Individual Variation in the Covering Behaviour of the Shallow Water Sea Urchin Paracentrotus lividus

Abstract. A number of recent field‐based studies have investigated the possible functions of covering behaviour in a range of echinoid species. Some of these have found important relationships between covering behaviour and environmental factors, such as light (in particular, ultra‐violet light) and covering item availability. However, these studies have often shown considerable within‐species variation, thus making covering functionality both within and between species difficult to interpret. The present study experimentally investigates individual variability in covering behaviour characteristics of the sea urchin Paracentrotus lividus (Lamarck). Data were collected bimonthly over a 1‐year period at Lough Hyne, Ireland, by observing P. lividus in outdoor aquaria. Significant variations in covering behaviour characteristics were observed as a function of individual size and mobility but not as a function of sex. A significant relationship between water temperature, mobility and covering behaviour was also found and is hypothesized to have implications for grazing activity in P. lividus. In contrast to previous field studies of P. lividus at Lough Hyne, no summer ‘peak’ in the proportion of individuals covering was observed. However, there was a significant relationship between local solar radiation and ‘time to first cover’. Overall, these data highlight (i) the ecological significance of individual variation in P. lividus covering behaviour and (ii) the importance of investigating multiple factors as potential determinants of echinoid covering behaviour.     

Structural analysis of the segmentation of the Central Indian Ridge between 20°30′S and 25°30′S (Rodriguez Triple Junction)

The morphological characteristics of the segmentation of the Central Indian Ridge (CIR) from the Indian Ocean Triple Junction (25°30S) to the Egeria Transform Fault system (20°30S) are analyzed. The compilation of Sea Beam data from R/VSonne cruises SO43 and SO52, and R/VCharcot cruises Rodriguez 1 and 2 provides an almost continuous bathymetric coverage of a 450-km-long section of the ridge axis. The bathymetric data are combined with a GLORIA side-scan sonar swath to visualize the fabric of the ridge and complement the coverage in some areas. This section of the CIR has a full spreading rate of about 50 mm yr^–1, increasing slightly from north to south. The morphology of the CIR is generally similar to that of a slow-spreading center, despite an intermediate spreading rate at these latitudes. The axis is marked by an axial valley 5–35 km wide and 500–1800 m deep, sometimes exhibiting a 100–600 m-high neovolcanic ridge. It is offset by only one 40km offset transform fault (at 22°40S), and by nine second-order discontinuities, with offsets varying from 4 to 21 km, separating segments 28 to 85 km long. The bathymetry analysis and an empirical orthogonal function analysis performed on across-axis profiles reveal morphologic variations in the axis and the second-order discontinuities. The ridge axis deepens and the relief across the axial valley increases from north to south. The discontinuities observed south of 22°S all have morphologies similar to those of the slow-spreading Mid-Atlantic Ridge. North of 22°S, two discontinuities have map geometries that have not been observed previously on slow-spreading ridges. The axial valleys overlap, and their tips curve toward the adjacent segment. The overlap distance is 2 to 4 times greater than the offset. Based on these characteristics, these discontinuities resemble overlapping spreading centers (OSCs) described on the fast-spreading EPR. The evolution of one such discontinuity appears to decapitate a nearby segment, as observed for the evolution of some OSCs on the EPR. These morphological variations of the CIR axis may be explained by an increase in the crustal thickness in the north of the study area relative to the Triple Junction area. Variations in crustal thickness could be related to a broad bathymetric anomaly centered at 19°S, 65°E, which probably reflects the effect of the nearby Réunion hotspot, or an anomaly in the composition of the mantle beneath the ridge near 19°S. Other explanations for the morphological variations include the termination of the CIR at the Rodriguez Triple Junction or the kinematic evolution of the triple junction and its resultant lengthening of the CIR. These latter effects are more likely to account for the axial morphology near the Triple Junction than for the long-wavelength morphological variation.     

