Southeast Indian Research Articles

The Kerguelen plateau: Records from a long-living/composite microcontinent

The Kerguelen Plateau lies in the Southern Indian Ocean and is classically considered as a Large Igneous Magmatic Province (LIP) emplaced above an oceanic crust. The interpretation of the gravimetric/bathymetric anomalies shows that the Plateau is structured by NNW- and NW-trending en échelon fractures along its north-eastern border and by WNW-trending faults along its western side which separate very differentiated zones (i.e. Elan Bank and Enderby Basin). A geochemical contamination by continental material has been evidenced for all the studied magmatic rocks of the southern and central part of the Plateau, only the basalts of the northern zone recording the signature of ridge emitted magmas. The facies evolution of the overlying sediments also shows that the deposition began in a subaerial environment for the southern part of the Plateau and that the subsidence notably increased after 60 Ma. All these evidences suggest that the Southern part of the Kerguelen Plateau is underlain by stretched continental crust as it is the case for Elan Bank, and that the Central Kerguelen Plateau is most probably also underlain by attenuated continental crust. In reconstructions, the South and Central Kerguelen Plateaus resulted from the interaction between these remnants of continental crust with the Kerguelen Plume while the latter interacted with the South East Indian Ridge to form the North Kerguelen Plateau. We thus propose a cinematic scenario characterised by the formation of a composite microcontinent composed of fragments of stretched continental crust, separated by ridge propagators which were emplaced successively or contemporaneously between India and Antarctica, during the rifting process, being triggered by the Kerguelen plume emplacement.

Constraints on the mantle temperature gradient along the Southeast Indian Ridge from crustal structure and isostasy: implications for the transition from an axial high to an axial valley

Axial and ridge flank depths on the Southeast Indian Ridge (SEIR) systematically increase for 2500 km from 90°E to 120°E approaching the Australian-Antarctic Discordance. The SEIR also experiences an abrupt change in ridge axis morphology near 103°30′E with axial highs found to the west and axial valleys to the east. Since the spreading rate is constant throughout this region, these variations have been ascribed to an along-axis gradient in mantle temperature. A seismic refraction experiment provides information on the crustal thickness and seismic velocity structure of two segments with differing axial morphology. Segment P2, centred near 102°E with an axial high has a mean crustal thickness of 5.9 ± 0.2 km, while the mean crustal thickness is 5.3 ± 0.3 km at Segment S1 with an axial valley and centred near 109°45′E. Isostatic compensation of the difference in crustal thickness and density structure between the two segments only accounts for 33 m of the 198 m difference in average ridge flank depth between the two segments. The remaining depth difference must result from a difference in mantle density. Melt production models imply a mantle temperature difference of 11–13.5 °C to produce the observed difference in crustal thickness. Isostatic compensation of the two segments requires that the resulting density difference must extend to about 300 km in the mantle. The transition in axial morphology along the SEIR is very abrupt occurring over a narrow zone within a single segment in which the transition is complete. If a linear mantle temperature gradient is assumed, the temperature difference across the transition segment is only 2.4 °C. The change in axial morphology is accompanied by abrupt changes in other parameters including abyssal hill height, magnetic anomaly amplitude, layer 2a thickness and the presence or absence of an axial magma lens. The abrupt, coincident change in a number of parameters with a very small change in mantle temperature strongly suggests a threshold change between two distinctly different modes of crustal accretion. The trigger for the transition appears to be whether a steady-state crustal magma lens can be maintained.

A study on the factors affecting the stable isotopic composition in precipitation of Tamil Nadu, India

AbstractStable isotope measurements in precipitation help us to form the basic inference about the origin and the state of water in different environments. The precipitation samples collected during the South West Monsoon (SWM) during August 2007 from 37 different locations in the state help in deriving the first preliminary local meteoric water line (LMWL) for the Tamil Nadu State. The study reveals that there are three main sources of water vapours, namely South East Arabian Sea (SEAS), Indian Ocean (IO) and evaporation from local water bodies. There are wide variations in the altitude and amount of rainfall received in the state. It is also noted that there is a significant variation in the latitude and the distance from the coast. Hence, an attempt is made in this paper to study the factors controlling the composition of stable isotopes in precipitation with reference to amount of rainfall, latitude, altitude and the continental effect. The altitude and continental effect shows good correlation to the variations in stable isotope composition. Copyright © 2009 John Wiley & Sons, Ltd.

