Ronald E. Martin

Cyclic and secular variation in microfossil biomineralization: clues to the biogeochemical evolution of Phanerozoic oceans

The stratigraphic occurrence and mineralogy of major protistan microfossil taxa tend to reflect evolutionary innovation in response to ocean chemistry and fertility. In foraminefera, the characteristic test composition—and, in some cases, ultrastructure—of each suborder is indicative of the degree of surface ocean CaCO3 saturation, which varied in a cyclic manner through the Phanerozoic, at the time of origin of the suborder. High dissolved phosphate and low CaCO3 saturation in late Precambrian-Early Cambrian surface waters may have prevented calcification in primitive non-calcareous (organic, agglutinated) foraminiferal stocks. Scattered reports of coccolithophorid-like microfossils from the Paleozoic are indicative of a secular trend in rising nutrient levels and marine productivity that controlled the initiation of calcareous oozes. Based on acritarch, carbon isotope, and phosphorite records, extremely low nutrient levels (“superligotrophic” conditions) in Cambrian-to-Devonian seas typically limited population densities of calcareous nannoplankton and prevented the formation of calcareous oozes. The overall “superoligotrophic” surface conditions of the Paleozoic were punctuated, though, by episodes of “catastrophic” eutrophication in the Late Ordovician, Late Devonia, and Late Carboniferous (Worsley et al., 1986). Following each episode, CaCO3 rain rates were presumably enhanced because Marine C:P (MCP) burial ratios increased permanently above previous levels (Worsley et al., 1986). Nevertheless, it was not until the Carboniferous that the CCD had deepened sufficiently (via erosion of cratonic limestones) to allow pelagic calcareous oozes to begin to accumulate. Prior to this time, surface waters appear to have been sufficiently corrosive (high atmospheric pCO2 and low CaCO3 saturation), and the CCD sufficiently shallow, to dissolve virtually all incipient calcareous nannofossils. Following Late Permian extinctions, plankton re-expanded in response to both eustatic sea level rise (increased habitat availability) and increased nutrient levels (“mesotrophic” conditions). As organic matter (Corg) and CaCO3 rain rates increased, bioturbation rates also increased, thereby recycling nutrients back to the surface and accentuating productivity and calcareous ooze formation. MCP episodes further accelerated nutrient cycling and productivity in the Neogene, as indicated by the expansion of diatoms, which prefer nutrient-rich (“eutrophic”) conditions. Ironically, while permanently increasing C:P burial ratios and productivity through the Phanerozoic, catastrophic fluctuations in nutrient levels may have also exacerbated mass extinctions via shortening of pelagic food chains. Nevertheless, re-expansion of the marine biosphere following each extinction episode resulted in a secular trend of increasing biomass and biotic diversity that may have contributed to the decline in background extinction rates through the Phanerozoic.

Experimental analysis of abrasion and dissolution resistance of modern reef-dwelling foraminifera; implications for the preservation of biogenic carbonate

Fringing coral reefs at Discovery Bay, Jamaica, exhibit a pronounced depth-related gradient in water turbulence and associated physicochemical taphonomic factors (abrasion, dissolution), and thus provide ideal settings for investigating the influence of taphonomic processes on the formation of fossil assemblages. Foraminifera are prominent constituents of bioclastic sediments at Discovery Bay, and exhibit a high diversity of test sizes, shapes, wall compositions, architectures, and microstructures which may potentially affect their post-mortem behavior. We have developed a taphofacies model for Jamaican north coast fringing reefs and associated environments that has allowed us to generate hypotheses about the formation of foraminiferal sediment assemblages

THE FORMATION OF HOLOCENE MARSH FORAMINIFERAL ASSEMBLAGES, MIDDLE ATLANTIC COAST, U.S.A.: IMPLICATIONS FOR HOLOCENE SEA-LEVEL CHANGE

Substantial spatio-temporal variation in foraminiferal inputs occur over short areal distances at the sediment-water interface and downcore as a result of patchy distributions and seasonal reproduction; foraminiferal assemblages are in turn diagenetically overprinted by seasonal, inter-seasonal, and inter-annual changes in pore-water chemistry. Seasonal surface and near-surface assemblages are typically unrepresentative of deeper assemblages that are more likely to be incorporated into the sedimentary record. Cluster analysis of “artificially time-averaged” (ATA) assemblages revealed a distinct change in assemblages at ~20 cm depth. Differential preservation of foraminifera in the upper 60 cm, and especially the upper 20 cm, of sediment may produce an apparent paleoenvironmental change that could potentially be misinterpreted as a rapid fall in sea-level over the last ~100–200 years.

