The termwatermassis used here as the marine equivalent for the terrestrial climate zone, or biome, and this seems appropriate because temperature and salinity effects in the oceans generally seem to have sharp boundaries, in contrast to the land surface climate zones which possess more gradual transitions. In today's oceans, surface water masses exert a controlling effect on biogeographic provinces, climate-sensitive sediments, and organic productivity, so it is important to understand the factors that generate and differentiate these water masses. The map shows generalised temperature and salinity for the Recent, with related effects like upwelling, sea ice, and surface runoff, compiled in a new attempt to define marine climates. Present-day climate-sensitive sediments are included to test their reliability as water mass indicators for application in the geological past.
The water mass approach is also relevant economically because certain types of deposits form under very specific marine conditions; these include evaporites, phosphorites and oil source rocks. The evaporites, including halite and gypsum, are deposited in areas where evaporation exceeds precipitation, and where a level of isolation from the ocean is established. Today, such areas are mainly limited to lagoons, but in the Permian for instance, evaporite basins achieved scales of millions of square kilometres. At the opposite end of the spectrum, basins with a positive precipitation balance may be sites for organic rich muds and "estuarine circulation". Here, nutrient-rich waters converge from surface runoff and also inflow from the sea creating a "nutrient trap" and enhancing surface productivity, while low-density brackish surface waters contribute to stratification of the water column, in turn limiting oxygenation of the bottom waters. Upwelling water masses also contribute to high surface productivity through nutrient recycling and have been implicated in oil source rock formation and phosphorite deposition. We have developed an eight-fold classification of Present-day water masses of the oceans using particular salinity, temperature and productivity related effects. Our water mass base map is shown above, together with sediments reflecting these three parameters.
1. Wet Tropical. Water masses of the wet tropical zone are defined as having relatively brackish conditions with salinities below 32% (average surface salinity of the ocean is about 34.5%). Such areas typically have high runoff from nearby land with accompanying high turbidity. The plumes from major deltas, like the Amazon and Niger, fall in this category as do the shallow equatorial seaways typified by the Indonesian region where the rainfall associated with the Intertropical Convergence Zone (ITCZ) is concentrated. The Bay of Bengal brackish conditions extend over the deep ocean due to the high river runoff and low mixing in a regime of light wind activity (Tomczak and Godfrey, 1994). There is a rather vague correlation of peat with low salinity only for restricted seas like Indonesia. By contrast, peat along the eastern coast of Brazil is associated with normal to high salinity showing that, along narrow shelves, coastal precipitation has little influence on the salinity of the marine environment. Organic rich shales (Figure 3) seem not to be associated with the wet tropical water masses at the present time, but they almost certainly were in the geological past (e.g., the Permian of South China and the Jurassic of the Arabian Peninsula). We can surmise this because of the dual correlations of these source rocks with low palaeolatitudes, and of the general coincidence of the ITCZ with the equator today. Our view is that these geographic settings, if associated with restricted basins, would be productive because high rainfall would result in estuarine water masses.
2. Tropical. Tropical water masses are herein limited by the 20 degrees C winter isotherm (Briggs, 1974) and possess average salinities. Additional requirements are that salinity is normal and the water is clear due to a lack of sediment or plankton in the water column. In low latitudes, light penetration to the bottom can occur because the zenith angle of the sun is sufficient for refraction to the sea floor throughout the annual cycle (Ziegler et al., 1984). This warms the water and allows for bottom productivity and carbonate build-ups in areas of good circulation, due mainly to the profusion of calcareous secreting algae and hermatypic corals. Our reef distribution is mainly confined to the Tropical and the Dry Tropical water masses as defined. In our experience, this is true of the geological past, such that carbonate build-ups rarely plot above 35 degrees palaeolatitude for areas whose positions are well-constrained by palaeomagnetic data (Hulver et al., 1997). Non-tropical carbonates can be developed poleward of reef trends where terrigenous sediment dilution is insignificant (Nelson, 1988), but Bahamian-type carbonates with reefs, algal mats and oolite shoals are the characterising features of the Tropical water masses.