Mantle melting and magma supply to the Southeast Indian Ridge: The roles of lithology and melting conditions from U-series disequilibria

Intermediate-spreading Southeast Indian Ridge (SEIR) basalts display geographic gradients from 90°E to 118°E in Th/U, ( 230Th/ 232Th) and 238U– 230Th– 226Ra disequilibria ( 230Th-excesses of 1 to 21% and 226Ra-excesses of 0 to 160%). Highly correlated ( 238U/ 232Th) and ( 230Th/ 232Th) ( r 2 = 0.94) span nearly the entire global MORB range; basalts from three of four ridge morphologies form subparallel, vertically stacked arrays on an equiline diagram. ( 226Ra/ 230Th) is inversely correlated with ( 230Th/ 238U) within sub-regions of the study area. There is a convex ( 230Th/ 238U) along-axis pattern (lower 230Th-excesses in the west and east, at shallowest and deepest depths), such that ( 230Th/ 238U) does not correlate with axial depth as described elsewhere, despite the ~ 2300 m west-to-east axial depth increase. κ and κ Pb also vary along axis from κ < κ Pb in the west to κ ≅ κ Pb in the east. Variations in axial depth, crustal thickness, U-series disequilibria, κ and κ Pb are best explained by forward model scenarios in which the physical characteristics of melting (such as melting rate, degree of melting, porosity and melt initiation depth) vary along axis in response to an inferred long wavelength temperature gradient and small variations in the underlying source lithology (pyroxenite veins). This leads to decreased melt supply from west to east, a concomitant change in axial morphology and axial magma chamber depth, and systematic long wavelength geochemical gradients. Melting rate covaries with melt supply in the western and central regions, decreasing eastward along the SEIR, but supply and rate vary inversely from the central to the eastern regions, because cooler eastern mantle contains a few percent of fusible pyroxenite veins that melt productively to generate melts with high Th–U–Ra concentration but low 230Th-excess and 226Ra-excess. The veins contribute only a small proportion of the total melt volume from mantle that is > 95% peridotite. Th, Pb and He isotopic gradients are consistent with a changing proportion of enriched pyroxenite veins along axis having a roughly 0.5 to 1 Gyr reservoir age.

Impact of lateral transport on organic proxies in the Southern Ocean

Lateral transport of fine-grained organic carbon particles can complicate the interpretation of paleoclimate records based on organic proxies. Here we investigated the effect of lateral transport on newly developed temperature and soil organic matter proxies, TEX 86 and BIT index, respectively, in core MD88–769 recovered from the South East Indian Ridge. Our results show that TEX 86 and BIT records in comparison to diatom and foraminifera records were representative for more local climate changes while alkenones and n-alkanes originated from distant areas by oceanic and atmospheric transport, respectively. This suggests that TEX 86 and BIT paleoclimate records are primarily influenced by local conditions and less subjected to long-distance lateral transport than other organic proxies in the Southern Ocean.

Crustal thickness variations along the Southeast Indian Ridge (100°–116°E) from 2‐D body wave tomography

