Adjacent Ocean Basins Research Articles

Structure and tectonic history of the Southern Kerguelen Plateau (Indian Ocean) deduced from seismic reflection data

Early single‐channel and recent multichannel seismic reflection data together reveal major tectonic events in the history of the Southern Kerguelen Plateau (SKP). The SKP was created by Early Cretaceous volcanism which was mainly subaerial. We estimate the age of the adjacent ocean basins to be similar to that of the plateau. Following termination of the main volcanism, the SKP began to subside and to accumulate sediment. A lower sedimentary megasequence was deposited at this time, primarily in depressions on the plateau. At about 88 Ma, tectonism affected the eastern Raggatt Basin; this tectonism may be related to faulting which created the steep eastern margin of the SKP. At about 72 Ma, predominant tectonic activity, characterized by widespread extension and uplift forming the tilted block morphology, occurred over large areas of the SKP. The extension centered along several NW‐SE trending rift systems, in places creating well‐preserved axial rifts. The largest structure of this kind, the Central SKP Uplift, lies along the center of the SKP; its relief increases from the southernmost part of the plateau toward the NW. The northernmost part of this uplift, the Banzare Bank, was elevated during rifting above sea level and was eroded. A second extensional structure, the SW Uplift, lies in the SW corner of the plateau, and other, smaller structures may also be present. The extension appears to have culminated in the initiation of the Southeast Indian Ridge at 43 Ma, but, at least in the Raggatt Basin, the two events were not continuous. Tectonic subsidence that was associated with the extension corresponds to deposition of a second megasequence, estimated to be about 1000 m thick on Banzare Bank. The subparallel trend of the rift systems on the SKP to the Southeast Indian Ridge and their timing suggest that the rearrangement of spreading in the South Indian Ocean at 43 Ma was not solely the result of the collision of India with Asia; it started earlier in association with other plate motions in the area.

Zircon fission track age for middle Pleistocene Rangitawa Tephra, New Zealand: stratigraphic and paleoclimatic significance

Rangitawa Tephra is an important stratigraphic marker in mid-Pleistocene marine and terrestrial sequences in New Zealand and adjacent ocean basins. Zircon fission track ages (ZFTA) on Rangitawa Tephra from five sites in the southern North Island yield mean site ages in the range 0.34 to 0.40 Ma with a weighted mean of 0.35±0.04 Ma (1 σ). On the basis of glass shard major-element chemistry, ferromagnesian mineralogy, ZFTA and similarity of paleomagnetic dates of proposed tephra correlatives in deep-sea cores, it is concluded that Rangitawa Tephra represents a major eruptive event in the Taupo Volcanic Zone most probably associated with eruption of the Whakamaru-group ignimbrites (0.35−0.39 Ma) or less likely the Paeroa Range Group Ignimbrites (0.36–0.38 Ma). Pollen analyses from two onshore sites, together with regional loess stratigraphy, show that Rangitawa Tephra was erupted during a glacial period. The ZFTA and previously reported oxygen isotope data from DSDP Site 594 indicate that Rangitawa Tephra was erupted near the end of oxygen isotope stage 10.

A geophysical study of the Hatton Bank volcanic margin: a summary of the results from a combined Seismic, gravity and magnetic experiment

This paper summarizes the results of a geophysical study of the Hatton Bank volcanic margin in the North Atlantic. Normal incidence and synthetic aperture profiles (SAPs) show seaward-dipping reflectors, interpreted as subaerial basalt flows, on the continental slope and in the adjacent ocean basin. Velocity-depth profiles, derived from expanding spread profiles (ESPs) shot along strike, show elevated velocities in the lower crust on the continental slope; the high velocities are interpreted as evidence for igneous underplating or intrusion. Gravity modelling shows high densities in the lower crust, supporting a model of igneous accretion. An ocean bottom Seismometer (OBS) and multichannel refraction line, shot perpendicularly to strike through the midpoints of the ESPs, further constrained the velocity structure across the margin. The velocity model was refined as each dataset was processed, clearly showing that multiple datasets were required in order to constrain the velocity structure tightly. In particular, ESPs were found to produce misleading results in places where the assumption that the sub-surface structure is one-dimensional, inherent in the ESP technique, was not valid. The final velocity model is used to infer the thickness of igneous material accreted across the Hatton margin, which is estimated to increase westwards from zero at the top of the bank to 18 km at the bottom of the continental slope and then thin rapidly to 11 km in the adjacent Iceland Basin. As the oceanic crust thins its Seismic velocity decreases, suggesting a decrease in percentage partial melting. These observations strongly support a model in which volcanic margins are produced by elevated asthenospheric temperatures.

