G.J. van der Zwaan

Benthic foraminifers: proxies or problems?: A review of paleocological concepts

In this paper, we review benthic foraminiferal distribution patterns in the context of their use as proxy to reconstruct paleoenvironments, in particular against the background of relevant biological data. These data suggest that benthic foraminifera, as most microbiota, are not stenotopic to most environmental variables. A more generalist strategy seems beneficial since the low degree of specialisation prevents rapid extinction. This immediately suggests that parameters like temperature and salinity are not very important in benthic foraminiferal distribution and thus not in proxy studies. Oxygen and organic flux, on the other hand, are of great ecological importance and it is not surprising that all viable proxies today are based on relationships with these parameters. Organic flux (food) is important in deciding on abundance, but is subordinate as soon as oxygen starts to be limiting. This is the case in almost all shallow water systems with muddy substrates, sometimes down to considerable waterdepth. Microhabitat patterns are shaped through the arrangement of species along a redox gradient, whereby species distribution seems to be correlated with specific redox levels. It cannot be excluded that a relationship with specific bacterial suites also plays a role here. However, the coupling to the successive redox stages provides a means for very detailed reconstructions of oxygenation. On the other hand, organic flux reconstructions can be distorted due to the fact that the same flux levels, though with different oxygenation, are characterised by different suites of species. Therefore, assemblage characteristics are discussed as additional means to constrain organic flux reconstructions. It is concluded that species distribution with depth is mainly a function of organic flux and oxygenation. In this sense, paleobathymetry should not be based on individual marker species, but preferably on quantitative assemblage characteristics such as P/B ratios. However, more generalised schemes of assemblage successions with depth are clearly helpful. It is further concluded that benthic foraminiferal distribution, and microhabitat occupation, are regulated by the interplay of organic flux, oxygen and competition. Each of these factors is acting in a different way and leads to the complex pattern as found in living associations.

The depth dependency of planktonic/benthic foraminiferal ratios: Constraints and applications

Abstract Study of the ratio between planktonic and benthic foraminifera in a great number of areas shows that variation of this ratio with depth can be described using organic matter flux equations. Essentially, the share of benthic foraminifera in the total association is inversely proportional to depth, as is the amount of organic matter resulting from primary productivity reaching the sea bottom. Variation in the P B ratio from area to area appears to be strongly dependent on the amount of inbenthic living foraminifera. If the ratios are corrected for this, the regressions between the proportion of planktonic foraminifera (%P) and depth appear to be near-identical in the Gulf of Mexico, the Gulf of California, the west coast of the USA and the Adriatic Sea. This regression can be described by: Depth = e (3.58718+(0.03534%P)) A study of the behaviour of this function in nine cores from the Adriatic Upper Quaternary reveals that the palaeodepth reconstructions are not influenced by fluctuations in productivity and temperature but that they are significantly influenced by redeposition of fine-grained deposits.

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.

Living (Rose Bengal stained) benthic foraminifera from the Pakistan continental margin (northern Arabian Sea)

The Arabian Sea is characterized by one of the world’s most pronounced oxygen minimum zones (OMZ) (<0.1 O2 ml/l), which impinges on the seafloor at 200–1000 m depths. The OMZ in the Arabian Sea results from extremely high surface water productivity and moderate thermocline ventilation. Nine box cores were taken on two parallel down-slope transects covering depths from 500 to 2000 m. From these nine box cores living (Rose Bengal stained) benthic foraminifera were studied in detail. Within the upper part of the OMZ, Bolivina dilatata and Bulimina exilis are the most abundant species. In the lower part of the OMZ, Uvigerina peregrina and B. exilis are the most abundant. Just below the OMZ, at a water depth of about 1250 m, the assemblage is typically dominated by Rotaliatinopsis semiinvoluta and U. peregrina; in still deeper waters (1500–2000 m) Bulimina aculeata and Epistominella exigua are the most prominent species. Transect II was sampled three weeks after transect I; on the average, standing stocks were four times higher in transect II. However, down-slope species distributions are similar in the two transects, both in the 63 μm and the 150 μm size fractions. Also vertical (in-sediment) distributions are remarkably similar. This indicates that standing stock differences between the two transects can be ascribed either to the effect of patchiness or, more likely, to the nature and the amount of organic flux. We found unambiguous evidence that in and below the OMZ many benthic foraminifera persist in suboxic to anoxic microhabitats. This regards surface as well as subsurface habitats. Evidently, oxygen is not a limiting factor for a considerable number of species. The obvious relationship between species distribution and the OMZ might then be explained in terms of preferences for amount or type of organic flux. An alternative explanation involves favourable effects resulting from the absence of larger predators. The various models pertaining to the limiting balance between organic flux and oxygen are discussed.