Axial morphology along the Southeast Indian Ridge (SEIR) systematically changes from an axial high to a deep rift valley at a nearly uniform intermediate spreading rate between 100°–116°E, west of the Australian‐Antarctic Discordance (AAD). Basalt geochemistry has a consistent Indian–mid‐ocean ridge basalt (MORB) type isotopic signature, so changes in axial topography are attributed to variations in both mantle temperature and melt supply. Wide‐angle seismic refraction lines were shot to four ocean bottom hydrophones within SEIR segments P1, P2, S1, and T, where each segment is characterized by a different morphology. We constructed 2‐D crustal velocity models by jointly inverting hand‐picked P wave refraction (Pg) and Moho reflection (PmP) traveltime data using a top‐down, minimum‐structure methodology. The results show a 1.5 km eastward decrease in crustal thickness across the study area, with segment averages ranging from 6.1 km at P1 to 4.6 km at T. Melt generation models require a ∼30°C decrease in mantle temperature toward the AAD to account for the crustal thickness trend. Significant changes in axial morphology accompany small‐scale variations in crustal thickness, consistent with models of crustal accretion where ridge topography is determined by a balance between mantle temperature, melt supply, and cooling from hydrothermal circulation. Layer 3 thins by 3.0 km as layer 2 thickens by 1.4 km between segments P1 and T, reflecting the eastward decrease in melt supply and increase in melt lens depth. The trade‐off in seismic layers may be explained by models relating the increase in overburden pressure on a deepening melt lens to the volume of magma erupted into the upper crust rather than cooling at depth to form new lower crustal material.

Warming and Freshening in the Abyssal Southeastern Indian Ocean*

Abstract Warming and freshening of abyssal waters in the eastern Indian Ocean between 1994/95 and 2007 are quantified using data from two closely sampled high-quality occupations of a hydrographic section extending from Antarctica northward to the equator. These changes are limited to abyssal waters in the Princess Elizabeth Trough and the Australian–Antarctic Basin, with little abyssal change evident north of the Southeast Indian Ridge. As in previous studies, significant cooling and freshening is observed in the bottom potential temperature–salinity relations in these two southern basins. In addition, analysis on pressure surfaces shows abyssal warming of about 0.05°C and freshening of about 0.01 Practical Salinity Scale 1978 (PSS-78) in the Princess Elizabeth Trough, and warming of 0.1°C with freshening of about 0.005 in the abyssal Australian–Antarctic Basin. These 12-yr differences are statistically significant from zero at 95% confidence intervals over the bottom few to several hundred decibars of the water column in both deep basins. Both warming and freshening reduce the density of seawater, contributing to the vertical expansion of the water column. The changes below 3000 dbar in these basins suggest local contributions approaching 1 and 4 cm of sea level rise, respectively. Transient tracer data from the 2007 occupation qualitatively suggest that the abyssal waters in the two southern basins exhibiting changes have significant components that have been exposed to the ocean surface within the last few decades, whereas north of the Southeast Indian Ridge, where changes are not found, the component of abyssal waters that have undergone such ventilation is much reduced.

South Australian rainfall variability and climate extremes

Rainfall extremes over South Australia are connected with broad-scale atmospheric rearrangements associated with strong meridional sea surface temperature (SST) gradients in the eastern Indian Ocean. Thirty-seven years of winter radiosonde data is used to calculate a time series of precipitable water (PW) and convective available potential energy (CAPE) in the atmosphere. Principle component analysis on the parameters of CAPE and PW identify key modes of variability that are spatially and seasonally consistent with tropospheric processes over Australia. The correlation of the leading principle component of winter PW to winter rainfall anomalies reveal the spatial structure of the northwest cloudband and fronts that cross the southern half of the continent during winter. Similarly the second and third principle components, respectively, reveal the structures of the less frequent northern and continental cloudbands with remarkable consistency. 850 hPa-level wind analysis shows that during dry seasons, anomalous offshore flow over the northwest of Australia inhibits advection of moisture into the northwest, while enhanced subsidence from stronger anticyclonic circulation over the southern half of the continent reduces CAPE. This coincides with a southward shift of the subtropical ridge resulting in frontal systems passing well to the south of the continent, thus producing less frequent interaction with moist air advected from the tropics. Wet winters are the reverse, where a weaker meridional pressure gradient to the south of the continent allows rain-bearing fronts to reach lower latitudes. The analysis of SSTs in the Indian Ocean indicate that anomalous warm (cool) waters in the southeast Indian Ocean coincide with a southward (northward) shift in the subtropical ridge during dry (wet) seasons.

