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Modern and Ancient Continental Shelf Anoxia: an Overview

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Modern and Ancient Continental Shelf Anoxia: an Overview Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021 Modern and ancient continental shelf anoxia: an overview R. V. TYSON 1 & T. H. PEARSON 2 1 Newcastle Research Group, Fossil Fuels and Environmental Geochemistry (Postgraduate Institute), Drummond Building, The University, Newcastle upon Tyne, NE1 7RU, UK. 2 Scottish Environmental Advisory Services Limited, c/o Dunstaffnage Marine Research Laboratory, P.O. Box 3, Oban, Argyll, PA34 4AD, UK. Abstract: The characteristics of mid-latitude shelf seas are primarily controlled by seasonal cyclicity in wind-driven mixing. Seasonal dysoxic-anoxic conditions may occur in summer in salinity-stratified estuarine or pro-delta settings, or more extensively in those open shelf areas where a total depth of ~<60 m and the seasonal thcrmocline result in a bottom water layer ~<10 m thick. When bottom circulation is limited the oxygen stored in this layer may become periodically exhausted if climatic factors extend the stratified period to seven or more months, or if there is additional organic loading (via flagellate blooms and/or pollution). This causes widespread mortalities and a shift to soft-bodied, non-fossilizing benthic faunas. Our review supports seasonal dysoxia-anoxia as being the best model to account for the key characteristics of many ancient epeiric sea black shales. It would appear that the latter rarely represent true continuous anoxia except locally in more- confined deeper sub-basins. We recommend that the following terms should be applied to environments, facies, or oxygenation levels: oxic (8.0-2.0 ml/1 O2), dysoxic (2.0-0.2 ml/1), suboxic (0.2-0.0 ml/1) and anoxic (0.0 ml/I). The corresponding biofacies terms are: aerobic, dysaerobic, quasi- anaerobic (laminated, without macrofauna, but with in situ benthic microfauna) and anaerobic. Hypoxic and normoxic should only be used with regard to the physiological responses of living organisms. The phenomenon of severe oxygen depletion in groups of scientists, an examination of the rel- continental shelf waters is of great significance evant literature indicates that rather little inter- to both geologists and marine biologists. For change of ideas, perspectives or terminology geologists its importance lies in the face that has hitherto occurred. In this introduction we most, perhaps 80%, of the world's petroleum attempt to integrate these different perspec- has been generated from ancient organic-rich tives, and show how a greater understanding of sediments whose sedimentological, geochemi- modern processes can enhance interpretation cal, and palaeontological characteristics indicate of the geological record, and how that record formation in a regime where oxygen-depleted can aid our analyses of modern anoxic or oxygen-free conditions prevailed at the environments. sediment-water interface. Most of these organic-rich petroleum source rocks are marine and were deposited on the continental shelf or Some fundamental characteristics of the upper slope. For the marine biologist, the in- shelf sea environment creasing frequency of bottom water anoxia in coastal waters and the severity of the resultant During the last twenty years the stimuli of the summer mass mortalities, with its disastrous Deep Sea Drilling Project and the Ocean Drill- commercial consequences for coastal fisheries, ing Project have resulted in a rather oceano- makes the phenomenon of continental shelf graphic perspective on the geological analysis of anoxia an area of urgent research. It is essential anoxic environments. However, the shelves are for the marine biologist to distinguish the rela- very distinct and often rather separate systems, tive importance of natural processes from the influenced by their own internal hydrographic effects of eutrophication caused by pollution, controls, rather than by external oceanographic and to forecast the likely response to global factors (although both are influenced by global warming and the increased urbanization of the climate and sea level). Table I summarizes some world's coastlines. of the key differences between modern conti- Despite the great mutual interest to these two nental shelves and oceans. Modern shelves are From TYSON, R. V. & PEARSON, T. H. (eds), 1991, Modern and Ancient Continental ShelfAnoxia Geological Society Special Publication No 58, pp 1-24. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021 2 R.V. TYSON & T. H. PEARSON Table 1. Comparison of modern shelves and oceans (from Tyson, in preparation, based on multiple sources) Shelf Ocean Percent of global surface area 4.5% 66.3% Relative marine area 7.6% (~<200 m) 92.4% Average depth ~<133 m 3,730 m Long-term sediment accumulation 10-200 1-4 m/MY Mean TOC (wt%) of sediment 1.02 (0.5-3.0) 0.34 Mean primary productivity 164 57 g/C/m2/yr Global marine productivity 27% (~<200 m) 73% Mean levels of new production 30% (~<50%) 6% Recycling of production ~<50% 80-90% Carbon burial effic!ency (oxic) 8-70% 0.5-5% Preservation of productivity 1-25% 0.4-1.0% Oxygen consumption rate 1-100 0.01-0.1 gM/l Global benthic oxygen consumption 83% (<~200 m) 17% Global sulphate reduction />90% ~<10% Relative global carbon burial 80-90% 10-20% Global benthic biomass 83% 17% Mean thickness of bioturbated layer 20 cm 10 cm Bioturbation coefficients 1-10 0.04-0.05 covered by relatively shallow marginal seas that during settling, and the much higher sediment occur only around the periphery of the conti- accumulation rates, shelf sediments are more nents and are generally only -+70 km wide and organic-rich and more reducing than their less than 130 m deep. However, sea level has oceanic counterparts, leading to a greater burial shown an approximately 400 m range of vari- of the sedimented organic matter (greater car- ation over geological time, and prior to around bon burial efficiency), despite the fact that there 65 million years ago (the beginning of the is greater exploitation by infaunal benthos. Cenozoic area), extensive epeiric seas period- Being shallow, shelf waters are generally ically covered large areas of the continents. It better mixed and are seldom deep enough to be was in these large extensive epeiric seas that beyond the reach of storm mixing (probably oxygen deficient conditions were most severe ~<100 m, and certainly no more than 200 m). and widespread; during the last 90 million years Although tides are locally of great importance, (i.e. since the early Turonian stage of the Late most of the energy for mixing is provided by the Cretaceous) they have occurred only on a much wind, and thus varies seasonally. This regular smaller, localized, and sporadic scale. variation in the extent of wind-driven mixing is From Table 1 we can see that the mean the single most important characteristic of shelf primary productivity of modern shelf waters is seas, as it controls the water column chemistry, over three times greater than the mean for the the hydrodynamic regime, and the dynamics oceans. The difference in productivity between and life histories of both the plankton and adjacent shelves and oceans in fact varies benthos (Tyson 1985). The seasonal stratifi- with latitude between a factor of 5 and 20 cation cycle is a well known and well docu- (Romankevitch 1984). Furthermore, a much mented phenomenon and is described in many higher proportion of shelf productivity rep- oceanography and marine ecology texts (e.g. resents 'new production' that can be exported Thurman & Webber 1984). from the euphotic zone (see Berger et al. 1989). In temperate shallow shelf waters convection This higher productivity reflects the shallower and wind-driven turbulence result in a complete depth and thus the higher proportion of the mixing of the whole water column during the water column that lies within the euphotic zone, winter months. This homogenizes the distri- the greater frequency and extent of mixing in bution of nutrients throughout the water shallow waters (which provides 'new' nutrients column. However, the growth of phytoplankton to the surface layer), and the greater proximity shows an annual minimum because the depth of to riverine sources of nutrients. Because the the wind-mixed surface layer exceeds the depth water column is so shallow, much of the primary of the euphotic zone, and the downward com- production is sedimented to the sea floor where ponent of the turbulent mixing prevents the it supports a much greater benthic biomass. phytoplankton from making effective use of the Because of the higher productivity, lower losses available light and nutrients. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021 MODERN & ANCIENT CONTINENTAL SHELF ANOXIA 3 Turbulent mixing decreases in the spring as water from the upper layers of the bottom mean wind intensity and storm frequency de- water. Along with the reduced levels of grazing, crease, and the greater insolation increases the this nutrient supply may permit a secondary buoyancy of the surface water and progressively autumn bloom while sufficient light is still avail- inhibits the vertical extent of mixing. Once the able (= new production). In low latitudes this whole of the mixed layer lies within the euphotic destratification may lead to the annual peak zone, the rapid growth of large chain-forming in primary production (e.g. Beers & Herman diatoms results in the 'spring bloom' (new pro- 1969). In the late autumn falling temperatures duction, the annual peak in primary pro- reduce the buoyancy of the mixed layer, the ductivity). Losses due to grazing are initially gradient of the thermocline/pyconocline de: insignificant as these diatoms multiply much creases, and turbulence and convective mixing faster than the zooplankton. The rapid phyto- eventually destroy the stratification completely. plankton growth is initially fuelled by the nutri- Winter storms mix the water column to the ents that were mixed into the surface waters bottom (or to the depth of the permanent ther- during the winter.
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