubere). Diskette may be ordered from AmericanGeophysical Union, 2000 Florida Avenue, N.W., Washington, DC20009;

Benthic Foraminifera and the flux of organic carbon to the seabed

15.00. Payment must accompany order. Quantitative estimation of surface ocean productivity and bottom water oxygen concentration with benthic foraminifera was attempted using 70 samples from equatorial and North Pacific surface sediments. These samples come from a well defined depth range in the ocean, between 2200 and 3200 m, so that depth related factors do not interfere with the estimation. Samples were selected so that foraminifera were well preserved in the sediments and temperature and salinity were nearly uniform (T = 1.5° C; S = 34.6‰). The sample set was also assembled so as to minimize the correlation often seen between surface ocean productivity and bottom water oxygen values (r² = 0.23 for prediction purposes in this case). This procedure reduced the chances of spurious results due to correlations between the environmental variables. The samples encompass a range of productivities from about 25 to >300 gC m−2 yr−1, and a bottom water oxygen range from 1.8 to 3.5 ml/L. Benthic foraminiferal assemblages were quantified using the >62 µm fraction of the sediments and 46 taxon categories. MANOVA multivariate regression was used to project the faunal matrix onto the two environmental dimensions using published values for productivity and bottom water oxygen to calibrate this operation. The success of this regression was measured with the multivariate r² which was 0.98 for the productivity dimension and 0.96 for the oxygen dimension. These high coefficients indicate that both environmental variables are strongly imbedded in the faunal data matrix. Analysis of the beta regression coefficients shows that the environmental signals are carried by groups of taxa which are consistent with previous work characterizing benthic foraminiferal responses to productivity and bottom water oxygen. The results of this study suggest that benthic foraminiferal assemblages can be used for quantitative reconstruction of surface ocean productivity and bottom water oxygen concentrations if suitable surface sediment calibration data sets are developed and appropriate means for detecting no-analog samples are found.

Quantitative estimation of global patterns of surface ocean biological productivity and its seasonal variation on timescales from centuries to millennia

Organic carbon supply is a fundamental aspect of all biological communities, and the organic carbon flux is generally limited for marine benthic organisms. Benthic organisms shrouded in darkness, at water depths greater than a few tens of meters, are entirely dependent on imported organic carbon for their energy requirements. As we will show in this review, areal variations in the flux of organic carbon to the seafloor have a pervasive effect on the benthic community, including the Foraminifera. Because the processes and ecology are so different, we will examine separately the marine environments within and below the euphotic zone. Our review will be presented in four sections. The first will examine the production of organic matter in the marine realm and its delivery to the ocean floor. The second will summarize observations concerning Foraminifera and food supply. The third will review our basic understanding of the benthic response to organic carbon flux, and present ideas on and models of the response of benthic Foraminifera to that flux. These models will be examined using data concerning foraminiferal microecology, spatial distributions, morphology and abundances. The last section will survey the state of our knowledge and raise important questions which limit our understanding of benthic Foraminifera and their geologic record.

A multiproxy reconstruction of biological productivity and oceanography in the eastern equatorial Pacific for the past 30,000 years

We present a quantitative method, based on the relative abundances of benthic foraminifera in deep-sea sediments, for estimating surface ocean biological productivity over the timescale of centuries to millennia. We calibrate the method using a global data set composed of 207 samples from the Atlantic, Pacific, and Indian Oceans from a water depth range between 2300 and 3600 m. The sample set was developed so that other, potentially significant, environmental variables would be uncorrelated to overlying surface ocean productivity. A regression of assemblages against productivity yielded an r2 = 0.89 demonstrating a strong productivity signal in the faunal data. In addition, we examined assemblage response to annual variability in biological productivity (seasonality). Our data set included a range of seasonalities which we quantified into a seasonality index using the pigment color bands from the coastal zone color scanner (CZCS). The response of benthic foraminiferal assemblage composition to our seasonality index was tested with regression analysis. We obtained a statistically highly significant r2 = 0.75. Further, discriminant function analysis revealed a clear separation among sample groups based on surface ocean productivity and our seasonality index. Finally, we tested the response of benthic foraminiferal assemblages to three different modes of seasonality. We observed a distinct separation of our samples into groups representing low seasonal variability, strong seasonality with a single main productivity event in the year, and strong seasonality with multiple productivity events in the year. Reconstructing surface ocean biological productivity with benthic foraminifera will aid in modeling marine biogeochemical cycles. Also, estimating mode and range of annual seasonality will provide insight to changing oceanic processes, allowing the examination of the mechanisms causing changes in the marine biotic system over time. This article contains supplementary material.