Melvicalathis, a new brachiopod genus (Terebratulida: Chlidonophoridae) fromdeep sea volcanic substrates, and the biogeographic significance of the mid-oceanridge system

Brachiopods form a small but significant component of the deep-sea benthos in all oceans. Almost half of the 40 brachiopod species so far described from depths greater than 2000 m are small, short-looped terebratulides assigned to two superfamilies, Terebratuloidea and Cancellothyridoidea. In this study we describe Melvicalathis, a new genus of cancellothyridoid brachiopod (Family Chlidonophoridae; Subfamily Eucalathinae) from ocean ridge localities in the south and southeast Pacific Ocean, and cryptic habitats within lava caves in glassy basalt dredged from the Southeast Indian Ridge, Indian Ocean. These small, punctate, strongly-ribbed, highly spiculate brachiopods occur at depths between 2009 m and 4900 m, and appear to be primary colonisers on the inhospitable volcanic rock substrate. The ecology and life-history of Melvicalathis and related deep-sea brachiopods are discussed. Brachiopods are rarely reported from the much-studied but localised hydrothermal vent faunas of the mid ocean ridge systems. They are, however, widespread members of a poorly known deep-sea benthos of attached, suspension-feeding epibionts that live along the rarely sampled basalt substrates associated with mid-ocean ridge systems. We suggest that these basalt rocks of the mid-ocean ridge system act as deep-sea “superhighways” for certain groups of deep-sea animals, including brachiopods, along which they may migrate and disperse. Although the mid-ocean ridges form the most extensive, continuous, essentially uniform habitat on Earth, their biogeographic significance may not have been fully appreciated.

High‐resolution surveys along the hot spot–affected Galápagos Spreading Center: 1. Distribution of hydrothermal activity

The spatial density of hydrothermal activity along most mid‐ocean ridges is a robust linear function of spreading rate (or magmatic budget), but extreme crustal properties may alter this relationship. In 2005–2006 we tested the effect of thickened crust on hydrothermal activity using high‐resolution mapping of plumes overlying the hot spot–affected Galápagos Spreading Center from 95° to 89°42′W (∼560 km of ridge crest). Plume mapping discovered only two active, high‐temperature vent fields, subsequently confirmed by camera tows, though strong plume evidence indicated minor venting from at least six other locations. Total plume incidence (ph), the fraction of ridge crest overlain by significant plumes, was 0.11 ± 0.014, about half that expected for a non–hot spot mid‐ocean ridge with a similar magmatic budget. Plume distributions on the Galápagos Spreading Center were uncorrelated with abrupt variations in the depth of the along‐axis melt lens, so these variations are apparently not controlled by hydrothermal cooling differences. We also found no statistical difference (for a significance level of 0.05) in plume incidence between where the seismically imaged melt lens is shallow (2 ± 0.56 km, ph = 0.108 ± 0.045) and where it is deep (3.4 ± 0.7 km, ph = 0.121 ± 0.015). The Galápagos Spreading Center thus joins mid‐ocean ridges near the Iceland (Reykjanes Ridge), St. Paul‐Amsterdam (South East Indian Ridge), and Ascension (Mid‐Atlantic Ridge) hot spots as locations of anomalously scarce high‐temperature venting. This scarcity implies that convective cooling along hot spot–affected ridge sections occurs primarily by undetected diffuse flow or is permanently or episodically reduced compared to normal mid‐ocean ridges.