The fossil record of biodiversity: nutrients, productivity, habitat area and differential preservation

It is hypothesized that the Phanerozoic record of fossil diversity is a function of a secular increase in nutrient availability and productivity (food, energy), and cyclic changes in sea level and habitat area due to supercontinent assembly and rifting. Both variables may have affected biodiversity through the combined variable of {productivity × area}. {Productivity × area} remained relatively constant after the Cambro-Ordovician until the end of the Permian, as did the traditional curve for biodiversity. During assembly of Pangea, decreasing sea level and habitat area were counteracted by increasing nutrient inputs due to uplift and the spread of vascular plants and enhanced continental weathering. As Pangea underwent its final assembly, interior drainage increased, so that by the end of the Permian both habitat area and nutrient runoff decreased. Following the end-Permian extinctions, the traditional curve of diversity began to increase, habitat area, nutrient levels and productivity all increased. Despite the confounding factors of differential preservation and sampling bias toward the present, the fossil record reflects a real response by the marine biosphere to tectonism, sea level, paleoceanographic regime and climate, and the spread of terrestrial floras, and their influence on habitat area, nutrient inputs, and productivity through time.

INTERANNUAL VARIATION OF MARSH FORAMINIFERAL ASSEMBLAGES (BOMBAY HOOK NATIONAL WILDLIFE REFUGE, SMYRNA, DE): DO FORAMINIFERAL ASSEMBLAGES HAVE A MEMORY?

Seasonal reproduction and preservation of foraminifera were monitored for three years at Bombay Hook National Wildlife Refuge (Smyrna, DE). Cluster analysis of “seasonally artificially time-averaged” (SATA) assemblages indicates that assemblages reflect the most recent test inputs, and that test inputs record subtle variations in porewater chemistry related to rates of sulfate reduction, pyrite oxidation, and vertical differentiation of geochemical gradients. Changes in geochemical gradients are caused by interannual variation of rainfall: more rainfall damps pyrite oxidation and weakens geochemical gradients, whereas less rainfall allows more pyrite oxidation and strengthens vertical gradients. Geochemical overprinting of assemblages related to rainfall tended to occur during the summer and early fall. Therefore, as assemblages pass into the historical layer, they may have a “memory” of the most recent test inputs and environmental conditions.

Taphonomy and time-averaging of foraminiferal assemblages in Holocene tidal flat sediments, Bahia la Choya, Sonora, Mexico (northern Gulf of California)

Abstract Foraminiferal reproduction and preservation have been studied in Holocene tidal flat sediments of Bahia la Choya, Sonora, Mexico (northern Gulf of California). Foraminiferal reproduction at Choya Bay tends to occur in discrete (~ a few weeks) seasonal pulses, which are then followed by periods of homogenization and dissolution of several months duration. Foraminiferal number (number of tests/gram sediment) increases northward across the flat primarily because of decreasing intensity of bioturbation and increasing total carbonate weight percent (shell content) of sediments. Despite intensive dissolution of foraminiferal reproductive pulses, tests which appear to be relatively fresh are actually quite old (up to ~ 2000 years based on 14C dates). We hypothesize that after reproduction some tests survive dissolution because of rapid advection (burial) downward by conveyor belt deposit feeders (e.g., callianassid shrimp, polychaete worms) into a subsurface shell layer, where tests are preserved until exhumation much later by biological activity or storms. Thus, taphonomic grade (surface condition) of foraminiferal tests in these sediments is not an infallible indicator of shell age (time since death). The condition of the test surface is indicative of the residence time of the test at the sediment-water interface (“taphonomically active zone”) and not test age.

Taphonomy and temporal resolution of foraminiferal assemblages

The surface mixed layer of sediment, which ranges in thickness from a few centimeters to as much as a meter, acts as a low-pass filter that damps high frequency signals before their incorporation into the historical record. This damping mechanism is referred to as time-averaging, which is the process by which fossils of different ages are mixed into a single assemblage. Because of time-averaging, a fossil assemblage represents a minimal duration of (at best) a few decades, and more likely hundreds to thousands of years, unless the assemblage is rapidly preserved by unusual conditions ( Lagerstatten). Time-averaging occurs because the generation times of organisms are inherently much shorter than net rates of sediment accumulation and burial (Kidwell, 1993). The mixed layer is also a taphonomically active zone (Davies et al., 1989) in which stratigraphic signals are damped by biological mixing (bioturbation) and dissolution. This chapter reviews what little is really known about the taphonomy and temporal resolution of foraminiferal assemblages in major depositional settings. Although post-mortem transportation of tests has been demonstrated to vary with respect to such factors as test size, shape, and density (e.g. Kontrovitz et al., 1978, 1979; Zhang et al., 1993), studies of assemblages suggest that little post-mortem transport of Foraminifera normally occurs (e.g. Snyder et al., 1990a,b). Therefore, the effects of bioturbation and dissolution on the formation of foraminiferal assemblages are emphasized.

