The Underwater Photic Environment of Cape Maclear, Lake Malawi: Comparison Between Rock- and Sand-Bottom Habitats and Implications for Cichlid Fish Vision

Lake Malawi boasts the highest diversity of freshwater fishes in the world. Nearshore sites are categorized according to their bottom substrate, rock or sand, and these habitats host divergent assemblages of cichlid fishes. Sexual selection driven by mate choice in cichlids led to spectacular diversification in male nuptial coloration. This suggests that the spectral radiance contrast of fish, the main determinant of visibility under water, plays a crucial role in cichlid visual communication. This study provides the first detailed description of underwater irradiance, radiance and beam attenuation at selected sites representing two major habitats in Lake Malawi. These quantities are essential for estimating radiance contrast and, thus, the constraints imposed on fish body coloration. Irradiance spectra in the sand habitat were shifted to longer wavelengths compared with those in the rock habitat. Beam attenuation in the sand habitat was higher than in the rock habitat. The effects of water depth, bottom depth and proximity to the lake bottom on radiometric quantities are discussed. The radiance contrast of targets exhibiting diffused and spectrally uniform reflectance depended on habitat type in deep water but not in shallow water. In deep water, radiance contrast of such targets was maximal at long wavelengths in the sand habitat and at short wavelengths in the rock habitat. Thus, to achieve conspicuousness, color patterns of rock-and sand-dwelling cichlids would be restricted to short and long wavelengths, respectively. This study provides a useful platform for the examination of cichlid visual communication.

Spectroscopy and detectability of liquid brines on mars

Recent geomorphological observations as well as chemical and thermodynamic studies demonstrate that liquid water should be stable today on the Martian surface at some times of the day. In Martian conditions, brines would be particularly more stable than pure water because salts can depress the freezing point and lower the evaporation rate of water. Despite this evidence, no clear spectral signature of liquid has been observed so far by the hyperspectral imaging spectrometers OMEGA and CRISM. However, past spectral analysis lacks a good characterization of brines׳ spectral signatures. This study thus aims to determine how liquid brines can be detected on Mars by spectroscopy. In this way, laboratory experiments were performed for reproducing hydration and dehydration cycles of various brines while measuring their spectral signatures. The resulting spectra first reveal a very similar spectral evolution for the various brine types and pure water, with the main difference observed at the end of the dehydration with the crystallization of various hydrated minerals from brines. The main characteristic of this spectral behavior is an important decoupling between the evolution of albedo and hydration bands depths. During most of the wetting/drying processes, spectra usually display a low albedo associated with shallow water absorption band depths. Strong water absorption band depth and high albedo are respectively only observed when the surface is very wet and when the surface is very dry. These experiments can thus explain why the currently active Martian features attributed to the action of a liquid are only associated with low albedo and very weak spectral signatures. Hydration experiments also reveal that deliquescence occurs easily even at low temperature and moderate soil water vapor pressure and could thus cause seasonal darkening on Mars. These experiments demonstrate that the absence of water absorptions in CRISM in the middle afternoon does not rule out water activity and suggest future spectral investigations to identify water on the Martian surface.

Constraints on barium isotope fractionation during aragonite precipitation by corals

We present a barium (Ba) isotope fractionation study of marine biogenic carbonates (aragonitic corals). The major aim is to provide first constraints on the Ba isotope fractionation between modern surface sea water and coral skele- ton. Mediterranean surface sea water was found to be enriched in the heavy Ba isotopes compared to previously reported values for marine open ocean authi- genic and terrestrial minerals. In aquarium experiments with a continuous sup- ply of Mediterranean surface water, the Ba isotopic composition of the bulk sample originating from cultured, aragonitic scleractinian corals (d137/134Ba between +0.16 +/- 0.12permil and +0.41 +/-0.12permil) were isotopically identical or lighter than that of the ambient Mediterranean surface sea water (d137/134Ba = +0.42 +/- 0.07permil, 2SD), which corresponds to an empirical maximum value of Ba isotope fractionation of D137/134Bacoral-seawater = -0.26 +/- 0.14permil at 25°C. This maximum Ba isotope fractionation is close and identical in direction to previous results from inorganic precipitation experiments with aragonite- structured pure BaCO3 (witherite). The variability in measured Ba concentrations of the cultured corals is at odds with a uniform distribution coefficient, DBa/Ca, thus indicating stronger vital effects on isotope than element discrimination. This observation supports the hypothesis that the Ba isotopic compositions of these corals do not result from simple equilibrium between the skeleton and the bulk sea water. Complementary coral samples from natural settings (tropical shallow-water corals from the Bahamas and Florida and cold- water corals from the Norwegian continental shelf) show an even wider range in d137/134Ba values (+0.14 +/- 0.08permil and +0.77 +/- 0.11permil), most probably due to additional spatial and/or temporal sea water heterogeneity, as indicated by recent publications.