Attempt to identify sources of atmospheric methane and carbon dioxide concentrations found in in situ aircraft measurements over Southern Australia

Identifying the sources and sinks of methane and carbon dioxide is important for understanding processes within the Earth's climate system. This paper attempts to use back trajectories to identify sources of atmospheric methane and carbon dioxide as measured by high resolution in situ gas analyzers during aircraft ascents and descents in Southern Australia. Results from the back trajectory analysis were confirmed by also performing a forward trajectory analysis on some of the data. The in situ aircraft measurements were part of a joint Japanese‐Australia field campaign in March and April 2007 near Adelaide, South Australia. The vertical profiles showed considerable variation in methane and carbon dioxide content above the planetary boundary layer. We used back trajectories based on an atmospheric transport model to derive the origin of the air masses which enabled speculation about sources of the gases. We were thus able to identify emission from the volcanoes on Réunion Island in the Indian Ocean and the seafloor hydrothermal activity in the Southeast Indian Ridge, confirming speculations published earlier by other research teams.

Toward a minimum change model for recent plate motions: calibrating seafloor spreading rates for outward displacement

SUMMARY We use seafloor spreading distances derived from reconstructions of more than 7000 crossings of young magnetic anomalies along seven plate boundaries to study outward displacement, a source of systematic bias in estimates of seafloor spreading rates, which is caused by a combination of processes that shift magnetic polarity transition zones away from their idealized locations. Linear regressions of 81 independent sequences of seafloor opening distances as a function of their magnetic reversal ages for anomalies younger than C3n.1 (4.19 Ma) yield 75 positively valued intercepts for zero seafloor age, confirming the ubiquitous outward shift of magnetic reversals reported by previous authors. Grouping these data into 29 locally consistent clusters yields better constrained zero-age intercepts that are uniformly positive and average 2.2 ± 0.3 km globally. These values, which are 1–3 km at most locations and are significantly larger (3–5 km) along the well-surveyed Reykjanes and Carlsberg ridges, agree well with published magnetic polarity zone transition widths, which are estimated directly from nearbottom seafloor magnetic measurements. Significant variations in outward displacement along the Southeast Indian Ridge are strongly correlated with changes in axial morphology and axial depth; however, a similar correlation is not observed along other ridges. Forward magnetic anomaly modelling suggests that variations in outward displacement can be explained by differences in the magnetic source layers that are assumed to characterize different spreading centres. If not corrected for outward displacement, the implied systematic upward biases in seafloor spreading rates, which are averaged over the width of Anomaly 1—the youngest reversal that is used for plate reconstructions—range from 6 mm yr −1 along the Reykjanes Ridge, where outward displacement is 4–5 km, to 3 mm yr −1 along ridges where outward displacement approximates the global average of 2 km.

