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After: The Murchinson Symposium; proceedings of an international conference on the Silurian System, M. G. Bassett, P. D. Lane, and D. Edwards (eds.), Special Papers in Palaeontology, v. 44, p. 123-143. (1991). Modified for the WWW April 1997

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ABSTRACT

The physical and chemical oceanography of the Silurian is developed from the relationship among atmospheric, oceanic, and tectonic models calibrated by lithofacies studies, inventories of major cyclic elements and global isotopic signatures. Specific interpretations for the Silurian are done in the context of global signatures for the Palaeozoic. The Early Silurian was characterized climatically as a return to non-glacial conditions after the major glaciation of the Late Ordovician. Carbonate lithofacies was dominate along an equatorial band of warm water, fringed by dark shaley limestones at the margins of the continental blocks. Shaley facies dominated in temperate and southern-polar latitudes. Due to relatively low atmospheric oxygen of from 65 to 35% of the present atmospheric level, waters below about 100 meters were anoxic so that most of the bottom waters of the middle and outer shelf during high-stands of sea level were anoxic. The pycnocline expanded and became increasingly more anoxic during the warmer climates of the Silurian, which caused anoxic waters to invade shelf waters. Late in the Silurian, lands began to emerge in the tropics at the expense of shelf seas, which reduced the area of deposition. Such shoaling produced conditions conducive to dolomite and evaporite deposition. Two major ocean basins existed (1) a Boreal-Pacific basin, and (2) a Proto-Tethyian basin bounded latitudinally by Equatorial and Austral landmasses and meridionally by the temperate west coast of Gondawana and the east coast of Laurentia. Iapetus, at this time was at most an embayment on the west side of Proto-Tethys. By the end of the Silurian, the connection on the west to the Pacific was closed. Equatorial circulation was dominated by westward flowing currents in the Pacific which fed into strong western intensification currents flowing anti-cyclonically about oceanic high pressure systems centered in middle latitudes. Circulation in Proto-Tethys also was dominated by oceanic high pressure in middle southern latitudes but was modified by monsoonal conditions generated by the large landmass of Gondawana. Major warm water masses included tropical Equatorial Waters, paralleled by sub- tropical to temperate large paired Central Water Masses in the North and South Pacific and a smaller Southern Central Water Mass in ProtoTethys. Due to the presence of a large southern polar land mass, the southern-most areas of Proto-Tethys lay within the boundaries of cool South SubPolar water. The northern hemispheric zonal ocean north of 30°, also was covered by cool to cold water. In general, warm Central Water Masses expanded during the Silurian at the expense of the SubPolar waters. Concentrations of nutrient-related trace metals declined in the waters below the photic zone with increasing anoxia resulting in decreased productivity reflected in the global inventory of organic carbon. Stable isotopes reflecting chemical conditions in the oceans show trends consistent with the return to more anoxic conditions after the Late Ordovician glaciation. During the Silurian isotopic signatures show a del ¹³C decline from 1 to .2 per mil, whereas del ³⁴S and ⁸⁷Sr/⁸⁶Sr increase from 24.5 to 25, and .70825 to .7088 respectively. The end of the Silurian corresponds to troughs in the del ¹³C curve, and peaks in the del ³⁴S and ⁸⁷Sr/⁸⁶Sr curves. These trends are coincident with minima in both the atmospheric oxygen and carbon dioxide concentrations. The rise in the strontium ratios also appear related to more exposure of continental rocks to erosion.

INTRODUCTION

The Silurian Period, with its relatively short duration of about 30 million years, is an ideal time segment in which to interpret the physical and chemical environment of the atmosphere and the oceans. As with most of the rock record, in the Silurian there are no direct Fossils air or sea water samples, bathymetric charts, barometric or weather maps, wind or water current measurements etc., which are used in the modern world to characterize the present environment. In the following discussion, we will use the only evidence available, the rock record interpreted with biological, chemical and physical analyses to produce models of the expected environment. Such an interdisciplinary approach is fraught with uncertainties almost as formidable as the imperfections of the Geologic Record itself. However, we feel that the various contributory sciences and analyses have reach a state that a synthesis is both feasible and desirable. To be honest, we have been amazed that a consistent picture of events in the Silurian can be produced from such seemingly disparate data.