Structure of the lower crust beneath the Carolina trough, U.S. Atlantic continental margin

Data from three large‐offset seismic profiles provide information on the crustal structure beneath the Carolina trough. The profiles, obtained by the U.S. Geological Survey, the Naval Oceanographic Research Development Agency, and the Scripps Institution of Oceanography in 1985, were oriented parallel to the trough and were located (1) seaward of the East Coast Magnetic Anomaly (ECMA), which is generally thought to represent the boundary between oceanic and continental crust; (2) along the axis of the trough between the ECMA and the hinge zone, which is thought to reflect the landward limit of highly stretched and altered transitional crust; and (3) along the Carolina platform landward of the basement hinge zone on crust thought to have been thinned only slightly during rifting. These data constrain the velocity structure of the lower crust and provide evidence for a thick lens of high‐velocity (>7.1 km/s) lower crustal material that extends beneath the Carolina trough and the adjacent ocean basin. This lens reaches a maximum thickness of about 13 km beneath the deepest part of the trough, thins to about 5 km seaward of the ECMA, and is either very thin or absent landward of the hinge zone. It is interpreted to represent material that was underplated beneath and/or intruded into the crust during the late stage of continental rifting and that led to an anomalously thick plutonic layer during the early seafloor spreading phase. These data thus support the recent conclusions of White et al. (1987b) and Mutter et al. (1988) that the initiation of seafloor spreading is attended in many, if not most, cases by the generation of an anomalously large volume of melt.

Meridional heat transport across the Sub-Tropical Convergence by a warm eddy

The heat transport of the ocean–atmosphere system is strongly dependent on the meridional transport of heat in the oceans and in particular that between the Southern Ocean and adjacent ocean basins. This ocean process may possibly be dominated by the propagation of energetic mesoscale eddies at certain distinct sectors of the Sub-Tropical Convergence such as those bordering the western boundary currents where eddies could be preferentially spawned. I report here for the first time the shedding of a warm ocean eddy from the Agulhas Current across the Sub-Tropical Convergence, its internal structure as well as rotational frequency. The subsequent drift of the eddy and its further interaction with the Sub-Tropical Convergence is documented by sequential satellite infrared measurements as well as the behaviour of a free-drifting buoy placed in the feature. Based on simultaneous hydrographic measurements, the total heat flux across the Sub-Tropical Convergence achieved by the eddy is estimated at 26.7 x 1017 J.

Cretaceous-Tertiary magmatic and tectonic events in North Greenland and the history of adjacent ocean basins

Greenland emerged as a separate entity during the break-up of the Laurasian plate in Palaeogene time. Research in the last decade has revealed some of the magmatic and tectonic events which were associated with the development of Greenland's northern margin: late Cretaceous basic dyke swarms, an explosive volcanic province (Kap Washington Group) dated as end-Cretaceous, a thrust zone of late Paleocene - Eocene age and a later Tertiary period of fault reactivation related to regional uplift. The north-eastern (Wandel Sea) margin was affected by compressional and extensional deformations. The evidence for this sequence of events is outlined together with chemical data which characterise the igneous activity as alkalic, of ‘within-plate' type. A coherent pattern can be discerned between the onshore geological history and the spreading history of adjacent ocean basins, when the revised polarity time scale of Hailwood et al. (1979) is used as a link. We show how the geological evidence from North Greenland provides a number of important constraints on the timing and magnitude of displacements between Greenland and Canada in the Labrador Sea and Baffin Bay and also along Nares Strait.

Structure profonde du plateau de Madagascar: Relations avec le plateau de Crozet

A study of the crust beneath the Madagascar Ridge and the submarine Crozet Plateau has established clear similarities between these two structures, located in symmetrical positions with respect to the Southwest Indian Ridge. Seismic refraction interpretation confirms the division of the Madagascar Ridge in a northern and a southern domain along latitude 31°S. No argument in favour of a continental nature of the northern domain can be put forward: this domain would rather correspond to a strongly anomalous oceanic crust. The deep structure of the southern domain ressembles that of the submarine Crozet Plateau; seismic velocities beneath these two structures are close to those encountered in the oceanic crust. The southern domain of the Madagascar Ridge and the submarine Crozet Plateau probably have a common origin and were later separated since the early Paleocene by spreading from the Southwest Indian Ridge. Local isostatic equilibrium of the northern domain of the Madagascar Ridge, achieved by crustal thickening, is confirmed by gravity models the depth to the Moho being close to that deduced from seismic refraction results. Gravity also confirms that the southern domain is unbalanced with respect to the northern one, and to the adjacent oceanic basins.