LATE NEOGENE PLANKTIC FORAMINIFERAL BIOCHRONOLOGY OF THE ODP SITE 763A, EXMOUTH PLATEAU, SOUTHEAST INDIAN OCEAN

Research Article| July 01, 2008 LATE NEOGENE PLANKTIC FORAMINIFERAL BIOCHRONOLOGY OF THE ODP SITE 763A, EXMOUTH PLATEAU, SOUTHEAST INDIAN OCEAN Devesh K. Sinha; Devesh K. Sinha 3 1Department of Geology, Centre of Advanced Studies, Delhi University, Delhi-110007, India. 3Correspondence author. E-mail: devasinha@yahoo.com Search for other works by this author on: GSW Google Scholar Ashutosh Kumar Singh Ashutosh Kumar Singh 2Planetary and Geosciences Division, Physical Research Laboratory, Ahmadabad 380 009, India. Search for other works by this author on: GSW Google Scholar Author and Article Information Devesh K. Sinha 3 1Department of Geology, Centre of Advanced Studies, Delhi University, Delhi-110007, India. Ashutosh Kumar Singh 2Planetary and Geosciences Division, Physical Research Laboratory, Ahmadabad 380 009, India. 3Correspondence author. E-mail: devasinha@yahoo.com Publisher: Cushman Foundation for Foraminiferal Research Received: 11 Oct 2006 Accepted: 23 Feb 2008 First Online: 03 Mar 2017 Online ISSN: 1943-264X Print ISSN: 0096-1191 © 2008 Journal of Foraminiferal Research Journal of Foraminiferal Research (2008) 38 (3): 251–270. https://doi.org/10.2113/gsjfr.38.3.251 Article history Received: 11 Oct 2006 Accepted: 23 Feb 2008 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Devesh K. Sinha, Ashutosh Kumar Singh; LATE NEOGENE PLANKTIC FORAMINIFERAL BIOCHRONOLOGY OF THE ODP SITE 763A, EXMOUTH PLATEAU, SOUTHEAST INDIAN OCEAN. Journal of Foraminiferal Research 2008;; 38 (3): 251–270. doi: https://doi.org/10.2113/gsjfr.38.3.251 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyJournal of Foraminiferal Research Search Advanced Search Abstract The late Neogene section of ODP Hole 763A, Exmouth Plateau, southeast Indian Ocean, has been biostratigraphically subdivided into eight planktic foraminiferal zones. The zones are similar to those of the tropical northern Indian and tropical western Pacific Oceans. The sequential order of planktic foraminiferal events has been determined, and 32 late Neogene planktic foraminiferal events have been identified. A major faunal turnover (22 events) occurs between 80 and 45 meters below seafloor (mbsf), spanning 1.6 million years between 3.6 Ma and 2 Ma and may be related to the stresses transmitted to the upper ocean due to major cooling associated with Northern Hemisphere glaciation.Based on integration with magnetostratigraphy, numerical age estimates of planktic foraminiferal events have been determined for the first time from the southeast Indian Ocean. A comparison with the published dates for these events from other parts of the world suggests that 14 events are reliable for correlation over a wide geographic range. The useful synchronous events are the Globorotalia tosaensis FO (2.84 Ma), the Globorotalia tosaensis LO (0.59 Ma), the Globigerinoides extremus LO (1.87 Ma), the Globigerinoides fistulosus FO (3.26 Ma), the Globigerinoides fistulosus LO (1.73 Ma), the Dentoglobigerina altispira LO (3.05 Ma), the Dentoglobigerina globosa LO (3.18), the Neogloboquadrina dutertrei FO (2.77 Ma), the Sphaeroidinellopsis LO (3.35), the Globorotalia margaritae LO (3.38 Ma), the Globorotalia crassaformis FO (4.41 Ma), the Globoturborotalita nepenthes LO (3.44 Ma), the Globorotalia tumida tumida FO (5.10 Ma) and the Pulleniatina primalis FO (5.70 Ma). In contrast, the other events are diachronous and considered unsuitable for inter-oceanic correlation. The Pliocene-Pleistocene boundary is marked at ODP Hole 763A by the LO of Globigerinoides fistulosus, which is a synchronous event over a wide latitudinal range and occurs close to the top of the Olduvai Event (Chron C2n; 1.77 Ma). The Miocene-Pliocene boundary is approximated by the first evolutionary appearance of Globorotalia tumida tumida from its ancestor Globorotalia plesiotumida and occurs close to base of Thvera magnetic event (C3n.4n; 5.23 Ma). Recognition of both of these epoch boundaries is in accord with the International Commission on Stratigraphy. The detailed planktic foraminiferal biochronology developed for Hole 763A will aid correlation and dating of various paleoceanographic events over the last 6 million years in the southeast Indian Ocean. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Heat flow from the Southeast Indian Ridge flanks between 80°E and 140°E: Data review and analysis

We analyze available heat flow data from the flanks of the Southeast Indian Ridge adjacent to or within the Australian‐Antarctic Discordance (AAD), an area with patchy sediment cover and highly fractured seafloor as dissected by ridge‐ and fracture‐parallel faults. The data set includes 23 new data points collected along a 14‐Ma old isochron and 19 existing measurements from the 20‐ to 24‐Ma old crust. Most sites of measurements exhibit low heat flux (from 2 to 50 mW m−2) with near‐linear temperature‐depth profiles except at a few sites, where recent bottom water temperature change may have caused nonlinearity toward the sediment surface. Because the igneous basement is expected to outcrop a short distance away from any measurement site, we hypothesize that horizontally channelized water circulation within the uppermost crust is the primary process for the widespread low heat flow values. The process may be further influenced by vertical fluid flow along numerous fault zones that crisscross the AAD seafloor. Systematic measurements along and across the fault zones of interest as well as seismic profiling for sediment distribution are required to confirm this possible, suspected effect.