Comparative carbon cycle dynamics of the present and last interglacial

Changes in temperature and carbon dioxide during glacial cycles recorded in Antarctic ice cores are tightly coupled. However, this relationship does not hold for interglacials. While climate cooled towards the end of both the last (Eemian) and present (Holocene) interglacials, CO₂ remained stable during the Eemian while rising in the Holocene. We identify and review twelve biogeochemical mechanisms of terrestrial (vegetation dynamics and CO₂ fertilization, land use, wild fire, accumulation of peat, changes in permafrost carbon, subaerial volcanic outgassing) and marine origin (changes in sea surface temperature, carbonate compensation to deglaciation and terrestrial biosphere regrowth, shallow-water carbonate sedimentation, changes in the soft tissue pump, and methane hydrates), which potentially may have contributed to the CO₂ dynamics during interglacials but which remain not well quantified. We use three Earth System Models (ESMs) of intermediate complexity to compare effects of selected mechanisms on the interglacial CO₂ and δ¹³ CO₂ changes, focusing on those with substantial potential impacts: namely carbonate sedimentation in shallow waters, peat growth, and (in the case of the Holocene) human land use. A set of specified carbon cycle forcings could qualitatively explain atmospheric CO₂ dynamics from 8ka BP to the pre-industrial. However, when applied to Eemian boundary conditions from 126 to 115 ka BP, the same set of forcings led to disagreement with the observed direction of CO₂ changes after 122 ka BP. This failure to simulate late-Eemian CO₂ dynamics could be a result of the imposed forcings such as prescribed CaCO₃ accumulation and/or an incorrect response of simulated terrestrial carbon to the surface cooling at the end of the interglacial. These experiments also reveal that key natural processes of interglacial CO₂ dynamics eshallow water CaCO₃ accumulation, peat and permafrost carbon dynamics are not well represented in the current ESMs. Global-scale modeling of these long-term carbon cycle components started only in the last decade, and uncertainty in parameterization of these mechanisms is a main limitation in the successful modeling of interglacial CO₂ dynamics.

Reproductive Biology of the Deep-Sea Polychaete Gorgoniapolynoe Caeciliae (Polynoidae), a Commensal Species Associated with Octocorals

Some aspects of the reproductive biology of the polychaete Gorgoniapolynoe caeciliae have been described for the first time. Gorgoniapolynoe caeciliae is a deep-sea commensal species associated with Candidella imbricala, all octocoral that populates the New England Seamount chain. Gorgoniapolynoe caeciliae is a dioccious species with an equal sex ratio and fertile segments throughout most of the adult body. The gonads of both sexes are associated with genital blood vessels emerging from the posterior surface of most intersegmental septa. In the female, oogenesis is intraovarian with oocytes being retained within the ovary until vitellogenesis is completed. The largest female examined contained over 3000 eggs with a maximum diameter of 80-90 mu m. In the male, the testes are repeated in numerous segments and consist of small clusters of spermatogonia, spermatocytes and early spermatids associated with the walls of the genital blood vessels. Early spermatids are shed into the coelom where they complete differentiation into mature ect-aquasperm with a spherical head (4 mu m), a small cap-like acrosome, and a short mid-piece with four mitochondria. Indirect evidence suggests that this species is an annual breeder that releases its gametes into seawater and produces a planktotrophic larva following fertilization. The reproductive biology of G. caeciliae is consistent with that of most other polynoids including many shallow water species suggesting that phylogenetic history strongly shapes its biology.