The effect of oceanographic variability and interspecific competition on juvenile pollock (Theragra chalcogramma) and capelin (Mallotus villosus) distributions on the Gulf of Alaska shelf

Results from this study suggest that small-scale variability in the Alaska Coastal Current (ACC) and competition between juvenile pollock and capelin are potential mechanisms affecting the distribution and abundance of fishes in the Gulf of Alaska (GOA). Fish distributions in Barnabus Trough, off the east coast of Kodiak Island, were assessed using acoustic data collected with a calibrated echosounder during August–September 2002 and 2004. Trawl hauls were conducted to determine the species composition of the fish making up the acoustic backscatter. Oceanographic data were collected from moorings, conductivity–temperature–depth (CTD) probes, trawl-mounted microbathythermographs (MBT) and expendable bathythermographs (XBT). National Centers for Environmental Prediction (NCEP) reanalysis data were used to assess area winds, and information on regional transport was derived from current meters deployed on moorings north and south of Kodiak Island. The distribution of water-mass properties and fish during 2002 showed variability at the temporal scale of weeks. Juvenile pollock (age-1 and age-2) were initially most abundant in warm, low-salinity water on the inner shelf, whereas capelin were distributed primarily on the outer shelf in cool, high-salinity waters. During a 2-week period juvenile pollock distribution expanded with the offshore expansion of warm, low-salinity water, and capelin abundance in outer-shelf waters decreased. We hypothesize that wind-driven pulsing of the ACC resulted in increased transport of warm, low-salinity water through the study area. In 2004, warm, low-salinity water characterized the inner shelf and cool, high-salinity water was found on the outer shelf. However, the distribution of water-mass properties did not show the weekly scale variability observed in 2002. Area winds were consistently toward the southwest during 2004, such that we would not expect to see the wind-driven pulsing of ACC water that occurred in 2002. Age-1 and age-2 pollock were not observed in Barnabus Trough in 2004. Instead, the midwater acoustic backscatter was composed of capelin mixed with age-0 pollock, and these capelin were not restricted to the outer-shelf waters, but were found primarily in warm, low-salinity inner-shelf waters that had been previously occupied exclusively by age-1 and age-2 pollock. We suggest that this is consistent with inner-shelf waters being preferred foraging habitat for juvenile pollock and capelin. Further study of the mechanisms linking climate change with variability in the ACC is needed, as are studies of the potential for competition between juvenile pollock and capelin.     

Coupling parameters of the MIT OBS at two nearshore sites

A model representing the coupling of an ocean-bottom seismometer (OBS) to the seafloor as a mass-spring-dashpot system satisfactorily explains the results of transient tests performed on different instruments during the Lopez Island intercomparison test. In this paper, we compare the results obtained for the MIT OBS at Lopez Island to results from similar tests at a dockside site at Woods Hole, Massachusetts. The vertical instrument response at the Lopez Island site shows a highly damped resonance at a frequency of 22 Hz, whereas the response at the Woods Hole site shows a marked resonance at 13 Hz. The difference between the responses at the two sites can be qualitatively attributed to the difference between the surficial sediments.     

Volcanic stratigraphy of DSDP/ODP Hole 395A: An interpretation using well-logging data

Deep Sea Drilling Project/Ocean Drilling Program Hole 395A was drilled approximately 500 m deep into young oceanic crust west of the Mid-Atlantic Ridge. Core recovery is very poor in this hole and therefore continuous downhole measurements are important to understand the drilled lithology. Geophysical downhole measurements were carried out during several cruises. A new set of logs was recorded during Leg 174B in summer 1997. The new logging data show a significant improvement in data quality compared to older measurements from Leg 109. The lithostratigraphy established from cores gives only limited information because of the poor core recovery in this hole. The gaps in the core lithostratigraphy are filled by reconstructing a synthetic lithological profile using the standard well-logging data. Three types of lava morphologies, massive basalts, altered lava flows, and pillow basalts, may be distinguished using the logs because the lava morphologies show differences in their physical properties due to differences in fracturing and alteration. The synthetic lithological profile gives a more detailed and precise vertical definition of single layers than the core profile. The integration of further logging and core data enables a detailed reconstruction of the accretion history at the drill site. Cyclic, upward decreasing trends in the resistivity logs were already observed during earlier cruises and were referred to magmatic cycles. Similar trends occur in the density log and, inversely, in the total gamma ray log. The trends reflect gradual changes in fracturing, porosity, permeability, and alteration and cover depth intervals of several tens of meters. Boundaries between cycles are interpreted to correspond to periods of volcanic quiescence. Two types of boundaries may be identified. Boundaries correlating with reversals in the magnetic field and/or changes in the geochemical composition of the basalts are interpreted as long pauses. Basalts separated by these boundaries were probably fed by separate magma reservoirs. Boundaries identified only by changes in alteration but not in geochemistry are interpreted to represent shorter pauses. They separate basalts that were probably fed by the same magma chamber. This revised version was published online in November 2006 with corrections to the Cover Date.     

