Frans Jorissen

A conceptual model explaining benthic foraminiferal microhabitats

Abstract We present a conceptual model which explains benthic foraminiferal microhabitat preferences in terms of differences in the downward organic flux. We argue that under oligotrophic conditions the microhabitat depth is controlled by the availability of metabolizable food particles in the sediment. Under more eutrophic conditions, the ecosystem is no longer food-controlled, but instead, a critical oxygen level determines down to what depth we find a living fauna. Under food-limited conditions, anaerobic degradation of organic matter may provide an additional food source around the redox front, which could explain deep infaunal maxima reported in the literature. In a sample transect through the Adriatic Sea, both microhabitat controls (food-limited and oxygen-limited) are present. On the shelf and the upper part of the slope, the rather shallow depth of the microhabitat is controlled by a critical oxygen level. In the 1250 m deep southern basin and on the lower part of the slope, on the contrary, the availability of metabolizable organic matter, and not a critical oxygen level, determines down to what depth living foraminifera are found.

Magnitudes of sea-level lowstands of the past 500,000 years

Existing techniques for estimating natural fluctuations of sea level and global ice-volume from the recent geological past exploit fossil coral-reef terraces or oxygen-isotope records from benthic foraminifera. Fossil reefs reveal the magnitude of sea-level peaks (highstands) of the past million years, but fail to produce significant values for minima (lowstands) before the Last Glacial Maximum (LGM) about 20,000 years ago, a time at which sea level was about 120 m lower than it is today. The isotope method provides a continuous sea-level record for the past 140,000 years (ref. 5) (calibrated with fossil-reef data), but the realistic uncertainty in the sea-level estimates is around ±20 m. Here we present improved lowstand estimates—extending the record back to 500,000 years before present—using an independent method based on combining evidence of extreme high-salinity conditions in the glacial Red Sea with a simple hydraulic control model of water flow through the Strait of Bab-el-Mandab, which links the Red Sea to the open ocean. We find that the world can glaciate more intensely than during the LGM by up to an additional 20-m lowering of global sea-level. Such a 20-m difference is equivalent to a change in global ice-volume of the order of todays Greenland and West Antarctic ice-sheets.

Letters to Nature: Magnitudes of sea-level lowstands of the past 500,000 years

Existing techniques for estimating natural fluctuations of sea level and global ice-volume from the recent geological past exploit fossil coral-reef terraces or oxygen-isotope records from benthic foraminifera. Fossil reefs reveal the magnitude of sea-level peaks (highstands) of the past million years, but fail to produce significant values for minima (lowstands) before the Last Glacial Maximum (LGM) about 20,000 years ago, a time at which sea level was about 120 m lower than it is today. The isotope method provides a continuous sea-level record for the past 140,000 years (ref. 5) (calibrated with fossil-reef data), but the realistic uncertainty in the sea-level estimates is around ±20 m. Here we present improved lowstand estimates—extending the record back to 500,000 years before present—using an independent method based on combining evidence of extreme high-salinity conditions in the glacial Red Sea with a simple hydraulic control model of water flow through the Strait of Bab-el-Mandab, which links the Red Sea to the open ocean. We find that the world can glaciate more intensely than during the LGM by up to an additional 20-m lowering of global sea-level. Such a 20-m difference is equivalent to a change in global ice-volume of the order of todays Greenland and West Antarctic ice-sheets.

Live benthic foraminiferal faunas from the Bay of Biscay: faunal density, composition, and microhabitats

In the meso-oligotrophic Bay of Biscay, a diminishing downward organic matter flux with depth is accompanied by an important decrease of the live foraminiferal density. Although bottom water oxygenation is not directly influenced by organic matter input, the oxygenation of interstitial waters and the primary redox fronts do change in response to variations of the organic matter flux. The occurrence of deep and intermediate infaunal taxa can be linked to fundamental redox fronts and putative associated bacterial consortia. Our data are in agreement with the TROX-model, which explains the benthic foraminiferal microhabitat as a function of organic flux and benthic ecosystem oxygenation. Both the depth of the principle redox fronts and the microhabitat of deep infaunal species show important increases with depth. At the deepest oligotrophic stations, deep infaunal faunas become relatively poor. Therefore, the exported flux of organic matter appears to be the main parameter controlling the composition and the vertical distribution of benthic foraminiferal faunas below the sediment-water interface. The oxygenation of pore waters plays only a minor role. A species-level adaptation of the TROX-model is presented for the Bay of Biscay.