LITHOFACIES

Sedimentary rocks bear the record of past environments. They contain the permanent record of both biologic and physical changes through time. Because they do, an understanding the distributions of and changes' in the major lithofacies through time is essential to documenting past atmospheric and oceanic conditions. The first step in ascertaining the distributions in major lithof acies is accurate time correlations of rock units. Once these correlations have been developed, time interval by time interval, then the lithofaces may be plotted on palaeogeographic maps.

Boucot and others (1968) compiled a lithofacies map for the Silurian based upon preliminary time correlations of Silurian strata worldwide. Subsequently a series of Silurian correlation charts were compiled. The following Silurian correlations have been published: North America (Berry and Boucot, 1970), South America (Berry and Boucot, 1972a), Southeast Asia and the Near East (Berry and Boucot, 1972b), Australia, New Zealand and New Guinea (Talent, Berry and Boucot, 1975), the British Isles (Ziegler, Rickards and McKerrow, 1974), and China (Mu En- zhi and others, 1986). As well, detailed Silurian correlations for Scandinavia, the remainder of Europe, and Asiatic Russia have been compiled in manuscript by Berry and Boucot with the assistance of numerous European and Russian colleagues. These correlations provide an exceptionally refined data base for Silurian lithofacies and biofacies useful in analyses of atmospheric and oceanic chemistry.

Silurian plate positions have been described in a series of palaeogeographic maps (Scotese. 1986). Plotting the Silurian lithofacies based on Boucot and others (1968) and the subsequent series of Silurian correlation charts, three major features become apparent: 1) much of the Northern Hemisphere north of the tropics was open ocean; 2) large areas of shelf seas lay within the tropics; and 3) a large land mass (Gondwana) was centered over the south polar area, with a part of it extending northward into the tropics. Tropical shelf sea lithofacies are characterized by carbonates. Algae are rich in the shallowest environments in tropical shelf seas. McKerrow (1978) summarized a number of studies of marine benthic organism, showing that associations he called ecogroups tended to form depth-reflective bands. These bands paralleled shorelines of the time (see Berry and Boucot, 1970; Ziegler and others, 1974). Brachiopods are the prominent organism in the tropical shelf sea benthic faunas. They are joined by a number of other organisms that secreted calcium carbonate shells, of which crinoids and corals were especially prominent. Shorelines around Gondwana were dominantly sandstones. These pass laterally into dark mudstones and shales that formed in shelf seas ((Berry and Boucot, 1972 a, b; Mu En-zhi and others, 1986). These shelf sea mud rocks bear some brachiopods, tracks and trails of unknown organisms, and bivalves. Associations of clams are prominent in many areas. Kriz (personal communication) indicates that certain clam-dominated benthic faunas seemingly reflect oxygen-poor benthic environments. Boucot (this symposium) draws attention to the differences between marine benthic fauna from the tropical shelves with those from the mud rocks deposited in temperate zones. Mu En- zhi and others (1986) and Chen Xu (personal communication) noted that the Early Silurian (Llandovery age) moderate depth and deeper shelf environments in China were probably anoxic. Certain of these mud rocks are rich in graptolites, but they lack traces of benthic organisms (Mu En- zhi and others, 1986; Chen Xu (personal communication). The faunal evidence from the temperate zone shelf sea mudrocks suggests that the environments in which these sediments accumulated were sites of low to no oxygen content. The faunal data are consistent with geochemical observations of Silurian graptolite-bearing strata carried out by the authors (wilds and others, 1990, in press).

