Wolfgang Schlager

Nutrient excess and the demise of coral reefs and carbonate platforms

Growth rates of corals on Holocene reefs indicate that carbonate platforms should easily keep pace with long-term subsidence and sea-level changes, yet drowned reefs and platforms are common in the geologic record. Recognition of the negative influence of nutrients on reef communities provides a clue to that paradox. The primary carbonate-sediment producers of the coral reef community are highly adapted to nutrient-deficient environments. Input of nitrates and phosphates stimulates growth of plankton, which reduces water transparency, limiting depth ranges of zooxanthellate corals and calcareous algae and thereby reducing carbonate production. Higher nutrient concentrations and plankton densities also stimulate growth of fleshy algae and ahermatypic suspension-feeding animals in the benthos. Besides displacing hermatypic algae and corals, many of these fastgrowing competitors are bioeroders that actively destroy thze reefal structure. Because rates of carbonate production and bioerosion are similar, even modest increases in nutrient availability can shift a reef community from net production to net erosion. In the geologic record, drowned reefs and carbonate platforms typically exhibit evidence of nondeposition, bioerosion, and reduced redox potential, which indicate excess nutrient availability during drowning. Drowned reefs overlain by shales are possible victims of nutrients in terrestrial runoff that suppressed reef growth before arrival of siliciclastic sediments. Other drowned platforms may have succumbed during rapid pulses of sea-level rise that flooded previously subaerial platforms. Nutrients in the soils of the flooding platform were mixed into surface waters, suppressing reef growth. The reef drowned if submergence proceeded beyond the critical depth before the excess nutrients were exported from the system. Other mechanisms for reef drowning by excess nutrients include changes in local or regional upwelling patterns or mid-ocean overturn.

The paradox of drowned reefs and carbonate platforms

Shallow-water carbonate platforms and reefs are drowned when tectonic subsidence or rising sea level outpaces carbonate accumulation, and benthonic carbonate production ceases. Drowned platforms are common in the geologic record, but they present a paradox if one considers rates of processes involved. During the early Holocene, when sea level rose at rates of 6,000 to 10,000 µm/yr (= mm/1,000 yr), most reefs and platforms were outpaced by the rising sea. During the late Holocene with sea level rising 500 to 3,000 µm/yr in the Atlantic-Caribbean area, reefs and platforms started to recover, built to sea level, and prograded seaward, 1,000 µm/yr is thus a conservative estimate of the average growth potential of modern reefs and platforms. Independently, accumulation rates of prograding platforms in the geologic record suggest growth potential in excess of several hundred microns per year. The growth potential of 1,000 µm/yr exceeds any relative rise of sea level caused by long-term processes in the geologic record. Newly formed ocean crust subsides at a maximum of 250 µm/yr, basin subsidence averages 10 to 100 µm/yr, and sea level rises due to increased sea-floor spreading amount to less than 10 µm/yr. Rapid pulses of relative rise of sea level or reduction of benthic growth by deterioration of the environment remain the only plausible explanations of drowning. The geologic record shows examples of both of these processes. Global mass extinctions of reefs and platforms occurred in the middle Cretaceous (eustatic rise due to submarine volcanism or desiccation of a small ocean basin?) and the Late Devonian (global crisis of ocean environment, extraterrestrial cause?). Drowning controlled by regional tectonics prevailed in the Jurassic and Early Cretaceous of the Tethyan realm, and the drowning of Mesozoic platforms in the western North Atlantic seems to have been dictated by plate-tectonic drift to higher latitudes.

Carbonates and relative changes in sea level

Abstract In the geologic record some of the most accurate gauges of changes in sea level are the sediment type, geometry and diagenesis of carbonate shelves and platforms. This is because carbonates frequently occur at or very near sea level and are usually less compacted than siliciclastics. World-wide changes in relative sea level (the sum of eustatic sea-level changes, sedimentation and crustal movements) have occurred repeatedly and cyclicly through geologic time, producing characteristic responses in carbonates. 1. (1) Relative rises in sea level (usually caused by the cumulative effect of tectonic subsidence and eustatic rise) may result in the following: 1.1. (A) Drowned carbonate reefs or platforms. Here carbonate growth potential is exceeded by relative sea-level rise, and is characterized by shallow-water sediments, overlain by hardgrounds and/or deep-water sediments, some of which may be condensed sequences. 1.2. (B) Platforms where only the fast growing rim and patches of the interior are able to match sea-level rise while the remainder of the platform is drowned (temporarily). 1.3