Vertical distribution of benthic foraminifera in the northern Adriatic Sea: The relation with the organic flux

Abstract In October 1989 the distribution of benthic foraminifera stained by Bengal Rose in the uppermost seven centimeters of the sediment has been determined in 14 sample stations in the northern Adriatic Sea. The downward organic flux, which controls the complex relation between food and oxygen availability in the benthic environment, appears to be the main factor determining the distribution of benthic foraminifera. In most bottom environments especially the oxygen concentration is limiting benthic life; low values are responsible for the low faunal densities within the sediment of some of the most organic-rich areas. As soon as the oxygen level surpasses a critical treshold value, food availability becomes the limiting factor, regulating abundance and species composition of the benthic faunas. The areas with the highest downward organic flux are typified by a number of very opportunistic taxa, which can be epifaunal as well as potentially (mobile) infaunal. These taxa are most able to profit from the combination of a high food availability and fair oxygen levels after the reoxygenation of the bottom environment in autumn. The areas with lower organic fluxes are characterized by a more stable fauna, consisting of less stress-tolerant epifaunal taxa in combination with less mobile infaunal species, which lack the possibility to track critical oxygen levels. The present data confirm that microhabitat differentiation is minimal in months of low oxygen values in the sediment. However, fossil records of such seasonal environments will mainly be determined by production in the well-oxygenated season, and therefore knowledge about microhabitat behaviour of individual taxa can certainly be useful for the reconstruction of ancient organic fluxes.

Organic flux control on bathymetric zonation of Mediterranean benthic foraminifera

A data set of benthic foraminiferal faunas counted in 138 surface samples from the Mediterranean Sea has been used to investigate whether the bathymetrical distribution of the dominant taxa is controlled by the amount of labile organic matter transported to the sea floor. We find that most of the major taxa show a clear W to E shallowing of their upper or lower depth limit, coinciding with a W to E decrease in the surface water primary production, and in the estimated flux of the labile organic matter to the sea floor. This observation implies that the bathymetrical succession of these taxa is indeed determined by the organic flux. In the western Mediterranean, we find successions from more oligotrophic taxa at greater water depths to more eutrophic taxa in more shallow water. Towards the eastern Mediterranean, most eutrophic taxa tend to become increasingly rare, or even to disappear, whereas the more oligotrophic taxa show a clear shoaling of their depth range. Deep infaunal taxa are mainly limited to the western part of the Mediterranean. This is explained by their dependency on a relatively elevated organic flux, and by the fact that the bacterial stocks on which they feed may become unattainable when the redox front is positioned too deep in the sediment. The close similarity between the flux level controlling our main faunal boundary, and the flux levels coinciding with important faunal changes in other parts of the world ocean, suggests that a flux level of about 2‐3 g labile Cm 22 y 21 level corresponds to a benthic ecosystem threshold value of global importance. q 2000 Elsevier Science B.V. All

Live benthic foraminiferal faunas off Cape Blanc, NW-Africa : Community structure and microhabitats

Live (Rose-Bengal stained) benthic foraminifera were studied along a transect across the main area of organic matter deposition in the Cape Blanc upwelling region. The faunal analyses suggest that at the shallowest station (1200 m) the benthic ecosystem is permanently influenced by the upwelling, whereas at the deepest stations (3010 and 2530 m depth) the ocean bottom is subject to significant organic influxes only in summer. The vertical zonation of foraminiferal species in the sediment shows a close correspondence with the depth distribution of oxic respiration, nitrate and sulphate reduction. It is suggested that this linkage is caused by the presence of various stocks of anaerobic and sulphate- and nitrate-reducing bacteria. Deep infaunal foraminiferal species are thought to feed selectively, either on the bacterial stocks or on nutritious particles produced by bacterial degradation of more refractory organic matter. As such, foramininiferal microhabitats are only indirectly controlled by pore water oxygen concentrations.