The lithofacies shown on Figs. 1 and 2 document changes in areal extent of the major facies during the Silurian. Volcanism increased during the latter part of the Silurian, notably in eastern Australia (latent and others, 1975). Approximately coeval volcanic rocks are known in the Urals and in Kazakhstan (Boucot and others, 1968). The area of shelf seas of Gondwanaland declined during the Silurian. Direct connections to the open ocean became restricted in some regions of the tropics so that the central part of the Laurentian platform (modern North America and part of Siberia) became sites of accumulation of dolomite and anhydrite. Algal decay in shallow oxygen- poor environments could produce anoxic marine embayments in sub- tropical areas of high evaporation from 20° to 30°. The embayments replenished by episodic incursion of magnesium and sulfate-rich seawater from the open ocean may have led to deposition of minerals of the dolomitic and evaporitic facies. Palaeogeographic reconstructions by Scotese (1988) indicate that part of modern Siberia moved northward into the Northern Hemisphere temperate zone. Plate motion resulting in the classical Caledonide Orogeny resulted in rising land mass across what is now Wales and part of Scotland (Ziegler and others, 1974). That land was the site of early vascular land plants such as Cooksonia and related forms (Edwards and Feenan, 1980). Shelf margins near that land were sites of one of the most impressive developments of reef growth during the Silurian. The reefs flourished markedly for a short time in the late Wenlock (Ziegler and others, 1974; McKerrow, 1978). Silurian correlations indicate that progradation of shoreline environments across shelf seas along Gondwanaland's borders and shallowing of tropical shelf sea environments commenced in the middle Silurian and became marked during the Late Silurian. These changes and the onset of significant volcanic activity at about the same time may indicate renewed activity along oceanic ridges in the latter part of the Silurian. Widespread rise of sea level following deglaciation and absence of marked volcanic activity early in the Silurian suggests relative oceanic ridge quiescence during the early part of the Silurian.

Early Silurian shelf seas in temperate zones and those in deep shelf or slope settings in the tropics were sites of accumulation of dark mudrocks. These mud rocks from tropical shelf margins are very calcareous. Faunas in the mudrocks, both from the tropical and temperate zones, commonly are thinly-laminated, lack any signs of tracks, trails or bioturbation, include thin sews of pyrite, and bear primarily the remains of planktic organisms, graptolites. As noted, the mudrocks, those bearing clam-dominated faunas, appear to have accumulated in low oxygen- content environments. The faunal and lithologic evidence suggests that the greatest abundance of the mudrocks formed in either anoxic or oxygen-poor environments on shelves or on shelf margins. Shelf sea dark mudstones were widespread early in the Silurian. At that time sea level rose following deglaciation. To ascertain the environmental conditions under which graptolite-bearing shales could have accumulated, Berry and others (1987) and Wilde and others (1989) reviewed water chemistry in the Eastern Tropical Pacific and the Cariaco Trench. In both of these settings, dark mudstones are accumulating in anoxic depositional environments. In both, the upper part of the oceanic water column is oxic. With depth, coincident with oxygen consumption because of decomposition of organic matter and lack of replacement of significant quantities of oxygen, anoxic waters form. Organic decomposition continues in anoxic waters with less efficient oxidants such as nitrate, nitrite, and sulfate replacing dissolved oxygen as the electron donor. Thermodynamically, for dominant oxidant species in sea water (Quinby-Hunt and Wilde, 1987), the sequence of reduction is oxygen, oxidized nitrogen compounds [nitrate, nitrite] and oxidized sulfur compounds [sulfate] (Stumm and Morgan, 1981). Organic matter thus decomposes, with depth with at least three different oxidants [oxygen, nitrate, sulfate] producing three redox zones with different rates of decomposition of organic matter each reflecting decreasing redox and decreasing ability to oxidize. Such conditions may be analogues for Silurian shelf sea environments in which graptolite-bearing mud rocks accumulated. Many of the shelf sea environments, below the surface oxic layer, may have been sites of nitrate reduction zone waters analogous to the denitrification zone of the modern Eastern Tropical Pacific (Berry, Wilde and Quinby- Hunt, 1987). The clam- prominent benthic associations in some dark mudstone lithofacies suggests that episodically the bottom waters there became slightly oxic. The surface sediment returned to anoxic conditions if sufficient organic matter was present to consume small amount of oxygen available in the pore waters to initiate nitrate then sulfate reduction.

CIRCULATION:

Major open ocean surface circulation patterns are fundamentally driven by the oceanic pressure systems produced by the planetary winds. At any given time interval the actual patterns are based on palaeogeographic positions of continental blocks. These blocks present meridional barriers converting general zonal (east-west) circulation patterns into meridional (north-south) flow. Wilde and Berry (1986) and Wilde (1991) discuss the mechanics for the development Of such patterns and their attendant surface water masses for the Ordovician. That approach will be continued here bracketing the Silurian using the palaeogeographic reconstruction of Scotese (1986). This procedure yields five sets of seasonal [summer and winter] maps depicting oceanic surface circulation for the Late Ordovician: Ashgill (Fig. 3); Lower Silurian: Llandovery (Fig. 4); Middle Silurian: Wenlock (Fig. 5); Upper Silurian: Ludlow (Fig. 6); and Earliest Devonian: Gedinnian] (Fig. 7).

