Pat Wilde

Turbidity currents generated by Hurricane Iwa

Off southwest Oahu, Hawaii, an array of current sensors recorded four successive episodes of downslope displacement associated with high-speed near-bottom currents of up to 200 cm/s and elevated water temperatures. These episodes coincided with the maximum storm effects of hurricane Iwa. Sensors from four moorings recorded increases in depth of as much as 220 m, implying downslope movement of as much as 2.4 km at speeds up to 300 cm/s. A succession of slope failures at or above the 110-m shelf break, each resulting in a turbidity current event, is the favored explanation.

Iridium Abundances Across the Ordovician-Silurian Stratotype

Chemostratigraphic analyses in the Ordovician-Silurian boundary stratotype section, bracketing a major extinction event in the graptolitic shale section at Dobs Linn, Scotland, show persistently high iridium concentrations of 0.050 to 0.250 parts per billion. There is no iridiumn concentration spike in the boundary interval or elsewhere in the 13 graptolite zones examined encompassing about 20 million years. Iridium correlated with chromium, both elements showing a gradual decrease with time into the middle part of the Lower Silurian. The chromium-iridium ratio averages about 106. Paleogeographic and geologic reconstructions coupled with the occurrence of ophiolites and other deep crustal rocks in the source area suggest that the high iridium and chromium concentrations observed in the shales result from terrestrial erosion of exposed upper mantle ultramafic rocks rather than from a cataclysmic extraterrestrial event.

Palaeo-oceanography and biogeography in the Tremadoc (Ordovician) Iapetus Ocean and the origin of the chemostratigraphy of Dictyonema flabelliforme black shales

High concentrations of vanadium, molybdenum, uranium, arsenic, antimony with low concentrations of manganese, iron and cobalt heretofore restricted to Dictyonema flabelliforme-bearing Tremadoc black shales in Balto-Scandia, have been found in coeval black shales in the Saint John, New Brunswick area. Prior palaeogeographic reconstructions place these areas about 400 km. apart in high southern latitudes in the Iapetus Ocean, with New Brunswick in proximity to Avalonia (southeastern Newfoundland). These geochemical similarities are not found in coeval Tremadoc black shales of Bolivia, New York, Quebec, Wales, and Belgium. Palaeo-oceanographic reconstructions of Iapetus support the proximity of Balto-Scandia and the Saint John area during the early Tremadoc and Geesx (1981) suggestion that the signature is a feature of eastern Iapetus. Furthermore, first-order modelling of the major surface currents and related primary productivity in the Tremadoc Iapetus Ocean explain the apparent wide latitudinal range of D. flabelliforme (Fortey, 1984) and the anomalous trace metal content of certain black shales of that time. Variations in the elemental content of these black shales is produced by oceanographic and geologic conditions unique to the geographic site. The distinctive Balto-Scandic geochemical signature resulted from the coincidence of anoxic waters transgressing the shelf at latitudes of high organic productivity at the polar Ekman planetary divergence. This produces the conditions for concentrations of V, U, and Mo in the shales. Metal enriched anoxic bottom waters produced by leaching of volcanics or through hydrothermal activity may be the source of the other enhanced signature elements such as As and Sb. The absence of this geochemical signature in younger non-D. flabelliforme Tremadoc and later black shales in Balto-Scandia and other areas suggests that the closing of Iapetus moved the depositional sites into less productive oceanic areas.

Monterey Fan, Pacific Ocean

Monterey Fan is the largest modern fan off the California shore. Two main submarine canyon systems feed it via a complex pattern of fan valleys and channels. The northern Ascension Canyon system is relatively inactive during high sea-level periods. In contrast, Monterey Canyon and its tributaries to the south cut across the shelf and remain active during high sea level. Deposition on the upper fan is controlled primarily by the relative activity within these two canyon systems. Deposition over the rest of the fan is controlled by the oceanic crust topography, resulting in an irregular fan shape and periodic major shifts in the locus of deposition.