Effects of the 2004 Great Sumatra Tsunami: Southeast Indian Coast

A large earthquake (Mw 9.3) struck northern Sumatra on December 26, 2004. The generated tsunami caused significant casualties and damages in many countries around the Indian Ocean. This is a report on the subsequent rapid reconnaissance survey of the Southeast Indian Coast. A wide variety of field observations—from hydrodynamics to human behaviors—made within 2 weeks of the event are discussed. We found that individual tsunami effects of this event were not extraordinary but similar to previous smaller events; what makes the December 26 event distinct is the extent of the affected area.

Decadal ventilation and mixing of Indian Ocean waters

Chlorofluorocarbon (CFC) and hydrographic data from the World Ocean Circulation Experiment (WOCE) Indian Ocean expedition are used to evaluate contributions to decadal ventilation of water masses. At a given density, CFC-derived ages increase and concentrations decrease from the south to north, with lowest concentrations and oldest ages in Bay of Bengal. Average ages for thermocline water are 0–40 years, and for intermediate water they are less than 10 years to more than 40 years. As compared with the marginal seas or throughflow, the most significant source of CFCs for the Indian Ocean south of 12°N is the Southern Hemisphere. A simple calculation is used to show this is the case even at intermediate levels due to differences in gas solubilities and mixing of Antarctic Intermediate Water and Red Sea Water. Bottom water in the Australia–Antarctic Basin is higher in CFC concentrations than that to the west in the Enderby Basin, due to the shorter distance of this water to the Adelie Land coast and Ross Sea sources. However, by 40°S, CFC concentrations in the bottom water of the Crozet Basin originating from the Weddell Sea are similar to those in the South Australia Basin. Independent observations, which show that bottom water undergoes elevated mixing between the Australia–Antarctic Basin and before entering the subtropics, are consistent with high CFC dilutions (3–14-fold) and a substantial concentration decrease (factor of 5) south to north of the Southeast Indian Ridge. CFC-bearing bottom waters with ages 30 years or more are transported into the subtropical South Indian Ocean by three western boundary currents, and highest concentrations are observed in the westernmost current. During WOCE, CFC-bearing bottom water reaches to about 30°S in the Perth Basin, and to 20°S in the Mascarene Basin. Comparing subtropical bottom water-CFC concentrations with those of the South Pacific and Atlantic oceans, at comparable latitudes, Indian Ocean bottom water-CFC concentrations are lower, consistent with its high dissipation rates from tidal mixing and current fluctuations as shown elsewhere. Thus, the generally high dilutions and low CFC concentrations in bottom water of the Indian Ocean are due to distance to the water mass source regions and the relative effectiveness of mixing. While it is not surprising that at thermocline, intermediate, and bottom levels, the significant ventilation sources on decadal time scales are all from the south, the CFCs show how local sources and mixing within the ocean affect the ventilation.

Thermal regime of the Southeast Indian Ridge between 88°E and 140°E: Remarks on the subsidence of the ridge flanks

The flanks of the Southeast Indian Ridge are characterized by anomalously low subsidence rates for the 0–25 Ma period: less than 300 m Ma−1/2 between 101°E and 120°E and less than 260 m Ma−1/2 within the Australian‐Antarctic Discordance (AAD), between 120°E and 128°E. The expected along‐axis variation in mantle temperature (∼50°C) is too small to explain this observation, even when the temperature dependence of the mantle physical properties is accounted for. We successively analyze the effect on subsidence of different factors, such as variations in crustal thickness; the dynamic contribution of an old, detached slab supposedly present within the mantle below the AAD; and depletion in ϕm, a parameter here defined as the “ubiquitously distributed melt fraction” within the asthenosphere. These effects may all contribute to the observed, anomalously low subsidence rate of the ridge flanks, with the most significant contribution being probably related to the depletion in ϕm. However, these effects have a deep‐seated origin that cannot explain the abruptness of the transition across the fracture zones that delineate the boundaries of the AAD, near 120°E and near 128°E, respectively.