Extrapolation of physical properties of sediments from a localized area in the Gulf of Mexico, based on a conceptual geological model

Conceptual models of depositional systems may permit extensive extrapolation of physical properties for a relatively small amount of detailed correlation among physical, sedimentological and acoustic properties. For example, in a 9-m midslope core (IG19-39, lat. 28°51.2′N, long. 87°14.0′W, depth 1006 m) in the northeastern Gulf of Mexico there are two approximately 60-cm zones (at the core top and from 750 to 810 cm) which have distinctly higher foraminiferal sand and clay contents than the rest of the core. In the foram-rich zones, the relative increase in both clay- and sand-sized particles leads to poorer sorting, lower silt/clay ratios, more negative skewness, less platykurtosis, and higher fine-clay/total-clay ratios than in the silty clay which comprises the rest of the core. Within the silty clay, bulk densities and in situ acoustic velocities increase with depth from 100 to 425 cm, remain at or near peak values to 700 cm, and then drop sharply into the lower foram-rich zone. Although high impedence contrasts within this sequence do not correlate well with the 3.5-kHz reflection pattern, the pattern is distinctive, and its extent suggests that the core's sedimentary pattern extends over about 4000 km 2 on the northeastern continental slope of the Gulf of Mexico. The foram-rich zones have a predominance of warm-water species and correlate closely with higher carbonate content (in the mud as well as in the sand fraction) and more positive δ 13C values (for organic carbon), which are typical of marine-derived organic matter. On the other hand, the silty clay has few if any warm water indicating forams and much less carbonate. It has organic carbon with more negative δ 13C values, indicative of a terrestrial origin. Several investigators have concluded that, in the Gulf of Mexico, the foram-rich zones represent warm, interglacial or interstadial periods dominated by pelagic sedimentation in the deep-sea, whereas the siltier zones represent glacial periods of low sea-level stands during which more terrigenous sediment was delivered to the continental margins and transported to adjacent ocean basins by various gravity flow mechanisms. Since the causes of the changes in depositional processes are glacio-eustatic, the change in sediment types should be widespread with good lithostratigraphic correlations. Published data exist for over 100 cores from the deep Gulf of Mexico wherein only one or a few sediment properties are reported. In a large number of these cores the reported property correlates well with the pattern in the core studied and is explained by the glacio-eustatic model. This suggests that the relationships between physical, chemical, textural, and acoustic properties in core IG19-39 may be used to predict similar relationships over a widespread area. The model seems to apply to the abyssal plains of the Gulf of Mexico, the margins of the Mississippi Fan, and various parts of the northern continental slope.

A comparison of tidal harmonic constants computed at and near an inlet

Time series of bottom pressure and surface water level are used to compute harmonic constants and thus compare tidal conditions at and near an inlet along Florida's Atlantic coast. Supportive hydrographic and surface atmospheric pressure data are incorporated to estimate the probable maximum errors in comparing pressure fluctuations with water level variations. The M2 tidal amplitude decreases from 0.45 decibar (0.45 m±0.02 m) over the inner shelf to 0.28 m in the nearby inlet. There is an 11° phase lag of the M2 tide in the inlet. The K1 and O1 constituent amplitudes show essentially no decrease from the shelf to the inlet, though the tidal wave form is delayed by 9° and 18° respectively. Locally damped amplitudes and phase lags are attributed to estuarine-shelf exchanges through the inlet. Local perturbations in the tidal harmonic constants may have a significant effect on the construction of co-tidal and co-range charts for the adjacent ocean basin.