Accommodation and supply—a dual control on stratigraphic sequences

Abstract It is widely accepted that both eustatic and tectonically controlled regional changes of sea level have contributed to the record of stratigraphic sequences. I suggest that environmental change be added as a third, autonomous control. Sedimentologic principles clearly indicate that sequences and their systems tracts are controlled by the interplay of two rates —the rate of change in accommodation (space available for sedimentation) and the rate of sediment supply. Sea level has direct control on accommodation, but its influence on sediment supply is remote and easily overshadowed by environmental factors. For instance, the record of the most recent sea-level rise is a transgressive systems tract where supply is low; it is a prograding highstand systems tract in deltas where the supply is high. Examples of sequence boundaries generated by changes in sediment supply include tectonically driven shifts in sediment input into basins, changes in ocean currents, pulsating supply from failure of submarine slopes and drowning of carbonate platforms by environmental stress. Furthermore, the stratigraphic sequences in fluviatile continental basins are physically removed from sea-level induced changes in accommodation and must have formed by changes in the rate and pattern of supply. Subaerial exposure of marine sediments at the sequence boundary is a most important criterion for recognizing sea level cycles as opposed to supply cycles. Other criteria include downstepping of shelf breaks and characteristic patterns in the spacing of time lines within sequences. Some third-order cycles (ca. 0.5–3 Ma duration) meet these criteria, others do not. Cycle-stacking patterns and the shifting facies belts on cratons indicate that many second- and third-order cycles lack pronounced exposure unconformities and represent gradual changes superimposed on more rapid, shorter oscillations. Seismic data yield poor images of these gradational changes because they lack resolution. Seismic models of outcrops show that near the limits of resolution, the seismic tool tends to portray rapid changes in facies and bed thickness, i.e. lateral variations in supply, as unconformities. Seismic unconformities do not necessarily correspond to unconformities in outcrop. The succession of stratigraphic sequences is best considered a composite record of sea-level and environmental change. Separating the various controls requires carefully planned experiments. Work on the oxygen-isotope curve—another proxy for sea level—has set an example in this respect.

Glacial versus interglacial sedimentation rates and turbidite frequency in the Bahamas

The southern Tongue of the Ocean is a 1300-m-deep, flat-floored basin in the Bahamas that receives large amounts of sediment from the carbonate platforms surrounding it on three sides. We have examined five 8–13-m-long piston cores and determined bulk sedimentation rates, turbidite frequency, and turbidite accumulation rates for the past two glacial and interglacial periods. The mean of bulk sedimentation rates is four to six times higher in interglacial periods; average accumulation rates of recognizable turbidites are higher by a factor of 21 to 45, and interglacial turbidite frequency is higher by a factor of 6 to 14. Sediment composition indicates that increased interglacial rates are due to higher accumulation of platform-derived material. Additional data from other Bahamian basins as well as published material from the Caribbean strongly suggest that highstand shedding is a general trend in pure carbonate depositional systems. Carbonate platforms without a siliciclastic component export more material during highstands of sea level when the platform tops are flooded and produce sediment. The response of carbonate platforms to Quaternary sea-level cycles is opposed to that of siliciclastic ocean margins, where sediment is stored on the inner shelf during highstands and passed on to continental rises and abyssal plains during lowstands of sea level.

Submarine slope angles, drowning unconformities, and self-erosion of limestone escarpments

We measured angle and height of carbonate platform slopes in the Pacific and the Atlantic and of siliciclastic continental slopes in the Atlantic. We also determined depositional regime and sediment budget of these slopes by using seismic profiles, morphology, and sediment cover. Slopes of carbonate platforms are significantly steeper than submarine slopes in siliciclastic terrains. Carbonate slopes are concave in profile; siliciclastic slopes are slightly convex or straight. The angle of carbonate slopes increases with height; the angle of siliciclastic slopes does not. The increase in slope angle of upward-growing carbonate platforms changes the sediment budget and depositional regime from accretionary slopes that have positive budget to erosional slopes that have negative budget. Oversteepening of carbonate platforms has important implications. (1) Drowning and burial of carbonate platforms by siliciclastics can produce a “drowning unconformity” that simulates a fall in sea level with seaward shift in onlap. (2) Upper slopes of high-rising platforms can be so steep that accretion is impossible, and instead, the slope is being eroded by slumps and turbidity currents from the platforms9 own sediment. This self-erosion is the prime cause of the conspicuous limestone escarpments in modern oceans.

Depositional bias and environmental change—important factors in sequence stratigraphy

Abstract Differences among depositional systems, here called depositional bias, strongly influence sequence patterns. Siliciclastics and shallow-water carbonates, for instance, shed most of their sediment during opposite phases of a sea-level cycle (lowstand shedding and highstand shedding, respectively). Furthermore, the two systems generate their own, system-specific relief on the sea floor, disperse their sediment load along different avenues and differ in the way they are deactivated: reefs and carbonate platforms can be drowned, whereas siliciclastic deposition can be shut off and renewed at any depth. As a consequence of these differences, pronounced unconformities (drowning unconformities) develop where carbonate platforms are terminated and buried by siliciclastics (the siliciclastic-to-carbonate transition tends to be more gradual). Drowned platforms and drowning unconformities appeared world-wide in great abundance in the Miocene, Cretaceous (Valanginian-Turonian), Jurassic (Toarcian) and Devonian (Frasnian/Famennian). Examples of drowning unconformities interpreted as sequence boundaries, include those of the Early Cretaceous platforms off New Jersey and off Morocco, the mid-Cretaceous unconformity in the Gulf of Mexico and Miocene unconformities on top of reefs in the Far East. The eustatic cycles postulated from sequence stratigraphy are a very unlikely cause for the mass drownings of reefs and platforms: either the rates of rise are an order of magnitude lower than the growth potential of platforms, or cycle amplitudes are too small, or the cycles are too short and subsidence too slow to remove platforms permanently from the photic zone. The critical element in the mass drowning seems to be environmental stress that reduces the growth potential of carbonate systems. Thus, drowning unconformities demonstrate the importance of environmental change as a control on sequence development. Other examples of dominantly environment-controlled sequences include slope deposits shaped by shifting currents (e.g. Florida and Blake Plateau) and basin fills that were controlled by changes in sediment input related to tectonic alteration of the drainage in the hinterland (e.g. Gulf of Mexico). Environmental change must be considered as a third, independent factor that competes with eustatic and tectonically driven regional changes of sea-level for control of sequences. The definition of sequences and sequence boundaries should be broad enough to include the possibility of non-sea-level controls.

