Wolfgang H. Berger

Diversity of Planktonic Foraminifera in Deep-Sea Sediments

The diversity of a planktonic foraminiferal assemblage on the ocean floor depends on the state of preservation of that assemblage. As dissolution progresses, species diversity (number of species in the assemblage) decreases, but compound diversity (based on relative species abundance) first increases and then decreases; species dominance first decreases and then increases. The reason for these changes is that the species most susceptible to solution deliver moresediment to the ocean floor than do species with solution-resistant shells, possibly because the more soluble tests are produced in surface waters, where growth and production are greatest.

Isotope paleontology: growth and composition of extant calcareous species

Abstract Isotope paleontology uses the isotopic composition of fossil remains of organisms to make inferences about the physical surroundings of growth of the organisms (especially temperature), and to obtain clues about life history and modes of growth. In calcareous fossils, oxygen isotopes are mainly used in the former, and carbon isotopes in the latter. However, since physical surroundings and organism response are intimately associated, both types of information are contained in each of the isotopic signals. To explore the potential of isotope paleontology, and to provide a basis for reconstruction, a broad range of extant organisms has been studied, taking the pioneering work of Epstein and associates as a starting point. Results are summarized for a representative sampling of these studies, with emphasis on work at the laboratories of the authors, from the mid-seventies to the present. The organisms considered are nannoplankton, benthic algae, planktonic and benthic deep-sea foraminifera, “larger” foraminifera, sponges, corals, bryozoans, polychaetes, arthropods, bivalves, gastropods, cephalopods, and vertebrates (fish otoliths). The survey broadly suggests that, regarding oxygen isotopes, materials tend to be precipitated close to equilibrium with the surrounding seawater (as postulated by Urey), and that for carbon isotopes disequilibrium is the rule. Life spans, growth rates, differential seasonal growth, and age of reproductive activity can be extracted under favorable circumstances from individual shells and skeletal parts. In detail, the interpretation of isotope records of individual shells is quite complicated, and simple models will fail to give satisfactory results in many or most cases.

Planktonic Foraminifera: Selective solution and the lysocline

Abstract Pacific Ocean waters appear to be undersaturated with calcium carbonate at all depths except in the uppermost few hundred meters. This leads to continuous destruction of almost all calcareous sediment exposed on the Pacific Ocean floor. Samples derived from sediment and from plankton were exposed for four months on a taut wire buoy in the central Pacific, in order to assess the effects of solution on foraminiferal death assemblages. The rate of destruction varied for different species and for different variants within species. Sediment assemblages therefore should tend to become enriched with resistant (non-spinose) species and with opaque (usually thick-shelled), zero and negative forms, i.e., specimens with small terminal chambers. Results obtained by laboratory experiments compare well with those obtained from the solution experiments in the field. The distribution of resistant forms in surface sediment samples from the East Pacific Rise shows that there exists a level of rapid solution increase (lysocline) in this area. The surface of the lysocline is at approximately 4,000 m depth in the tropics on the western side of the East Pacific Rise. The lysocline surface slopes upward toward Antarctica and toward South America and apparently becomes less well defined in high latitudes and near the continent. The calcium carbonate compensation depth results from a balance of rates of solution and of rates of supply of calcareous matter and usually lies well below the lysocline. The existence of a lysocline implies an associated oxygen minimum roof, and is bound to both active bottom water production and excess calcite supply by planktonic organisms.

Paleoproductivity from benthic foraminifera abundance: Glacial to postglacial change in the west-equatorial Pacific

Surface productivity is correlated with the rate of accumulation of benthic foraminifera on the deep-sea floor. As a rule of thumb, for each 1 mg of organic carbon arriving at the sea floor, one benthic foram shell >150 μm is deposited. The correlation can be used to reconstruct organic flux to the sea floor in the past, and hence the productivity of past oceans. Applying the appropriate equations to box core data from the Ontong Java Plateau in the western equatorial Pacific, we found that productivity during the last glacial maximum exceeded present productivity by a factor of between 1.5 and 2.0, with intermediate values for the mid-transition period. Accumulation of benthic foraminifera was depressed on top of the plateau during the glacial and transitional period, presumably because increased winnowing removed part of the food supply.

