The Paleoecology of the Ostracodes of DSDP Leg 42A

Abstract

Twenty-three ostracode species, identified from 652 specimens, were found in 61 samples from seven of the eight sites of Leg 42A. Most of the samples were core fractions 5 cm long (50 cc). Eleven additional samples contained no ostracodes; these samples were generally concentrated in suspected azoic horizons. The highest rate of specimen recovery was in the Cyprideis Zone (Pannonian-Messinian), especially in the eastern Mediterranean (Site 376), where restricted marine conditions also produced the fewest ostracodes both before and after the late Miocene crisis. Site 372, on the west flank of the Algero-Provencal Basin, represents the most complete cored section in the western Mediterranean. It also yielded the most complete record of change in the marine ostracode faunas (24 samples from Burdigalian through Piacenzian). Only the Pleistocene and the lowermost Pliocene samples from this site (1-3, 51-57 cm and 4-2, 50-56 cm) contain no ostracodes. The post-salinity crisis fauna was best represented in the western Mediterranean at Site 371 and in the eastern Mediterranean at Site 376. Compared with the results of the ostracode study of Leg 13 (Benson, 1973b), which had one-fourth as many samples, yielding one-sixth as many specimens from nine species, the results of this report are considered reliable. Their value is best expressed in terms of the broader changes they reflect in bottom-water mass conditions that took place in the Mediterranean basins before and after the Messinian. Unfortunately, because the samples required were too large or the faunas poor, their poorest resolution is in representing the rapid events that occurred immediately before and after the salinity crisis; in these cases faunas from outcrops, such as Falconara in Sicily, can be examined for comparison. The results of this study can be expressed in two parts: those confirming the marine changes that affected the western and the eastern deep basins, which can be demonstrated to have existed throughout the record represented; and the Paratethyan influences during which produced the most abundant fossil record. In my opinion, the most important discovery of this study was the occurrence of a typical deep-sea Atlantic and World Ocean ostracode species {Bradleya dictyori) in the middle Miocene of Site 372. This is not the first time psychrospheric ostracodes have been found in the middle Miocene of the central or western Mediterranean (Benson, 1973a; Serravallian of Sicily). It confirms the existence of deep basins (estimated depth 1500 m or more) before the crisis, deeper oceanic influences than known before, and indicates further that the limiting threshold (Iberian Portal) was effective enough to sustain this fauna for more than 10 million years. For those who propound the deep-basin desiccation model (Ryan, Hsu, et al., 1972; Cita, 1973), the presence of Cyprideis faunas (in situ and well represented), in nine Messinian samples from Site 376 on the Florence Rise just west of Cyprus, provides further evidence of continental influence over marine influence (the existence of lac mer or caspibrackish conditions). Yet this evidence does not resolve the problem of the water depth in the basins. Some basins could have been deep and some shallow. Where it is known in the live state, Cyprideis is a moderately shallow water euryhaline ostracode. Whether it could have existed in well-ventilated brackish waters several hundred meters deep is simply not known. What is known is that when Cyprideis lives in shallow waters of moderate salinity ranges (near normal marine), it is almost always found with other well-known euryhaline marine ostracodes; this is not so in the Messinian faunas of the cores described in this report. That its salinity range also includes normal marine conditions is very weak evidence for the entrance of seawater into the Mediterranean during the Messinian. If the faunas of the clays between the evaporites represent the source of waters (brines?) containing the salts, they probably came from the vast continental lakes of Paratethys. No indigenous Atlantic, Red Sea, or Indian Ocean shallow-water, marine euryhaline ostracodes have yet been found or recognized in Mediterranean Messinian (post-marine) deposits. If the brines came from these sources, they were sterilized of these species (along the bottom, since these are benthic species) on their way into the basins. The presence of Paratethyan ostracodes may not indicate that Paratethys flowed into the Mediterranean basins. It is possible that Cyprideis can be flown in by birds (suggesting that at least the first invaders came in as shallow-water animals). However, it is easier to explain a massive faunal invasion by a direct connection, and perhaps this can be found. Marine ostracodes may be used as depth indicators, but water depth is not the only ecological variable affecting their distribution. Rapid temperature change is the most formidable gradient, after that of salinity change, for species to cross. Depth is of less ecologic