Monterey Fan: Growth pattern control by basin morphology and changing sea levels

Monterey Fan is the largest modern fan off the California shore. Two main submarine canyon systems feed it via a complex pattern of fan valleys and channels. The northern Ascension Canyon system is relatively inactive during high sea-level periods. In contrast, Monterey Canyon and its tributaries to the south cut across the shelf and remain active during high sea level. Deposition on the upper fan is controlled primarily by the relative activity within these two canyon systems. Deposition over the rest of the fan is controlled by the oceanic crust topography, resulting in an irregular fan shape and periodic major shifts in the locus of deposition.

Kōko seamount: A major guyot at the southern end of the Emperor seamounts

Abstract Kōko seamount is an exceptionally large guyot situated at 35°30′N, 171°45′E, at the southern end of the Emperor seamount chain. An accurate survey of Kōko seamount and two successful dredge hauls from near its summit were made by R/V “Thomas Washington” in August 1971. The dredge hauls included samples of a wide variety of unusual igneous rocks and coral reef debris. The significance of the occurrence of coral reef debris capping a guyot which is now both too deep and too far north for active reef growth is discussed briefly.

Electrochemical Meter for Activity Measurements in Natural Environments

An electrochemical meter designed for measurements in the marine environment but equally useful in other natural environments is described. Some features of the instrument are (1) all input channels are high impedance; (2) up to 11 reading electrodes can be monitored from one reference electrode; (3) voltages are measured with respect to solution ground; (4) a null balance circuit option; (5) an accuracy of better than 1 mV. With suitable operative ion‐selective electrodes, the instrument can monitor (a) operational activities, (b) activity ratios, (c) solubility products, or (d) states of saturation.

Potential Petroleum Reservoirs on Deep-Sea Fans off Central California

Evaluation of the petroleum potential of the Monterey and Delgada submarine fans based on single-channel seismic reflection profiles and surficial sediment data indicates two high, six good, and four moderate prospects. Sediment thicknesses in a northwest-southeast basement trough at the base of the continental slope generally exceed 1 km; within internal depressions thicknesses can reach 2 to 3 km. Organic carbon contents up to 1.3 weight percent occur on the upper fan. An average heat flow of 2 HFU suggests the occurrence of a thermal gradient sufficient to exceed catagenic conditions within the trough if potential source beds are old enough.

Quantitative measurements of deep-sea channels on the Cocos Ridge, East Central Pacific

Abstract Precision depth records taken during Criss-Cross Expedition 1963 show deep-sea channels, about 1 km wide, at depths of 1400 to 2800 m on the Cocos submarine ridge. The channels are near the thalwegs of broad (about 30 km wide) submarine valleys, which trend normal to the long axis of the ridge. A closed depression at the landward end of the ridge and a central saddle prevent continental material from being transported by bottom currents into the region of the channels. The currents that produce the channels carry only oceanic sediments. Local turbidity currents, laden with ash, erupted from the numerous submarine volcanoes on the ridge, possibly carve the channels. Hydraulic functions (Leopold and Maddock, 1953) for the channels calculated at bankfall stages are : (1) W = 11·2 Q ·39 ; (2) D = 0·.43 Q 0·32 : and (3) V = o·21 Q 0·29 . The hydraulic functions show that in a downslope direction the channel depth is maintained at the expense of the width, which suggest a cchannel flow that (1) has its sediment load concentrated near the base, but (2) deposits chiefly on its flanks probably by overbank spillage.

CATION EXCHANGE CAPACITY OF SUSPENDED MATERIAL FROM COASTAL SEA WATER OFF CENTRAL CALIFORNIA

Carter, R. C. and Wilde, P., 1972. Cation exchange capacity of suspended material from coastal sea water off central California. Mar. Geol., 13: 107-122. The cation exchange capacity (CEC) of suspended particles (> 0.5 ~) in sea water is measured by a described methylene blue (MB) absorption method which has the following characteristics: sample volume: minimum of 100 ml sea water; time of measurement: about 10 rain; useful range: 0.1-10.0 ~e/~; precision: -+ 0.05 ~te/~ at 0.7 ~e/~2; accuracy: function of organics and clay types. Test of the MB method with sea water samples from the central California off-shore show background values of about 1 ~te/~ with higher values up to 5 ~e/~ in the vicinity of stream discharge.