Subsurface activity of benthic foraminifera in relation to porewater oxygen content: laboratory experiments

In a series of laboratory experiments, we examined the migratory activity of some common benthic foraminifera in relation to subsurface oxygen concentrations in porewaters. Our results clearly demonstrate that foraminifera are capable of migrating through anoxic sediments, reinforcing the idea that some common benthic foraminifera are facultative anaerobes. We observed that migration was not completely random; there seemed to be a bias towards upward migration when buried without a subsurface oxic zone. Although this migration can be taken as a response to the absence of oxygen, it was not directly linked to gradients of oxygen as these were not present at that depth and a large portion of the assemblages continued to inhabit deeper sediment layers. The relatively higher percentage of soft-shelled foraminifera in the oxic zone suggests that they may be less tolerant to anoxic conditions than hard-shelled foraminifera. This possible differential tolerance may be one factor that will determine the survival or success of different species buried in deeper sediment layers.

150 years of eutrophication in the northern Adriatic Sea: Evidence from a benthic foraminiferal record

The vertical distribution of benthic foraminifera in a sediment core in front of the Po delta has been studied in detail. According to our age model, based on 210Pb and 137Cs analyses of another core from exactly the same locality, the studied core spans the past 160 years. The radio-isotope profiles further show that sediment mixing is largely restricted to the top centimeter, suggesting that the core should provide an extremely detailed record of the youngest history of the northern Adriatic Sea. Benthic foraminiferal patterns and grain-size analyses indicate a number of substantial changes in sedimentation rate and food/oxygen availability in the benthic ecosystem. Changes occurring at about 1840 and 1880 can be attributed to man-induced changes in the main outflow canals of the Po river. The first one led to an important reduction of the marine vegetation cover which probably was present up to that date. The second change resulted in the present-day situation in which the Po outflow is passing the studied core locality close by. The local benthic foraminiferal associations indicate a steadily increasing nutrient load from 1900 AD onwards. This trend is interpreted as the effect of anthropogenic eutrophication due to agriculture and waste water disposal, although the faunal record as discussed here only gives a limited impression of the basin-wide development. A marked faunal transition around 1930 indicates intensification of the eutrophication; around 1960 the first signs of an increasing importance of anoxic events can be recognized. The faunal changes in the last decade, which are ascribed to changes in preservation potential, indicate that more intense or more prolonged anoxia started about 10 years ago, and that the ecological health of this part of the northern Adriatic probably is still in decline.

Benthic foraminiferal response to variations in surface water productivity and oxygenation in the northern Arabian Sea

Abstract A benthic foraminiferal record of 120,000 year obtained from the base of the Oxygen Minimum Zone (OMZ) at a site in the northern Arabian Sea was studied. Quantitative benthic foraminiferal data were compared with palaeoproductivity indices (Corg and Globigerina bulloides) and indices for bottom water oxygen concentration (Pteropod Preservation Index, Mo/Al, V/Al and Mn/Al). The benthic foraminiferal record revealed two distinct assemblages that show variations in the precession frequency band. A high diversity, low equitability assemblage proliferates in isotopic stages 1, 4 and 5, and has been interpreted to reflect relatively oxygenated bottom water conditions. This assemblage shows a strong covariance with minima in summer monsoon productivity. A low diversity, low equitability fauna, typically with a few species showing high dominances, is considered to reflect low bottom water oxygen conditions related to eutrophic surface water conditions. Dominating species in this assemblage are bolivinids, buliminids and globobuliminids. This assemblage dominates during precession-driven maxima in summer surface water productivity, and also during isotopic stages 2 and 3. We suggest that an intensified and colder glacial winter (NE) monsoon led to increased winter production, which superimposed on the precession-driven changes in surface water productivity, resulted in overall poorly oxygenated bottom water during glacial stages 2 and 3. A promising proxy for tracing winter productivity in the northern Arabian Sea is the planktonic foraminifer Globigerina falconensis. The benthic foraminiferal species Rotaliatinopsis seminvoluta in particular, is prolific under glacial conditions.

On late miocene to recent vertical motions in the Cretan segment of the Hellenic arc

Abstract High-resolution foraminiferal and calcareous nannoplankton biostratigraphy and reliable paleobathymetry estimates based on plankton-benthos ratios in foraminifera make it possible to reconstruct in detail Late Neogene vertical motions along the Central Cretan segment of the Hellenic arc. These motions are considered to express the surficial effect of the roll-back process of the Hellenic subduction zone, which started about 12 Ma ago. In contrast to earlier views there is no sustained uplift since the late Middle Miocene. Successive paleotopographic profiles for central Crete show a predominance of subsidence, coupled with an increase of differential reliefs, from the latest Serravallian until the Messinian. Subsidence was most pronounced between the Tortonian-Messinian boundary interval and the early part of the Early Pliocene, locally up to a magnitude of more than 1000 m. A two-phased uplift history, coupled with tilting to the north or northeast, can be inferred from the late Early Pliocene-Recent record separated by a short, early Late Pliocene episode of subsidence. Rates of uplift were highest in the Early Pliocene up to about 125 cm/ka. The Central Cretan Late Neogene to Recent paleotopographic profiles are compared with those reconstructed from seismic interpretations and drilling data from the Cretan Sea. The combined results show that paleotopographic configurations between the Cyclades and Crete were fairly similar to those on central Crete until Messinian time. Foundering of the Cretan Basin started in the course of the Early Pliocene, but subsidence rates were less than the contemporaneous uplift rates of Crete. The timing and magnitude of the vertical motions along the Cretan Sea-Central Cretan transect put detailed geological constraints on tectonophysical modelling of the surficial effects of the roll-back process. These vertical motions are discussed in view of the model of southward translation of a large supracrustal slab, resulting in the origin of the Cretan Sea and in thrusting and uplift in the frontal parts of the slab, where it thrusted over the northern limb of the subducted Ionian Plate.