Planktonic Foraminifera: selective solution and paleoclimatic interpretation

Abstract Any sediment assemblage of planktonic Foraminifera contains at least three kinds of information: (1) the amount of solution it has experienced, (2) its geographic origin, (3) the range of depth habitats represented by the constituents species. By ranking the species with respect to their resistance to solution, their latitudinal occurence, and their preferred depth habitat, a portion of this information can be extracted. Selective solution can change the apparent latitudinal range of an assemblage and its depth habitat aspect. Corrections must be applied before drawing conclusions on the latter two ecological properties. It is then possible to trace in the sediments the movement of water bodies at different depth levels in the ocean. In the central Atlantic, the present current structure at the surface of the ocean is reflected in the bottom sediments with astonishing precision. The level at which the rate of ocean is reflected in the bottom sediments with astonishing precision. The level at which the rate of solution of calcium carbonate rapidly increases apparently coincides with the top of the Antarctic Bottom Water. Studies of older sediments suggest that the surface currents were wider, and the solution level stood higher in the past.

Sedimentation of planktonic foraminifera

Fossil assemblages of planktonic Foraminifera contain many valuable clues to paleoclimate and paleo-oceanography. Unfortunately, our understanding of production, dissolution, redeposition, and other processes of foraminiferal sedimentation is but rudimentary. Lacking direct observations, information largely rests on comparisons between abundance and composition patterns of life-, death-, and sediment-assemblages. Standing stock and production of life assemblages vary greatly, depending on the fertility of the water. Fertility is greatest near continents and along the equator. The shell supply in these fertile regions is several times greater than in the sterile subtropical areas. The distribution of living and empty shells in the water column suggests that virtually all production takes place within the mixed layer, possibly near its base. Size distributions suggest that most formainiferal production becomes food for predators and is not available for sedimentation. Morphology patterns of living and empty shells confirm this suggestion. Thus, very rapid turnover of populations is required to produce the sedimentation rates observed. A residence time of about one week for living Foraminifera larger than 150 μ is proposed for fertile regions, implying life spans of no more than about 2 weeks. Most of the foraminiferal shells reaching the ocean floor are redissolved. Dissolution is complete below the calcite compensation depth (CCD), but much solution takes place above this level also. This fact has long escaped notice, because percentages of calcite are poor indicators of solution patterns. Any reasonable solution profile, even a linear one, would produce a “sudden” decrease of calcite content at the level where the amount of calcite becomes comparable to the amount of non-calcareous dilutants. This is evident from discussion of the formula: L=100(1−RoR) where L is the loss of sediment necessary to increase the insoluble fraction R0 to R percent. (For example, set R0 = 5%; R = 10%, yielding L = 50%; i.e., 50% of the sediment, or more than half of the carbonate, was lost while the carbonate content went from 95 to 90%!). The same formula yields minimum dissolution in formainiferal assemblages, if resistant planktonic species (or alternatively, benthonic forms) are assumed insoluble. Resulting solution estimates improve understanding of the sedimentary lysocline, a depth level that separates well-preserved from poorly preserved assemblages. Calcite saturation levels inferred from thermodynamic calculations are conceptually and actually different from both lysocline and CCD. While the depth (i.e., pressure) control of solution patterns is obvious, circulation and fertility patterns also influence preservation of foraminiferal sediments. In general, fertile areas tend to have poorly preserved calcareous assemblages, even at shallow depths, because much CO2 is developed in the sediments. The associated CCD may be quite deep, however, because of high sedimentation rates of calcium carbonate. The ultimate reason for dissolution of shells is that organisms supply more CaCO3 to the ocean floor than is being introduced from external sources into the oceanic system. This excess supply leads to undersaturation. Excess supply and dissolution rates must balance for geochemical steady state to prevail. Undersaturation, therefore, proceeds to a value where dissolution rates are just right to provide this balance.