African monsoon variability during the previous interglacial maximum

Abstract Little is known about centennial- to millennial-scale climate variability during interglacial times, other than the Holocene. We here present high-resolution evidence from anoxic (unbioturbated) sediments in the eastern Mediterranean Sea that demonstrates a sustained ∼800-yr climate disturbance in the monsoonal latitudes during the Eemian interglacial maximum (∼125 ka BP). Results imply that before and after this event, the Intertropical Convergence Zone (ITCZ) penetrated sufficiently beyond the central Saharan watershed (∼21°N) during the summer monsoon to fuel flooding into the Mediterranean along the wider North African margin, through fossil river/wadi systems that to date have been considered only within a Holocene context. Relaxation in the ITCZ penetration during the intra-Eemian event curtailed this flux, but flow from the Nile – with its vast catchment area – was not affected. Previous work suggests a concomitant Eurasian cooling event, with intensified impact of the higher-latitude climate on the Mediterranean basin. The combined signals are very similar to those described for the Holocene cooling event around 8 ka BP. The apparent type of concurrent changes in the monsoon and higher-latitude climate may reflect a fundamental mechanism for variability in the transfer of energy (latent heat) between the tropics and higher latitudes.

Widespread occurrence of nitrate storage and denitrification among Foraminifera and Gromiida

Benthic foraminifers inhabit a wide range of aquatic environments including open marine, brackish, and freshwater environments. Here we show that several different and diverse foraminiferal groups (miliolids, rotaliids, textulariids) and Gromia, another taxon also belonging to Rhizaria, accumulate and respire nitrates through denitrification. The widespread occurrence among distantly related organisms suggests an ancient origin of the trait. The diverse metabolic capacity of these organisms, which enables them to respire with oxygen and nitrate and to sustain respiratory activity even when electron acceptors are absent from the environment, may be one of the reasons for their successful colonization of diverse marine sediment environments. The contribution of eukaryotes to the removal of fixed nitrogen by respiration may equal the importance of bacterial denitrification in ocean sediments.

200 Year interruption of Holocene sapropel formation in the Adriatic Sea

An interruption of Holocene sapropel S1 is found in cores from various subbasins of the eastern Mediterranean. In core IN68-9 from the Adriatic Sea, sapropel S1 is dated between 8300 and 6340 BP, interrupted between 7100 and 6900 BP (14C years uncorrected for reservoir age). Lithology and variations in the foraminiferal faunas suggest that the interruption is genuine, and not the result of resedimentation. The results indicate that S1 was deposited within a period of enhanced levels of productivity (resulting from increased seasonal contrasts) which started around 9300 BP and ended around 5200 BP. The onset, interruption, and final ending of S1 deposition in the Adriatic Sea, however, appear to have been triggered by changes in ventilation of the basin related to changes in sea surface temperature (SST). Although the rough estimates of SST change are relatively small (< 2°C), they still are significant when compared with the relative SST changes considered necessary to upset convection in the Adriatic. Moreover, recent studies show that the influence of the inferred temperature changes should be viewed in combination with that of reduced salinities due to (1) the deglaciation, and (2) increased humidity in the eastern Mediterranean area during the deposition of S1. The lithological and benthic foraminiferal evidence that sapropel formation in the Adriatic Sea ended around 6340 BP contrasts with the conclusion from a recent geochemical study that sapropel formation in the open eastern Mediterranean would have ended as late as 5000 BP. More significantly, the results of the present study combined with other reports on sapropel interruptions suggest that the process of sapropel formation is not a very stable mode in the basin, but that it may be relatively easily interrupted in response to subtle rearrangements in the balance between productivity and, especially, deep water ventilation.