Artificial time-averaging of marsh foraminiferal assemblages: linking the temporal scales of ecology and paleoecology

Abstract Multiple regression models were developed for seasonal test inputs to, and preservation of, marsh foraminiferal assemblages for a two-year period at Bombay Hook National Wildlife Refuge (BHNWR; Smyrna, Delaware). Seasonal assemblages were quite variable and yielded poor regression models. However, signal/noise ratios were amplified using artificially time-averaged (ATA) assemblages, in which separate dead and live abundances of the most abundant species were summed for all seasons. Regression models that used ATA species abundances to retrodict original sample depths accounted for up to ∼99% (p < 0.0001) and ∼91% (p < 0.023) of the variation of dead and live ATA assemblages, respectively, and usually retrodicted sample depths within 2–3 cm of actual depths. Artificially time-averaged assemblages were also used to extract multidecadal- to centennial-scale sea-level signals from near-surface assemblages at BHNWR formed during the past few centuries. The BHNWR sea-level curve closely resembles one previously published for marshes in Clinton, Connecticut (also based on foraminifera). The technique of artificial time-averaging therefore links the temporal scales of ecology and paleobiology by extracting high-resolution paleoenvironmental signals preserved in the fossil record.

Seafood through time revisited: the Phanerozoic increase in marine trophic resources and its macroevolutionary consequences

Abstract We review and synthesize multiple biotic and abiotic proxies for marine nutrient and food availability, primary productivity, and food quality (stoichiometry) and propose what their relationships may have been to macroevolutionary processes, especially speciation. This review confirms earlier suggestions that there has been an overall increase in marine primary productivity over the Phanerozoic, but indicates that the increase has been irregular and that present levels may not be the peak. We integrate these indicators into a new estimate of relative primary productivity in the global ocean through the Phanerozoic. We then combine multiple, frequently conflicting ecological-evolutionary hypotheses into a general model for how primary production may affect speciation over geological time scales. This model, an elaboration and extension of the “speciation cycle” previously proposed by Grant and Grant, attempts to explain why an increase in food supply sometimes is associated with decreased diversity, and at other times with increased diversification. We propose some simple tests for the application of this model to the fossil record.

Relation of counting methods to taphonomic gradients and biofacies zonation of foraminiferal sediment assemblages

Abstract The routine procedure for most distributional studies of foraminifera has been to count approximately 300 specimens of all sizes greater than some specified minimum (usually between 63 and 125 μm), and determine percent abundance of each species using total counts. This method fails to take into account that foraminiferal species may be found predominantly within certain size fractions, either because of species-specific size ranges or taphonomic processes (e.g., sorting, transport, abrasion, dissolution). Use of an alternative counting procedure (“sieve method”) takes into account foraminiferal size distributions. The sieve method utilizes counts of up to 300 specimens in each sand size fraction (0.125–0.25, 0.25–0.5, 0.5–1.0, 1–2 mm) of each sample. Counts are then totaled for each sample (up to 1200 specimens per site) and used in determination of species abundances for each site. The sieve method has been of considerable utility in recognition of foraminiferal bathymetric zonation preserved in sediment assemblages of fringing reef environments at Discovery Bay, Jamaica. Well-documented reef zones (originally based on corals and physiography) are relatively well-defined in Q-mode cluster analysis (UPGMA) of species abundances determined using the sieve method. In contrast, reef zones are not recognized in cluster analysis of foraminiferal species abundances based on the standard method, nor by cluster analysis of species abundances within individual size fractions. The sieve counting procedure compensates for operator bias in specimen counts introduced by large and unusually abundant species (e.g.,Amphistegina gibbosa) and by small but colorful forms (e.g.,Discorbis rosea), which mask the zonation using the standard procedure. The sieve method does not alter overall depth-related trends in species abundance, distribution, or diversity as determined by the standard method. Thus, foraminiferal sediment assemblages contain more paleoenvironmental information than had previously been thought.