SEPARATION OF CLIMATIC FLUCTUATIONS AND IMPACTS OF ENGINEERING ACTIVITIES IN ESTUARIES

Using salinity as an example of dissolved substances in estuarine waters it is shown how the long term trend of concentrations can be split up into a man-made and a climatic contribution. The understanding of long term mixing processes and adequate sampling techniques are essential for this purpose. The physical state of the estuary can be described in terms of 3 basic variables, the river discharge, the filtered water level and the filtered salinity. The river discharge represents the climatic fluctuations in the catchment area, and the filtered water level is a record of the large scale weather pattern over the adjacent ocean basin. Salinity trends which cannot be attributed to these two variables must originate from man-made actions, such as dredging or other engineering activities which change the geometry of an estuary. Two models are used for the trend analysis, the simplest possible mixing equation (which always holds for a sufficiently long time scale), and a salt flux consideration.

Geological setting and principal results of drilling on the margins of the Bay of Biscay and Rockall Plateau during Leg 48

During Leg 48 of the IPOD phase of the Deep Sea Drilling Project, seven holes were drilled on the margins of the Bay of Biscay and Rockall Plateau to compare the evolution of passive margins of contrasting age and structural development. The geological setting and principal results of the drilling are outlined for each margin. We discuss the history of rifting and subsidence on these margins in relation to the sea floor spreading history of the adjacent ocean basins. Implications of the results for the nature of unconformities at passive margins and anoxic episodes during the Lower Cretaceous are discussed briefly.

Mesozoic epeirogeny at the South Atlantic margin and the Tristan hot spot

Epeirogenic structures of the eastern margin of Brazil9s Parana basin seem related to the position of the proposed Tristan hot spot in Early Cretaceous time. Several lines of evidence indicate that the hot spot was located offshore from a major structural depression, the Torres syncline, which is flanked by two structural arches about 750 km apart. Structural relief between the syncline and arches is estimated to be 4 km. Synchronism of the formative epeirogenic movements with proto-Atlantic continental rifting is indicated by the occurrence of a prominent Early Cretaceous basaltic dike swarm on the crest of one of the arches. The Torres downflexure might have originated through volcanic loading of the continental margin prior to formation of the Rio Grande Rise in the adjacent ocean basin.

North American Geosynclines--Test of Continental-Drift Theory

North American Geosynclines--Test of Continental-Drift Theory

A seismic refraction study in the southern Indian Ocean

Abstract Seismic refraction profiles in the southern Indian Ocean are described. The profiles in the Transkei Basin show that its sediment thickness is 2 km over an area estimated as 300,000 km 2 . A long profile fired on the Mozambique Terrace shows that this structure is compensated, the masses of columns to depths of 30 km varying by only 2.3 per cent for the Terrace and the adjacent ocean basins.

Tsunami-Responses of San Pedro Bay and Shelf, Calif.

The lowest modes of free oscillation of the continental shelf off San Pedro Bay, CA, are determined analytically from geometric models which approximate the shelf as the sector of a cone overlying a vertical-convex circular-cylindrical continental slope. The shelf is also modeled as a sloping basin coupled with the deep San Pedro Channel between the Santa Catalina Island and the mainland. The lowest modes of oscillation of the adjacent oceanic basins most likely to affect San Pedro Bay are also determined by modeling them geometrically as open-mouth, semi-ellipsoidal (Gulf of Santa Catalina) and closed, ellipsoidal (Sant Cruz Basin to Gulf of Santa Catalina). Marigrams from several tide-stations in Los Angeles-Long Beach Harbors, which recorded the tsunamis of Nov. 11, 1922 (Chile), April 1, 1946 (Aleutian Trench), May 23, 1960 (Chile) and March 28, 1964 (Alaska), have been analyzed for their power spectra and reveal numerous peaks of high energy recurrent at the widely separated times. These periods of resonance agree quite closely with many of the calculated periods of free oscillations. Knowledge of these responses is important to harbor planning because of dangerous surge currents generated in the harbors.