Faunal and solution patterns of planktonic Foraminifera in surface sediments of the South Pacific

Abstract The distribution of planktonic Foraminifera in South Pacific sediments reflects the environments of production in surface waters and those of preservation on the ocean floor. Cluster analysis shows that distribution patterns have well-defined boundaries that correspond to the Subtropical Convergence, the Antarctic Polar Front, and the Peru-Chile Current in surface waters, and to the lysocline (level below which solution greatly increases) at depth. The interrelation of clusters is examined by temperature-solution rank analysis which shows how the great diversity of Foraminifera in tropical regions leads to a proliferation of clusters and how some clusters are derived from others by partial dissolution. The compensation depth, where Foraminifera disappear, is conceptually different from the lysocline which separates well-preserved from noticeably dissolved assemblages, and the two levels are not parallel. The variable thickness of the zone of partial dissolution between these levels suggests that the supply of calcareous shells and their dissolution tend to vary together, but in a non-linear fashion. Notes on selected species, including coiling-direction distributions, are appended.

Biogenous Deep-Sea Sediments: Fractionation by Deep-Sea Circulation

There are two types of marginal seas: the lagoonal type with deep water outflow, and the estuarine type with deep water inflow. Lagoonal basins are characterized by waters that have low nutrient concentrations and high salinity and are well aerated; their sediments are rich in calcium carbonate. Estuarine basins contain waters with high nutrient concentrations, low salinity, and relatively low oxygen values; their sediments are rich in silica and organic matter and tend toward being anaerobic. The Atlantic Ocean is “lagoonal” in that it has a deep water outflow and collects lime on its floor; the Pacific Ocean is “estuarine” with a deep water inflow and silica-rich sediments. The oceanic fractionation of silica from lime through basin-basin exchange of water at different depth levels is due to both supply and preservation patterns. Supply of biogenous precipitates to the ocean floor exceeds influx of solutes to the ocean, requiring dissolution on the ocean floor to maintain steady state. Supply is greatest in estuarine basins. Dissolution of lime proceeds fastest in estuarine basins, which have CO 2 -rich deep waters, whereas dissolution of silica is greatest in lagoonal basins, where the deep water is farthest from saturation because of low dissolved nutrient concentrations. Trace elements may conceivably be affected also by this fractionation mechanism. Some implications for the geological record are discussed.

Increase of carbon dioxide in the atmosphere during deglaciation: the coral reef hypothesis

Analyses of ice cores from Greenland and Antarctica [1, 2] have shown that the CO2 content of the atmosphere increased rather suddenly from a glacial low near 180 ppm to a postglacial high near 350 ppm early during deglaciation some 13 000 years ago. Here I present a hypothesis accounting for this observation. First, a look at the data to be explained. The various series of measurements on CO2 content exhibit considerable scatter (Fig. 1A). However, the most recent results have confirmed that a deglacial CO2 pulse exists [3]. The Dome 10 data (D10 in Fig. 1) of Delmas et al. [2] show the least scatter, and agree well with the new Camp Century profile of Neftel et al. [3]. I take the D 10 data as being representative, therefore (filled circles in Fig. 1A and B). A sharp step appears in this profile. What mechanisms can produce such a CO2 step? The CO2 reservoirs which are able to yield substantial quantities of CO2 to the atmosphere on short notice are (l) the biosphere (mainly forests), (2) the ocean, (3) sediments (carbonate and organic carbon). The biosphere was growing during deglacial warming of the globe [4]. Thus, it was not a source but a sink for COg. The ocean was a potential source of COa for two reasons : (1) warming the sea reduces its ability to hold C02 in solution, and (2) a drop in fertility, as indicated in decreasing deep-sea sedimentation rates and changing plankton composition [5], also reduces the COz content of the deep sea [6]. Warming apparently was unimportant, because from isotopic evidence and the composition of benthic faunas we know that the temperature of deep waters stayed about the same from glacial to postglacial

Rates of benthic mixing in deep-sea sediment as determined by radioactive tracers

A series of closely spaced radiocarbon measurements on a carbonate-rich box core from the western equatorial Pacific show a mixed layer at least 7 cm thick, with 14C ages between 4000 and 5000 years, and an orderly progression of ages below this layer, indicating an average sedimentation rate of about 2 cm/103 yr. The profile can be simulated using a numerical extension of the mixing model of Guinasso and Schink (1975) and a numerical exponential mixing model. The best-fit iteration indicates an apparent mixing coefficient of K = 120 cm2/103 yr which also fits well the excess 210Pb distribution. The best-fit also indicates that a small amount of sediment was lost on the top, and that there was a reduction in sedimentation rate within the early Holocene.