The major changes that affected open ocean circulation or water mass position and formation in the interval from the Late Ordovician through the Earliest Devonian were (1) the change in climate and (2) shifts in meridional barriers. The main climatic change occurred early with the transition from extensive glaciation in the Southern Hemisphere in the Late Ordovician to the generally warm non-glacial climates of the Silurian (Frakes, 1979). As Fig. 8 shows the mean global temperature remained relatively constant throughout most of the Silurian and into the early Devonian. Silurian plate motions that modified the Ordovician oceanographic regime were (a) the closing of the Iapetus Ocean; (b) the northward shift of Gondawana eventually merging with Laurentia, which eventually blocked zonal flow between the South Pacific and Proto-Tethys, and shrunk the Austral cool water mass of Proto-Tethys; and (c) the northward shift of Siberia reducing the zonal flow of the Boreal Ocean increasing the flow of cool water south along the west coast of Laurentia and the flow of warm water northward along the east coast of Siberia. Monsoonal conditions producing the reversal of the westward flowing South Equatorial Current continued throughout the Silurian due to the presence of the large land mass of eastern Gondawana. This would produce major seasonal shifts in the equatorial currents on the eastern side of Proto- Tethys. The northward movement of Siberia by the late Silurian might have developed monsoonal conditions in the Northern Hemispheric tropical waters off Kazakhstan. However, the position of the land mass on the western side of Proto-Tethys intensifying the Northern Tropical Current would prevent advection of boreal cool water toward the Equator. A comparison of Fig. 1 with Fig. 4 and Fig. 2 with Fig. 6 show that the general lithofacies patterns match the temperature defined surface water masses. Carbonate and evaporite facies are zonally related to the warm water masses of the tropical regions, whereas shaley facies are found adjacent to cool waters polarward of the Subtropical Convergence.

SILURIAN ENVIRONMENT ATMOSPHERE

Budyo and others (1987), using models based upon inventories of masses of major lithofacies proposed temperatures, carbon dioxide content and oxygen content for Palaeozoic atmospheres. Their model indicates a global Silurian temperature that was about 20 degrees centigrade, which was warmer than the modern global average of about 15 degrees centigrade. As shown in Figure 8, the proposed Silurian temperature is somewhat less than that suggested for the Ordovician and Devonian. This global change may be due to a combination of the increase in land area in the Northern Hemisphere as well as evaporative cooling in the shallow tropical embayments of the Silurian. Such changes might overcome the higher albedo of glacial regions in the southern hemisphere of the Late Ordovician as the equatorial regions contain a larger surface area than the polar regions.

Silurian atmospheric carbon dioxide, again although higher than modern values, was calculated to lie in mimina of the Palaeozoic curve with a value of about 900 parts per million or about 300 % of modern values. The Silurian atmosphere shows a decline from carbon dioxide values of 1200 ppm in the Upper Ordovician.

Budyko and others (1987) (see Fig. 8) indicated atmospheric oxygen declines from about 65 percent of PAL (Present Atmospheric Level) to about 35 percent of PAL in the Late Silurian. Berner (1989) using the same inventory as Budyko and others (1987) recomputed the Phanerozoic oxygen curve by adding recycling of organic matter to their model. The maxima and minima of Berner's curve are similar to those depicted in Figure 4, but the deviation from modern values is smaller. Thus the decline in atmospheric oxygen during the Silurian, according to Berner would be about 75% to 60% PAL. At this time considering, the uncertainties of the model, there is general agreement on the relative shape of the curve which shows a decline of atmospheric oxygen during the Silurian. The oxygen curves proposed by both Budyko and others (1987) and Berner (1989) are consistent with suggestions made by Raiswell and Berner (1986) that oxygen declined in the atmosphere in the mid-Paleozoic until the spread of plants across lands in the Devonian added a significant new non-oceanic source of atmospheric oxygen.