Reykjanes ridge crest: A detailed geophysical study

A geophysical survey employing satellite navigation was carried out over the Reykjanes submarine ridge southwest of Iceland. Water depth, sediment thickness, and the gravity and magnetic fields were continuously measured. In addition, bottom cores and measurements of sediment and water temperatures were obtained at stations. Expendable radio sonobuoys were used to make seismic refraction measurements. This paper combines these various geophysical data to obtain information about phenomena in the water layer, about details of crustal structure, and about mechanisms operating at the ridge axis. The satellite navigation results and water temperature data are used to deduce current directions and magnitude over the ridge. These currents play a role in the observed distribution of sediment. Variations in these currents are inferred from sediment temperature measurements. Magnetic profiles parallel to the ridge crest are used to demonstrate the presence of a thin, highly magnetized layer (termed layer 2A, since it constitutes the top of layer 2 of refraction seismology), as well as to directly infer the presence of normally and reversely magnetized rocks in bands on the ridge. Seismic refraction measurements reveal: (1) a 6.5-km/sec layer under the ridge; (2) a flankward increase in seismic velocity in the crust; and (3) evidence for a surface layer of relatively low velocity (about 3 km/sec) corresponding to layer 2A. Geothermal measurements revealed two zones of low heat flow, one within 10 km of the ridge axis and the other about 75 km from the axis. The maximum values of heat flow were observed in a zone from 15 to 50 km. The over-all average of heat flow over the ridge is not significantly different from that observed in the adjacent oceanic basins. Free-air gravity anomalies over the Reykjanes ridge range from +25 to +60 mgal. Compared to the mid-Atlantic ridge, the Bouguer anomalies over the Reykjanes ridge are about 60 mgal less, but the gradients are nearly the same. The narrow axial magnetic anomaly can be traced with minor offsets to the Reykjanes Peninsula. On Iceland, positive magnetic anomalies occur over much wider areas, implying that the active zone is much wider in Iceland than over the Reykjanes ridge.

Comparison of Continental Margins of Eastern North America at Cape Hatteras and Northwestern Africa at Cap Blanc

The opposing continental margins of Cape Hatteras and Cap Blanc appear broadly symmetric with respect to (1) distribution of physiographic provinces, with the exception of the lower continental rise hills which have not been observed off Cap Blanc, (2) early and middle Paleozoic, Mesozoic, and Cenozoic stratigraphic frameworks, (3) late Precambrian, Paleozoic, and Mesozoic tectonic frameworks, with certain qualifications, and (4) certain offshore residual magnetic anomalies, including a positive anomaly associated with the continental shelf edge and the boundary between smooth and rough magnetic fields. The third point must be qualified because (a) the absence of late Paleozoic (post-Devonian) strata in the Mauritanides fold belt of northwest Africa, as opposed to their i volvement in the Appalachian fold belt of eastern North America, limits resolution of the respective orogenic movements and (b) the existence of a system of Triassic fault-block basins parallel with the northwest African continental margin, corresponding to the system in eastern North America, has not been well documented. The opposing continental margins appear broadly asymmetric with respect to Cenozoic tectonic frameworks. Sedimentary strata underlying the coastal plain, continental terrace, and continental rise near Cape Hatteras are predominantly undeformed, except by superficial gravitational displacement processes acting above Horizon A (Late Cretaceous-Eocene). Sedimentary strata underlying the corresponding physiographic provinces off Cap Blanc are deformed by deep structural processes including (1) compressional folding related to Alpine diastrophism, (2) tensional faulting along west-northwest fracture trends, coincident with eastward projections of fracture zones which cross the Mid-Atlantic Ridge, and along inferred north-northeast fracture trends which regionally parallel the continental m rgin, (3) volcanism, at least of Oligocene to Holocene age, concentrated along the fracture zones, and (4) diapirism, probably produced by rock salt deposits beneath the continental terrace and continental rise correlative with Late Triassic and Jurassic evaporites in the northwest African coastal basins. Mesozoic and Cenozoic mean rates of subsidence and sequences of gross lithology generally correlate between the opposing continental margins. Similarities in the stratigraphic records of the opposing continental margins and the adjacent ocean basin indicate that the continental margins have behaved as if vertically, as well as horizontally, coupled to the ocean basin. Mesozoic and Cenozoic mean subsidence rates of the opposing continental margins (1-9 cm/1,000 year) are about 10-3 of inferred mean spreading rates of the intervening sea floor (1-4 cm/year). The mean subsidence rates vary in unison with the independently inferred mean spreading rates, to a first approximation. The limited data suggest that the epeirogenic subsidence of the opposing continental margins and the inferred spreading of the intervening sea floor are related genetically. The apparent symmetry in space and time of tensional rifting, orogenic compression, and epeirogenic subsidence on the opposing continental margins is consistent with a hypothesis of sea-floor spreading. These findings imply that geologic conditions relevant to the occurrence of petroleum can be predicted between the opposing continental margins.