The impact of seasonality on the benthos as reflected in the assemblages of deep-sea foraminifera

Abstract Surface ocean biological productivity was reconstructed for oxygen isotope stages 1 and 2 in 5 cores from the eastern equatorial Pacific using a newly developed proxy based on assemblages of benthic foraminifera. Along the equator and in the South Equatorial Current there was a drop in productivity during the early to mid-Holocene, a peak during the later stages of the glacial-interglacial transition, and full glacial values lower than those of the present. There was a decrease in biological productivity (new production) on the order of 25% along the equator during the last full glacial. The west to east productivity gradient in the east equatorial Pacific was maintained in glacial times, but was reduced compared to the present. In contrast, values to the south and north of the equatorial belt of high productivity may have increased modestly during the glacial. The equatorial decrease in surface ocean biological productivity during the Ice Age was tested using multispecies benthic foraminiferal carbon isotope data and carbonate preservation indices. These tests support the faunal interpretations. The paleoproductivity results presented here conflict with those based on the record of organic carbon in deep sea sediments. The nature of this conflict and reasons for it are examined. Finally, the results here may be compatible with geochemical reconstructions of surface water nutrient concentrations, nutrient utilization, paleo-pH estimates and CO2 efflux in the eastern equatorial Pacific. However, the nutrient reconstructions are not supported by planktonic foraminiferal multispecies carbon isotope values, and an alternative model of glacial age circulation and plankton communities may be needed to account for all the data.

Taphonomic process and species microhabitats in the living to fossil assemblage transition of deeper water benthic foraminifera

A surface sediment study of benthic foraminiferal assemblages in the eastern Pacific (Loubere, 1994) yielded a regression equation for estimating surface ocean productivity from assemblage composition with an r2=0.98. This equation was tested with samples from locations outside the calibration data set area and yielded estimates with an error of about 11% of the estimate. The estimation equation has an r2=0.86 for the non-calibration data set samples. This equation was then applied to surface sediment samples from the Indian Ocean, which represent benthic conditions identical to those of the Pacific calibration data set, except that surface ocean productivity in the Indian Ocean is highly seasonal. There is relatively little seasonal variation in productivity over the areas sampled for the Pacific calibration data. Although it is difficult to quantify average annual surface ocean productivity in the Indian Ocean, it appears that the benthic foraminiferal transfer function yields estimates of only qualitative value. At highest productivities it is likely that the equation underestimates productivity. Using Discriminant Function Analysis to compare sample groups from the Pacific and Indian Oceans representing identical environmental conditions, except for seasonality, shows that the Indian Ocean samples are increasingly anomalous, or “no-analog”, from the Pacific Ocean perspective as productivity increases. At higher productivities, Indian Ocean samples are deficient in Uvigerina species, Chilostomella sp., Pullenia sp. and E. tumidulus while having an excess of E. exigua, C. hooperi, Gyroidina grp. species, B. mexicana and Nonion species, from the Pacific viewpoint. The differences in benthic foraminiferal assemblages between the two oceans can be ascribed to changes in benthic community structure that occur as organic carbon flux to the seabed becomes increasingly episodic or seasonal. Overall, Indian Ocean foraminiferal assemblages follow compositional trends similar to those seen in the Pacific Ocean as surface productivity changes. Hence, a Pacific based benthic foraminiferal transfer function can still reasonably estimate productivity gradients from Indian Ocean assemblages. However, there are 4 sufficient assemblage differences between the two oceans to make seasonality a variable that should be incorporated in a calibration data set, and to make estimation of paleo-seasonality a possibility.

Nutrient and oceanographic changes in the Eastern Equatorial Pacific from the last full Glacial to the Present

A detailed analysis was made of living and death assemblage abundances of benthic foraminiferal species in the top 10 cm of three high quality box cores from the Gulf of Mexico (water depth=1170, 1050, and 1020 m). These data, combined with sedimentological studies and analysis of the biotic activity in the sediments (using Pb-210 profiles), were used to examine the processes controlling the transformation of living to fossil assemblages. These results have implications for the paleoceanic interpretation of benthic foraminiferal assemblage and isotopic data