Petrographic and chemical characteristics of abyssal tholeiites at DSDP Leg 63 Holes

Tholeiitic basalts were obtained from basaltic basement ranging in age from 6 to 17 m.y. on IPOD/DSDP Leg 63. The main rock types encountered at all sites but 473 are basaltic pillow lavas. Although many of these pillow basalts are highly or moderately altered, fresh glass is usually present. At Site 473, we recovered coarse-grained, massive basalts; no clearly defined pillowed forms were observed. Phenocrysts or microphenocrysts present in the Leg 63 basalts are Plagioclase and clinopyroxene at Site 469; olivine, Plagioclase, and spinel at Site 470; and olivine, Plagioclase, and clinopyroxene at Sites 472 and 473. Olivines of the basalts from Holes 470A and 472 (Fo85-88) are generally more magnesian than those of the Hole 473 basalts (Fo77-81). Also, plagioclases of Holes 470A and 472 basalts (An70-85) are generally more calcic than those of Holes 469 and 473 basalts (An66-72). Geochemical study of the Leg 63 basalts indicates that in all cases they are large-ion-lithophile (LIL) element depleted tholeiites like typical abyssal tholeiites. In particular, they are very similar in composition to those described from the eastern Pacific, although the degree of iron enrichment found in the Leg 63 basalts is not as extensive as in basalts from the Galapagos spreading center. Hence, the geochemical evidence of the Leg 63 basalts is compatible with their formation at a spreading center. Compositional variations in Leg 63 basalts from any single drill hole is small. Major and trace element data indicate that the samples from Holes 469 and 473 are more fractionated in chemical composition than are the samples from Holes 470A and 472; this compositional variation may be largely ascribed to differences in the extent of shallow-level fractional crystallization of similar parental magma. The Hole 472 samples, however, show a LIL element character distinct from the other Leg 63 samples.

Temperature and photoperiod effects on sex determination in Leuresthes tenuis (fish)

In some gonochoristic species, sex is influenced not only by genotype at conception but also by the environment that offspring experience during early ontogeny (termed environmental sex determination or ESD). ESD is thought to be adaptive when seasonal variations in environmental conditions provide a sex-specific fitness advantage. In vertebrates, temperature is the most common determinant of sex, and seasonal variation in temperature serves as a temporal cue of environmental quality such as length of the growing season. Some environments, however, lack strong seasonal temperature fluctuations and other cues, particularly photoperiod, may provide a more reliable indicator of the environment offspring enter. We tested this hypothesis by rearing the offspring of the California grunion (Leuresthes tenuis, Ayres), which experiences low seasonal temperature variation in nature, under common garden conditions at three temperature and two photoperiod treatments. Our experiments revealed that both temperature and photoperiod significantly affected sex ratios in L. tenuis. More females were produced at cooler temperatures and longer day lengths, which is consistent with female biased sex ratios early in the breeding season, and likely adaptive through increased female size and fecundity. To our knowledge, this is the first documented case of photoperiod-dependent sex determination in a gonochoristic vertebrate.