Geologic history of sea water : Rubey William W., 1964. In: The origin and evolution of atmospheric and oceans, P.J. Brancanzio and A.G.W. Cameron, Editors, John Wiley & Sons, 1–63

Paleontology and biochemistry together may yield fairly definite information, eventually, about the paleochemistry of sea water and atmosphere. Several less conclusive lines of evidence now available suggest that the composition of both sea water and atmosphere may have varied somewhat during the past; but the geologic record indicates that these variations have probably been within relatively narrow limits. A primary problem is how conditions could have remained so nearly constant for so long. It is clear, even from inadequate data on the quantities and compositions of ancient sediments, that the more volatile materials—H 2 O, CO 2 , Cl, N, and S— are much too abundant in the present atmosphere, hydrosphere, and biosphere and in ancient sediments to be explained, like the commoner rock-forming oxides, as the products of rock weathering alone. If the earth were once entirely gaseous or molten, these “excess” volatiles may be residual from a primitive atmosphere. But if so, certain corollaries should follow about the quantity of water dissolved in the molten earth and the expected chemical effects of a highly acid, primitive ocean. These corollaries appear to be contradicted by the geologic record, and doubt is therefore cast on this hypothesis of a dense primitive atmosphere. It seems more probable that only a small fraction of the total “excess” volatiles was ever present at one time in the early atmosphere and ocean. Carbon plays a significant part in the chemistry of sea water and in the realm of living matter. The amount now buried as carbonates and organic carbon in sedimentary rocks is about 600 times as great as that in today9s atmosphere, hydrosphere, and biosphere. If only 1/100 of this buried carbon were suddenly added to the present atmosphere and ocean, many species of marine organisms would probably be exterminated. Furthermore, unless CO 2 is being added continuously to the atmosphere-ocean system from some source other than rock weathering, the present rate of its subtraction by sedimentation would, in only a few million years, cause brucite to take the place of calcite as a common marine sediment. Apparently, the geologic record shows no evidence of such simultaneous extinctions of many species nor such deposits of brucite. Evidently the amount of CO 2 in the atmosphere and ocean has remained relatively constant throughout much of the geologic past. This calls for some source of gradual and continuous supply, over and above that from rock weathering and from the metamorphism of older sedimentary rocks. A clue to this source is afforded by the relative amounts of the different “excess” volatiles. These are similar to the relative amounts of the same materials in gases escaping from volcanoes, fumaroles, and hot springs and in gases occluded in igneous rocks. Conceivably, therefore, the hydrosphere and atmosphere may have come almost entirely from such plutonic gases. During the crystallization of magmas, volatiles such as H 2 O and CO 2 accumulate in the remaining melt and are largely expelled as part of the final fractions. Volcanic eruptions and lava flows have brought volatiles to the earth9s surface throughout the geologic past; but intrusive rocks are probably a much more adequate source of the constituents of the atmosphere and hydrosphere. Judged by the thermal springs of the United States, hot springs (carrying only 1 per cent or less of juvenile matter) may be the principal channels by which the “excess” volatiles have escaped from cooling magmas below. This mechanism fails to account for a continuous supply of volatiles unless it also provides for a continuous generation of new, volatile-rich magmas. Possibly such local magmas form by a continuous process of selective fusion of subcrustal rocks, to a depth of several hundred kilometers below the more mobile areas of the crust. This would imply that the volume of the ocean has grown with time. On this point, geologic evidence permits differences of interpretation; the record admittedly does not prove, but it seems consistent with, an increasing growth of the continental masses and a progressive sinking of oceanic basins. Perhaps something like the following mechanism could account for a continuous escape of volatiles to the earth9s surface and a relatively uniform composition of sea water through much of geologic time: (1) selective fusion of lower-melting fractions from deep-seated, nearly anhydrous rocks beneath the unstable continental margins and geosynclines; (2) rise of these selected fractions (as granitic and hydrous magmas) and their slow crystallization nearer the surface; (3) essentially continuous isostatic readjustment between the differentiating continental masses and adjacent ocean basins; and (4) renewed erosion and sedimentation, with resulting instability of continental margins and mountainous areas and a new round of selective fusion below.