Basic Types of Submarine Slope Curvature

ABSTRACT Curve fitting of the first-order morphology of recent slope profiles shows that with only three basic types of equations, a linear, an exponential, and a Gaussian distribution function, over 80% of a random collection of 150 seismic profiles, covering most of the worlds continental margins, can be quantified. Profiles from carbonate as well as siliciclastic margins, and from mud- to sand-dominated profiles, are present in each group. A small group of submarine slopes have a linear profile, and are interpreted to rest at the angle of repose. Oversteepening of this critical angle by sedimentation disturbs the entire planar surface, starting mass movements of variable style, number, and size. The exponential trend is attributed to the exponential decay of transport capacity or competence with increasing distance from the sediment source at the shelf break. The fact that the majority of slopes follow a Gaussian curve rather than an exponential one may be due to the disturbing effect of extrinsic processes. We propose as a working hypothesis that they represent regular exponential profiles whose upper parts have been disturbed by the interplay of wave-dominated transport with gravity-driven transport at the shelfbreak during base-level fluctuations. Gaussian profiles are also observed when sediment is eroded and redistributed at the shelfbreak by ocean currents during clinoform progradation. The geometry of slopes gives information on the depositional environment, whereas the shape parameters of the three governing equations offer some promising clues to deducing sediment composition. Mud-dominated slopes have a lower slope angle, curvature of exponential profiles, and peakedness of Gaussian curves than sand-dominated slopes.

Quaternary aragonite cycles and oxygen-isotope record in Bahamian carbonate ooze

Carbonate ooze in Bahamian troughs displays cyclic variation in the content of aragonite. A 10 m core devoid of turbidites shows 3.5 cycles, each defined by a sharp increase, followed by a gradual decrease of aragonite. The aragonite curve matches closely the oxygen-isotope curve of planktic foraminifera from the same core. Aragonite highs correspond to light oxygen-isotope values and thus to interglacial stages. Content of terrigenous material is low during interglacials and higher during glacial periods. Intermittent flooding and exposure of the Bahama Banks is most probably not the cause of the cycles, because the latest increase in aragonite precedes bank flooding by 8,000 yr and because the cycles are distinctly asymmetric, with rapid increase and gradual decrease of aragonitic content. Carbonate dissolution cycles tied to the glacial rhythm of Earth9s climate are a more likely explanation of the variations in aragonite content.

Bahama carbonate platforms — The deep and the past

Abstract Carbonate deposition prevailed for 150 Ma in the Bahama—Florida segment of the West Atlantic margin and provides an interesting test for actualism. Present-day facies patterns on the banks are controlled by the backward decrease of waves and tides and the concomitant increase of temperature and salinity variation. In the troughs, influx of sand and rubble from gravity flows varies with topography and distance from shallow-water sources and allows one to define facies belts: a rhythmic sequence of ooze and graded beds on the basin floors, subdivided into a basin-margin belt of coarse, thick turbidites and basin interior with fine turbidites; slope facies change with increase in height and declivity from accretionary to by-pass to erosional regimes. Stratigraphic history of the Bahamas is not simply a projection of the “Holo-Scene” back in time. Both long-term natural evolution (decrease in subsidence, upbuilding of the banks, submarine erosion) and outside factors (climate, eustacy) have caused significant changes. Since the Jurassic, the Bahamas seem to have evolved from a clastics-evaporite province to a single carbonate-evaporite platform and finally to an array of platforms and troughs. During the platform-trough stage, the rate of upbuilding of the platforms decreased, submarine canyon erosion increased. Platform flanks steepened as they grew higher and changed from accretionary to by-pass to erosional slopes. A change imposed by extraneous factors occurred in the Pliocene, when the Great Bahama Bank changed from a giant reef-rimmed atoll to a flat platform covered by oolites and peloid sands. The Bahamas share both long-term trends as well as random changes by extraneous factors with other platforms in the geologic record. Compared to ancient platforms, however, they are unusually long-lived and at a more advanced stage of growth, they are deeply dissected by erosion, their flanks are unusually high and steep and the troughs very narrow. The Neogene platform sequence is strongly controlled by eustatic sea-level fluctuations.