CONTINENTS

TECTONIC ACTIVITY

During the early part of the Silurian, sea level rose as a consequence of deglaciation. Plate motion appears to have been minimal initially as depicted by Scotese (1986). As Ziegler and others (1974) showed, a land mass rose in the area of present day Wales and western England, probably as a consequence of plate motion. By mid-Silurian time, the Southern Hemisphere land was the site of regressing seas. Shoreline deposits began to prograde across shelf sea environments (see Figs. 1 and 2). Gondwana became more emergent in the latter part of the Silurian, as indicated in Fig. 2. Ridge system activity probably increased at that time as suggested by the relationship between ridge activity and general sea- level propounded by Mackenzie and Piggot (1981). Latter part of the Silurian plate motion appears to have resulted primarily in plate doming such that land expanded and shelf seas shoaled. The only marked plate collision that resulted in mountains being formed was that of the Caledonide chain that included western Norway during the Silurian.

OCEANIC CHEMISTRY:
DISSOLVED GASES

The low levels of atmospheric oxygen combined with a warmer climate than at present indicate that oceanic surface mixed layer waters would have had less oxygen content during the Silurian than today. Anoxia in the pycnocline would have expanded significantly during the Early Silurian from that during the colder climate of the Late Ordovician. In the tropics, oxygen content of the surface waters may have been about 3 mL/L (65 % PAL) at the beginning of the Silurian declining to about 2 mL/L (35 % PAL) at the beginning of the Devonian. Figure 9 shows the redox profile with depth for an Equatorial position for an atmospheric value of 50% PAL (Isolde, 1987). Oxygen content in tropical waters would decline to zero by about 100 meters depth. Reducing conditions and anoxic waters extended downward for several hundred meters. Thus during high stands of sea-level, the bottom waters would be anoxic below depths of 100 meters.

The area of the continental shelf seas diminished from about 40% to 30% during the Silurian (Fig. 11). This would reduce the area of surface waters in contact with the atmosphere and thus the volume of sea water saturated with respect to atmospheric oxygen (depths less than 100 meters). That would increase the tendency for an expanded anoxic zone in the oceans particularly near shore.

As postulated above (Fig. 8), in the Silurian the atmospheric carbon dioxide levels were about 3 times that of modern times. In general, higher atmospheric carbon dioxide at a given temperature would produce a lower oceanic pH, which would inhibit calcium carbonate deposition. However, the global mean temperature was warmer than today (Fig. 8). Thus the saturation value of carbon dioxide in the Silurian Ocean was much less than would be expected if the Silurian temperatures were modern or much colder. The presence of stenohaline and presumably pH sensitive organisms such as brachiopods and crinoids in the Silurian indicate that the Silurian pH was not unlike that of modern times or about pH 8 (Smith and Hood, 1964). The oceans are a bigger sink for carbon dioxide than the atmosphere during the present cold climate. Accordingly, the high atmospheric values of carbon dioxide may reflect the much lower solubility of carbon dioxide in the ocean during the warm climates of the Silurian, which maintains a relatively constant slightly alkaline pH. Widespread deposition of calcium carbonate in the Silurian tropical shelf-seat also indicates a relatively alkaline pH.

DISSOLVED SOLIDS

In the present ocean, the concentration of dissolved ions may be classed as (1) conservative or those in constant proportion to each other and can be derived as a function of chlorinity, (2) nutrient and (2) non- conservative whose value is dependent on geography, depth, and/or biological activity. Non-conservative ions may be divided into (a) nutrient and nutrient related whose concentration is a function of growth and decay of marine phytoplankton, (b) nonnutrient related primarily to physical and non-biological chemical geologic processes such as volcanism, and weathering.

As the salinity of the Silurian ocean may be presumed to be similar to that of today, the concentration of the maj or conservative ions such as Na and Cl may be unchanged as they provide about 86% by weight of the total salinity (Sverdrup, Johnson and Fleming, 1942, p. 173). The next most abundant element S as sulfate makes up 7% by weight of the salinity or 2.5% as S. However, the presence of anoxic waters in the pycnocline suggest that the sulfate content may have been less with a significant amount of S as reduced species as sulfides. As Fig. 9 shows for a Silurian smear oxygen concentration of 50% PAL the mean redox is only about -2 mL/L which suggests only a relatively small proportion of the total S inventory as sulfide ions. Activity at oceanic ridges in the latter part of the Silurian implicit in the plate positions of the Scotese (1986) reconstructions would have increased anoxia of ocean waters near the ridge vents through thermal reduction of sulfate to sulfide (Edmond and others, 1979, Janecky and Seyfried, 1984). Increased ridge activity (that is recycling of ocean water through thermal vents) would extract Mg from sea water and release Ca and sulfide. Accordingly, modern conservative ions such as Mg (4%) and Ca (1%) may have had different ion/chlorinity ratios in the Silurian. Other processes such as dolomitization (lowering dissolved Mg inventory) and extensive evaporite deposition powering calcium and sulfate inventories) that occurred in the Late Silurian could contribute to the change in the ratios of these dissolved constituents in seawater.