GEOLOGIC HISTORY OF SEA WATER

Paleontology and biochemistry together may yield fairly definite information, eventually, about the paleochemistry of sea water and atmosphere. Several less conclusive lines of evidence now available suggest that the composition of both sea water and atmosphere may have varied somewhat during the past; but the geologic record indicates that these variations have probably been within relatively narrow limits. A primary problem is how conditions could have remained so nearly constant for so long. It is clear, even from inadequate data on the quantities and compositions of ancient sediments, that the more volatile materials—H 2 O, CO 2 , Cl, N, and S— are much too abundant in the present atmosphere, hydrosphere, and biosphere and in ancient sediments to be explained, like the commoner rock-forming oxides, as the products of rock weathering alone. If the earth were once entirely gaseous or molten, these “excess” volatiles may be residual from a primitive atmosphere. But if so, certain corollaries should follow about the quantity of water dissolved in the molten earth and the expected chemical effects of a highly acid, primitive ocean. These corollaries appear to be contradicted by the geologic record, and doubt is therefore cast on this hypothesis of a dense primitive atmosphere. It seems more probable that only a small fraction of the total “excess” volatiles was ever present at one time in the early atmosphere and ocean. Carbon plays a significant part in the chemistry of sea water and in the realm of living matter. The amount now buried as carbonates and organic carbon in sedimentary rocks is about 600 times as great as that in today9s atmosphere, hydrosphere, and biosphere. If only 1/100 of this buried carbon were suddenly added to the present atmosphere and ocean, many species of marine organisms would probably be exterminated. Furthermore, unless CO 2 is being added continuously to the atmosphere-ocean system from some source other than rock weathering, the present rate of its subtraction by sedimentation would, in only a few million years, cause brucite to take the place of calcite as a common marine sediment. Apparently, the geologic record shows no evidence of such simultaneous extinctions of many species nor such deposits of brucite. Evidently the amount of CO 2 in the atmosphere and ocean has remained relatively constant throughout much of the geologic past. This calls for some source of gradual and continuous supply, over and above that from rock weathering and from the metamorphism of older sedimentary rocks. A clue to this source is afforded by the relative amounts of the different “excess” volatiles. These are similar to the relative amounts of the same materials in gases escaping from volcanoes, fumaroles, and hot springs and in gases occluded in igneous rocks. Conceivably, therefore, the hydrosphere and atmosphere may have come almost entirely from such plutonic gases. During the crystallization of magmas, volatiles such as H 2 O and CO 2 accumulate in the remaining melt and are largely expelled as part of the final fractions. Volcanic eruptions and lava flows have brought volatiles to the earth9s surface throughout the geologic past; but intrusive rocks are probably a much more adequate source of the constituents of the atmosphere and hydrosphere. Judged by the thermal springs of the United States, hot springs (carrying only 1 per cent or less of juvenile matter) may be the principal channels by which the “excess” volatiles have escaped from cooling magmas below. This mechanism fails to account for a continuous supply of volatiles unless it also provides for a continuous generation of new, volatile-rich magmas. Possibly such local magmas form by a continuous process of selective fusion of subcrustal rocks, to a depth of several hundred kilometers below the more mobile areas of the crust. This would imply that the volume of the ocean has grown with time. On this point, geologic evidence permits differences of interpretation; the record admittedly does not prove, but it seems consistent with, an increasing growth of the continental masses and a progressive sinking of oceanic basins. Perhaps something like the following mechanism could account for a continuous escape of volatiles to the earth9s surface and a relatively uniform composition of sea water through much of geologic time: (1) selective fusion of lower-melting fractions from deep-seated, nearly anhydrous rocks beneath the unstable continental margins and geosynclines; (2) rise of these selected fractions (as granitic and hydrous magmas) and their slow crystallization nearer the surface; (3) essentially continuous isostatic readjustment between the differentiating continental masses and adjacent ocean basins; and (4) renewed erosion and sedimentation, with resulting instability of continental margins and mountainous areas and a new round of selective fusion below.

Notes on the Genus Doliolum

ABSTRACT There can be but little doubt that the distribution of ocean surface life is considerably influenced by the great systems of oceanic circulation which recent researches have disclosed. Such circulation probably tends to assimilate the inhabitants of adjacent ocean basins, and helps to explain the remarkable similarity in the faunas of even widely separated seas. But unless surface animals can live with equal facility at considerable depths, a sort of retentive selection will be exerted on those that drift in through narrow straits into seas like the Mediterranean, where the surface waters have an average inward flow, and where the denser outward current is in depths removed from the direct influence of light and air.