The impact of Paleocene/Eocene boundary events on middle neritic benthic foraminiferal assemblages from Egypt

Abstract A 25 m upper Paleocene to lower Eocene marly sequence exposed at Gebel Duwi, (Quseir, Red Sea coast of Egypt) provides a good opportunity for studying Paleocene/Eocene (P/E) boundary stratigraphy and events. Various bio-, eco-, and isotope stratigraphic correlations accurately delineate the position of the P/E boundary (i.e. the level of stable isotopic excursions and deep-sea benthic extinctions) in the middle part of this section, within Zones P5 and NP9. In this paper the impact of P/E boundary environmental perturbations on benthic foraminiferal communities of the shallow Tethyan shelf is examined. Benthic foraminiferal assemblages from Gebel Duwi, characterized by large Frondicularia phosphatica and F. nakkadyi in addition to a number of other neritic taxa, but without any typical outer neritic-bathyal taxa, suggest middle neritic (75–100 m) deposition throughout the studied interval and only minor relative sea-level variations (perhaps some 10–20 m). Across the boundary sea-level appears to have been stable. Yet, a distinct faunal turnover marks the P/E boundary of this shallow site, which, in contrast to deep-sea records, is less abrupt, being preceded by a gradual diversity decrease starting some four meters below the boundary. Moreover, most exits are either local or just temporary and thus not true extinctions: many (Lazarus) taxa reappear at some point above the boundary, while all but one of the disappearing taxa are known from other lower Eocene neritic deposits in Egypt. In general, benthic foraminiferal assemblages display low diversities and high dominance values (notably Anomalinoides aegyptiacus: up to 74%), suggesting high levels of environmental stress at the sea-floor. In combination with occasionally very high P B ratios (up to 98%P), and generally laminated sediments, the character of the benthic assemblage points to a strong influence of recurrent dysoxia and/or anoxia. We suggest that these resulted from surface eutrophication by (intermittent) coastal upwelling and reduced ventilation of the bottom environment. Although these conditions prevailed during most of the time-interval studied, they were most severe during the latest Paleocene and early Eocene when eutrophication intensified, and the organic carbon flux to the sea-floor increased. Enhanced atmospheric contrasts between the Tethys seaway and the African continent, resulting in intensified longshore tradewinds, may have controlled these changes. At the P/E boundary the bottom environment became minimally ventilated, perhaps due to reduced oxygen advection from deeper waters. Three meters above the boundary a succession of increasingly diverse assemblages indicate a gradual decrease of environmental stress and a return to more normal conditions, although the organic carbon flux to the sea-floor remained high.

Deep-water changes: the near-synchronous disappearance of a group of benthic foraminifera from the Late Miocene Mediterranean

Abstract Benthic foraminiferal and geochemical data from the Monte del Casino section in northern Italy are employed to reconstruct the sequence of events preceding the Late Miocene Mediterranean salinity crisis. We evaluate the effects of eustatic sea-level changes and tectonic events, affecting the Atlantic–Mediterranean connections. Changing benthic foraminiferal assemblages record changes in the deep-water environment, that can be explained by progressive isolation of the Mediterranean basin. The results of the analyses of stable oxygen- and carbon isotopes and redox-sensitive elements are in line with the benthic faunal trends. At 7.16 Ma, a first major step in isolation of the basin is indicated by the nearly simultaneous disappearance of a group of deeper-water benthic species usually found in middle to lower slope environments. At the same time, stable isotopes and redox-sensitive elements indicate cooling and decreasing bottom water oxygenation. After 6.8 Ma gradual development of water-mass stratification, probably accompanied by increasing bottom-water salinity, is indicated by all proxies. The cyclic pattern of homogeneous and sapropelitic sediments in the section is related to astronomical parameters, and allows a detailed correlation with Mediterranean sections at considerable geographic distances. We compared the benthic foraminiferal faunas with those from the Metochia section (Gavdos, Greece), deposited at a similar water depth (∼1000 m), to find that the benthic foraminifera at both sites indicate a similar, synchronous paleoenvironmental development. All proxies indicate that the onset of the Late Miocene Mediterranean salinity crisis dates back to the earliest Messinian. The development of the Mediterranean basin toward an evaporite trap is a result of interactions between a 400-kyr eccentricity related climatic effect superimposed upon gateway dynamics at the Atlantic–Mediterranean connections. Benthic foraminifera accurately record the steps in increasing isolation of the Mediterranean basin.