Nutrients and nutrient related metal concentrations are governed by uptake in the surface ocean by phytoplankton during photosynthesis and release during oxidative decay at depth. Thus in the modern ocean, nutrients such as phosphate, nitrate and silica and metals such as Cu. Cd, and Nit increase with depth in oxic waters. In anoxic basins such as the Cariocao Trench (Jacobs and others, 1987) and the Black Sea (Brewer and Spencer, 1974), the elements that form sulfides are depleted in anoxic waters while redox sensitive elements such as Fe, Mn and Co are at minimum values in oxic waters being scavenged as oxides in sediments (Froelich et al., 1979) but increase in anoxic waters. In the Silurian with low atmospheric oxygen values, at 50% PAL the ocean below 100 meters would be anoxic (Fig. 9). Also, in the oxygenated surface layer the trace metal content would be low due to the uptake by phytoplankton in the photic zone (0 to 50 meters) as shown in Fig. 10. Below the photic zone organic matter would be oxidized releasing the nutrient related metals. However at the oxic- anoxic boundary, the rate of oxidation and metal release would decrease as oxidation by nitrogen species such as nitrate is not as effective as by oxygen. Once these nitrogen compounds are depleted sulfate reduction would begin at about 150 meters (@50 % PAL). Once sulfides are formed in the water column, many of the released metals will form sulfides and their concentration will diminish rapidly with depth (Fig. 10). Metals such as Fe and Mn which are only partially nutrient related, but have strong redox affinities would behave differently than the nutrient- related metals. Using Mn as an example, the values in the oxygenated waters would be essentially zero due to the precipitation of oxides. Mn would be released to sea water just below the oxic-anoxic level and continue to increase until the level of sulfate reduction is reached. The concentration would decrease below the nitric-sulfate reduction boundary layer with the partial precipitation of Mn sulfides as the sulfide concentration increases below the boundary. However, the amount of sulfide generated is insufficient to precipitate all the dissolved Mn so a relatively uniform concentration is maintained with depth. In the Silurian due to the proximity of anoxic and sulfidic water in the top of the pycuocline, the amount of metals available to phytoplankton organisms via recycling by upwelling in the water column would be much less than in the modern ocean. Accordingly, although there would be sufficient N as ammonia and P as phosphate, the trace metals would be diminished. In essence, these metals would be scavenged by sulfide and sink to the bottom. This lack of available trace metals may limit significant primary productivity to (1) the near shore where the metals may be derived from run-off from the highly acid Silurian rain or (2) regions of planetary and seasonal upwelling where the recycling of metals would be less than is seen in the modern well-ventilated ocean.

MASS BALANCE

The inventory of volcanogenic rocks, carbonate carbon and organic carbon is shown in Fig. 11 after Budyko and others, 1987). These values are plotted against the areal percentage of seas overlying the continental blocks. As depicted, all these materials decline throughout the Silurian. Both volcanogenic rocks and carbonate carbon inventories show rapid decline in the Upper Silurian with the shoaling of the continental blocks especially in the tropics, which reduced the area of shelf deposition. Organic carbon burial however shows little change in the Upper Silurian and really does not begin to increase until later in the Devonian with the development of extensive continental sinks due to land plants. Thus, organic carbon sinks in the Silurian still were governed by marine redox conditions. The reduction in the organic carbon inventories during the Silurian suggests that the increased anoxia in the water column did have an inhibiting effect on primary productivity. At low atmospheric oxygen concentrations and correspondingly low surface dissolved oxygen values in the ocean such as in the Silurian, much less organic matter or overlying productivity is required to produce anoxic conditions in either the water column or in sediments. Accordingly, the presence of dark anoxic facies rocks, although having the potential to be excellent sinks for organic carbon, do not necessarily imply a high initial organic content. Thus dark, thinly laminated sediment characteristic of much of the Silurian mudrock lithofacies (Fig. 1 and 2) are not necessarily organic-rich. This relationship merits a caution in use of all dark mudstones as sinks for organic carbon.