3. Dry Subtropical. Areas with salinities above 37% are under the influence of a strong negative precipitation minus evaporation balance and do possess evaporite deposits in coastal lagoons. Admittedly, such indicators also occur along upwelling coasts, and are not uncommon along coasts characterised as having Mediterranean or Savannah climates, so a generally negative precipitation situation is adequate for evaporites. A subdivision of the Dry Subtropical water mass with above 42% salinity is shown, but is only seen in the Kara Bogaz Gulf on the east side of the Caspian Sea. This subdivision is proposed mainly for the geological past when broad evaporative seas existed in the subtropical zone. Areas like the Red Sea are not suitable for oil source rock formation because the saline waters generated in shallow waters form density currents which convey oxygen to the depths of the basin.
4. Cool Subtropical. This category is reserved for upwelling zones which are mainly limited to the subtropical belt where the consistency of wind direction and strength is available to drive Ekman transport. Our maps show upwelling for opposite seasons and were compiled from a variety of information ranging from temperature anomalies to organic productivity (Binet and Marchal, 1993; Gordon, 1967; Ittekot et al., 1992; Parrish et al., 1983; Sharma, 1978; van Andel, 1964; Zijlstra and Baars, 1990). Many organic rich muds and phosphorites are associated with these upwelling zones, especially along the eastern boundary currents of the Atlantic and Pacific oceans. The Arabian Sea is an interesting case because organic rich muds and upwelling are common there, but do not generally overlap in map distribution. Here, the upwelling does generate high organic productivity, the decay of which consumes the oxygen in this region. This allows for preservation of organic matter on the sea floor, which is isolated from the deep oxygenated waters of polar origin by sills related to sea floor spreading.
5. Temperate. This category ranges from the 20 degrees C to the 0 degrees C isotherms and is simply reserved for the broad temperate seas with average salinity. There seems to be no particularly distinctive climate indicator of this zone, although peats are quite common due to the low evaporation rates in mid to high latitudes.
6. Wet Temperate. As in the case of the Wet Tropical, this category is defined by brackish conditions, with salinities below 32% and, in extreme conditions like the Baltic and Black Seas, by salinities below 27%. These conditions are especially well developed in geographically or bathymetrically restricted basins associated with high rainfall zones. Peats are characteristically developed around the margins and organic rich muds are common in the centre, again due to the estuarine circulation induced by the fresh water cap effect.
7. Cold Temperate. These water masses are defined by the extent of sea ice in the winter season and this corresponds closely with the 0 degrees C isotherm (Zwally et al., 1983; Parkinson et al., 1987). A high degree of asymmetry is observed in the Northern Hemisphere with respect to both Atlantic and Pacific Oceans. This is because the high pressure cells over North America and Eurasia drive poleward currents (like the Norwegian Current) on the west sides of the continents and the equator ward currents (Labrador Current) on the east sides of the continents. Cold Temperate ice-dominated seas reach low latitudes (45 degrees) on the east sides of the continents while the ice flows are confined above 80 degrees by the Norwegian current, even in winter. The seasonal variability in air and sea surface temperature is at a maximum along the east sides of continents due to outflow of cold continental air, causing a stressful existence for organisms living there. In the Southern Hemisphere the sea ice extends about 1000 km from Antarctica in the winter, but virtually disappears in the summer, making this an extremely variable environment (Zwally et al., 1983). Diamictites would be indicative of this environment in the fossil record.
8. Glacial. Permanent ice floes, like most of the Arctic Ocean, or the Ronne and Weddell Ice Shelves, constitute this environment. Tills could be expected along mountainous coasts of these regions. The Glacial and Cold Temperate Water Masses today are generated by outflow of cold air mainly from ice sheets, so the question arises as to their existence during periods of the geological past when continental scale ice domes were lacking. The situation in the Northern Hemisphere is complicated by the confined nature of the Arctic Ocean; winds cannot effectively disperse the ice on an annual basis as happens in the Southern Hemisphere. From the meteorological point of view, the northern continents together with the Arctic Ocean constitute a polar supercontinent, producing frigid climates atypical of the geological past (Ziegler, 1998).