Age analysis, aragonite and high magnesium calcite content from different IODP Holes from Exp 310

The widespread occurrence of microbialites in the last deglacial reef frameworks (16-6 Ka BP) implies that the accurate study of their development patterns is of prime importance to unravel the evolution of reef architecture through time and to reconstruct the reef response to sea-level variations and environmental changes. The present study is based on the sedimentological and chronological analysis (14C AMS dating) of drill cores obtained during the IODP Expedition #310 "Tahiti Sea Level" on the successive terraces which typify the modern reef slopes from Tahiti. It provides a comprehensive data base to investigate the microbialite growth patterns (i.e. growth rates and habitats), to analyze their roles in reef frameworks and to reconstruct the evolution of the reef framework architecture during sea-level rise. The last deglacial reefs from Tahiti are composed of two distinctive biological communities: (1) the coralgal communities including seven assemblages characterized by various growth forms (branching, robust branching, massive, tabular and encrusting) that form the initial frameworks and (2) the microbial communities developed in the primary cavities of those frameworks, a few meters (1.5 to 6 m) below the living coral reef surface, where they heavily encrusted the coralgal assemblages to form microbialite crusts. The dating results demonstrate the occurrence of two distinctive generations of microbialites: the "reefal microbialites" which developed a few hundred years after coralgal communities in shallow-water environments, whereas the "slope microbialites" grew a few thousands of years later in significantly deeper water conditions after the demise of coralgal communities. The development of microbialites was controlled by the volume and the shape of the primary cavities of the initial reef frameworks determined by the morphology and the packing of coral colonies. The most widespread microbialite development occurred in frameworks dominated by branching, thin encrusting, tabular and robust branching coral colonies which built loose and open frameworks typified by a high porosity (> 50%). In contrast, their growth was minimal in compact coral frameworks formed by massive and thick encrusting corals where primary cavities yielded a low porosity (~ 30%) and could not host a significant microbialite expansion.

Geochemistry and mineralogy at DSDP Holes 61-462, 61-462A and 33-315A

Sedimentary rocks of Barremian through early Maestrichtian age recovered on Deep Sea Drilling Project Leg 61 had their principal source in the complex of igneous rocks with which they are interlayered in the Nauru Basin. Relict textures and primary sedimentary structures show these Cretaceous sediments to be of hyaloclastic origin, in part reworked and redeposited by slumps and currents. The dominant composition now is smectite, but locally iron, titanium, and manganese oxides, plagioclase, pyroxene, analcime, clinoptilolite, chalcedonic quartz, cristobalite, amphibole, nontronite, celadonite, and pyrite are also present. The mineral assemblages and the geochemistry reflect the original basaltic composition and its subsequent alteration by one or more processes of submarine weathering, authigenesis, hydrothermal circulation, and contact metamorphism. Hyaloclastitic sandstone, siltstone, and breccia within the sheet flows below 729 meters sub-bottom depth have Barremian fossils, thus establishing the age of the lower, or extrusive, complex of post-ridge-crest volcanism. Similar hyaloclastites between 564 and 729 meters are invaded by hypabyssal sills of the upper igneous complex, and fossil ages of Albian or Cenomanian set an older limit to the age of that second post-ridge-crest episode. Cenomanian to early Campanian sedimentary rocks between 490 and 564 meters have a substantial contribution of clays of submarine-weathered-basalt origin, as well as hydrothermal and pelagic components. The interval of reworked hyaloclastitic siltstone, sandstone, and breccias between 450 and 490 meters is of late Campanian and early Maestrichtian age. These sediments probably formed from glassy basalt that fragmented upon eruption nearby, when sills were being emplaced. In addition to pelagic elements, these Upper Cretaceous volcanogenic sediments include redeposited material of shallow-water origin, apparently derived from the Marshall Islands.

Geochemistry of manganese nodules from a British Columbia fjord

Concretions of iron and manganese oxides and hydrous oxidesóobjects commonly called manganese nodulesóare widely distributed not only on the deep-sea floor but also in shallow marine environments1. Such concretions were not known to occur north of Cape Mendocino in the shallow water zones bordering the North-East Pacific Ocean until the summer of 1966 when they were recovered by one of us (J. W. M.) in dredge samples from Jervis Inlet, a fjord approximately 50 miles north-west of Vancouver, British Columbia.