ISOTOPIC SIGNATURES

The relation among chemical and isotopic variations in the Phanerozoic have been discussed by Holser, Margaritz and Wright (1986) and Holser and others (1988). Variations with time in the Paleozoic (Fig. 12) of del ¹³C (Lindh and others, 1981), del ³⁴S (Saltzman and others, 1982; Lindh, 1983) and ^87/86Sr (Burke and others, 1982) reflecting oceanic conditions, show consistent trends during the Silurian. del ¹³C decline from 1 to .2 per mil, whereas del ³⁴S and ^87/86Sr increase from 24.5 to 25, and .70825 to .7088 respectively (Fig. 12). The end of the Silurian corresponds to a trough in the del ¹³C curve, and peaks in the del ³⁴S and ^87/86Sr curves. As noted by Veizer and others (1980) the carbon and sulfur isotopic curves are inversely proportional. The general high values of del ³⁴S in the pre-Carboniferous and the high values of the Cerium anomaly in apatites have been interpreted by Wright, Schrader and Holser (1986) and Holser and others (1988) as indication of extensive oceanic anoxia in the Early through Middle Paleozoic.

For the Silurian, the increase in del ³⁴S indicates an increased pyrite burial and/or in anoxia in the oceans (Holser and others, 1988), both of which are consistent with the decline in atmospheric oxygen shown in Fig. 8 resulting in lower dissolved oxygen concentrations in the surface ocean (Fig. 9). The decrease in del ¹³C suggests declining productivity as predicted by the model of Berger and Vincent (1986) where light ¹³C is "pumped" into waters below the photic zone as a function of primary productivity. Thus the del ¹³C decline in the Silurian is consistent with the productivity suppression expected from the diminished trace metals with depth as a result of lower atmospheric oxygen, increased anoxia and decreased organic carbon inventories (Fig. 11) in the Silurian. Holser and others (1988, p. 69) noted the correlation of the del ³⁴S curves with ^87/86Sr suggesting tentatively some common cause such as ridge activity. Koepnick and others (1988) state that a rise in ^87/86Sr values shows more contribution from a granitic (continental) as opposed to a basaltic (oceanic) source. This conclusion also is consistent with the reduction in the area of shelf seas in the latter Silurian (Fig. 11), when more of the continent were exposed.

SUMMARY

Table 1 gives a synopsis of the discussed environmental changes in the Silurian. The Silurian is unique among the geologic periods because various chemical signatures demonstrate uniform trends. Both these chemical trends and the surface oceanographic currents and water masses based on palaeogeographic reconstructions are consistent with the lithofacies developed from the classical bio-stratigraphy as initiated by Sir Roderick, himself.

ACKNOWLEDGEMENTS

As should be apparent, this paper is a inter-disciplinary synthesis of the activities of many divergent groups. In particular, for this study of the Silurian we thank R. A. Berner, A. J. Boucot, L. Hickey, W. T. Holser, J. Grey, J. Wright and the late R. M. Garrels for their special insights. M. Krup co-ordinated preparation of the figures and the manuscript with her usual competence and dispatch. This is contribution 89-001 of the Marine Sciences Group of the University of California, Berkeley.

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Table One
Silurian Environmental Values

	Lower	Upper
Atmosphere
Temperature	140%PAL	135%PAL
Carbon Dioxide	300%PAL	250%PAL
Oxygen	65%PAL	35%PAL
Continents	Lithofacies
Shales	+	-
Shaley Limestones	-	+
Limestones	+	-
Dolomites		+
Evaporites		+
	Mass Balance (*= x1021 grams)
Shelf Seas	37%	30%
Volcanogenic	8*	6*
Carbonate C	1.25*	0.75*
Organic C	0.15*	0.08*
Oceans	Water Mass Area
Equatorial	NSC	NSC
Central	NSC	NSC
Sub-Polar	+	-
	Chemistry
del13C	+1	+0.25
del34S	23.5	24
Log C/S	.45	.3
87Sr/86	.7079	.7088
		Anoxia
Tropical Surface O2	3.25 Ml/L	1.75 mL/L
Surface 0xic layer	0-125 meters	0-90 meters
Upper Nitric Zone	55 meters	50 meters
Top of Sulfatic Zone	180 meters	140 meters
Anoxic Layer	2250 meters	2500 meters