The evolution and orientation of early cycle 22 active regions

The evolution of six major active regions which appeared during the first phase of the present solar cycle (cycle 22) has been studied. It was found that the northern hemisphere regions exhibited a broad range of evolutionary behavior in which the commonly accepted normal pattern (whereby the follower flux moves preferentially polewards ahead of the leader flux) is represented only at one end of the range. At the other end of the range, the leader flux is displaced polewards of the follower flux. In the latter cases equatorward extensions of the polar coronal hole are noted.While it is emphasized that some of the regions in this study follow the more conventional pattern and that all regions in this study emerged during the early phase of cycle 22, the implications for theories of the solar polar field reversals are noted.     

Geochemical fluxes and weathering of volcanic terrains on high standing islands: Taranaki and Manawatu-Wanganui regions of New Zealand

Sediment fluxes from high standing oceanic islands (HSIs) such as New Zealand are some of the highest known [Milliman J. D. and Syvitski J. P. M. (1992) Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. J. Geol.100, 525-544]. Recent geochemical work has suggested that along with their extremely high physical weathering yields, many New Zealand watersheds also have very high chemical weathering yields. In New Zealand, the magnitude of both the physical and chemical weathering yields is related to the lithology of the watershed. Most of the previous work on this topic has been undertaken in Southern Alps watersheds of schist and greywacke and in East Cape watersheds of semi-consolidated marine sediments and greywacke. We recently sampled North Island watersheds in the Taranaki and Manawatu-Wanganui regions which have been subjected to volcanism since the Miocene. We sampled watersheds that contain both volcanic and sedimentary rocks. A series of water and sediment samples was collected and analyzed for major, minor and trace elements. This was done to quantify the weathering intensities in the watersheds and to establish the relationship between physical and chemical weathering yields in volcanic lithologies. Our results reveal distinct chemical signatures for the different regions. Waters draining the Taranaki region volcanics are significantly enriched in K⁺, and depleted in Ca²⁺ and Sr²⁺ compared to waters draining the Manawatu-Wanganui region volcanics, which also traverse expanses of sedimentary siltstones and mudstones. The Ca²⁺ and Sr²⁺ depletions may reflect the relative absence of CaCO₃ in the Taranaki region watersheds. In addition, sediment samples from the Taranaki region show significant enrichment in Ti, Al, Ca, Fe, Mn, Mg, Ca, and P and depletion in Si and Rb compared to those of the Manawatu-Wanganui region. From total dissolved solids concentrations and mean annual water discharge, we calculate chemical weathering yields of 60-240 tons km⁻² a⁻¹. These weathering yields fall within the middle to upper range of those previously documented for the Southern Alps (93-480 tons km⁻² a⁻¹) and East Cape (62-400 tons km⁻² a⁻¹). Calculated silicate weathering yields of 12-33.6 tons km⁻² a⁻¹ and CO₂ consumption of 852-2390 × 10³ mol km⁻² a⁻¹ for the rivers draining the Taranaki volcanic region are higher than those previously reported for watersheds hosted in sedimentary and metamorphosed rock terrains on HSIs. CO₂ consumption is found to be within the range previously measured for the basaltic terrains of the Deccan Traps (580-2450 × 10³ mol km⁻² a⁻¹) and Réunion Island(1300-4400 × 10³ mol km⁻² a⁻¹). Our calculated chemical weathering yields demonstrate the importance of HSIs, particularly those with volcanic terrains, when considering global geochemical fluxes.