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.

Isotopic survey of diagenetic carbonates, at DSDP Leg 63 Holes

Authigenic carbonates, principally calcium-rich dolomites, with extremely variable isotopic compositions were recovered in organic-rich marine sediments during Leg 63 drilling off southern California and Baja California. These carbonates occur as thin layers in fine-grained, diatomaceous sediments and siliceous rocks, mostly deposited during the Neogene. A combination of textural, geochemical, and isotopic evidence indicates these dolomites formed as cements and precipitates in shallow subsurface zones of high alkalinity spawned by abundant CO2 and methane production during progressive microbial decay of organic matter. Depths and approximate temperatures of formation estimated from oxygen isotopes are 87 to 658 meters and 10°C to 50°C, respectively. Within any sedimentary section, dolomites may form simultaneously at several depths or at different times within the same interval. Highly variable carbon isotopes (-30 to +16 per mil) reflect the isotopic reservoir in which the carbonates formed. Oxidation of organic matter through microbial reduction of sulfate at shallow depths favors light-carbon carbonates such as those at Sites 468 and 471; heavy-carbon carbonates at Site 467 most likely formed below this zone where HC**12O3**- is preferentially removed by reduction of CO2 to methane during methanogenesis. An important controlling factor is the sedimentation rate, which dictates both the preservation of organic matter on the sea floor and depth distribution of subsurface zones of organic-matter decay.

Geochemistry and minerals of glass shards at DSDP Leg 66 Holes

Deep Sea Drilling Project Leg 66 drilled eight sites along a transect across the Middle America Trench off Mexico, including continental (Sites 493 and 489), oceanic (Site 487), and trench (Site 486) reference sites and four sites (490, 492, 491, 488) in the trench inner wall. Because of their location - close to volcanic sources and subject to prevailing winds and marine currents (N to S, NW to SE) - analysis of airborne ashes intercalated within the sediments at these sites provides a reliable record of explosive volcanism in the area. Intense onshore volcanic activity in Mexico during the Oligo-Miocene has been well documented by the andesites and ignimbrites of the Sierra Madre Occidental and Sierra Madre del Sur and in the Plio-Quaternary by the andesites and basalts from the Trans-Mexican Neovolcanic Belt and the eastern border of Baja California.