Mapping absolute migration of global mid-ocean ridges since 80 Ma to Present

Abstract We have computed and mapped the absolute migration of global mid-ocean ridges in the past 80 m.y. and found that ridges have migrated substantially during that period. Presently, the faster-migrating ridges are Pacific-Antarctic, Central Indian, Southeast Indian, Juan de Fuca, Pacific-Nazca, Antarctic-Nazca and Australia- Antarctic ridges which migrate between 3.3 and 5.5 cm/yr. The slower-migrating ridges are Mid-Atlantic and Southwest Indian ridges which migrate between 0.3 and 2.0 cm/yr. Comparing these results with mantle tomography and geochemistry suggests that slower-migrating ridges have deeper depth of origin than faster-migrating ridges, implying a correlation between migration velocity and depth of origin of ridges. The reconstructed Southwestern Indian ridge (near 44°E) between 60 Ma and Present, lies atop reconstructed Central Indian ridge between 80 and 50 Ma, and the present-day Antarctic-Nazca or (Chile) ridge lies atop the reconstructed East Pacific Rise at 70 Ma. Furthermore, the South Mid Atlantic Ridge and the East Pacific Rise near 10?S appear to have been stationary relative to the mantle for the last 80 m.y. These observations suggest that different portions of the mantle have undergone different recycling history, and may explain the origin of mantle heterogeneities.

Influence of the Amsterdam/St. Paul hot spot along the Southeast Indian Ridge between 77° and 88°E: Correlations of Sr, Nd, Pb, and He isotopic variations with ridge segmentation

The submarine Amsterdam‐St. Paul (ASP) Plateau, bisected by the Southeast Indian Ridge (SEIR), is a bathymetric high rising ∼2 km above the surrounding seafloor that includes the islands of Amsterdam and St. Paul; this excess volcanism is attributed to a mantle hot spot. We obtained new Sr, Nd, and Pb (n = 37) and He isotopic (n = 10) ratios for basalt glasses from 11 SEIR segments on and adjacent to the plateau and from three seamounts on the plateau. The results show systematic spatial variations in these isotopic ratios that correlate with physical segmentation of the ridge. Specifically, lavas from the four ridge segments on the ASP Plateau have higher 208Pb/204Pb at a given 206Pb/204Pb than SEIR basalts distant from ASP Plateau. Surprisingly, lavas from the ridge segment 100 km north of the ASP Plateau are distinguished by the most radiogenic 206Pb/204Pb (up to 19.4) and highest 3He/4He ratios (up to 14.1 RA). These are characteristics of lavas erupted at Amsterdam and St Paul Islands. The isotopic data for SEIR basalts erupted on or adjacent to the ASP Plateau provide equivocal evidence for a mantle component derived from the distant Kerguelen hot spot. Overall, the Pb‐Nd‐Sr‐He isotope variations within this data set are explained well by three mantle end‐members: (1) depleted mantle having relatively low 206Pb/204Pb and 87Sr/86Sr and high 143Nd/144Nd, which has been variably mixed with (2) material having relatively high 208Pb/204Pb and 87Sr/86Sr and low 143Nd/144Nd, a signature commonly ascribed to detached or eroded metasomatized continental lithosphere, and (3) hot spot–related mantle having elevated 3He/4He and 206Pb/204Pb but intermediate 87Sr/86Sr and 143Nd/144Nd, similar to the common or C material observed in hot spots globally. These results suggest either that the ASP hot spot is isotopically heterogeneous or that the shallow mantle or lithosphere beneath the ASP Plateau contains more continentally derived material than the SEIR mantle ≥500 km away. Perhaps, like the 39°–41°E section of the Southwest Indian Ridge, beneath the ASP Plateau there are rafts of continental material stranded within a local “tectonic corridor,” possibly present since the opening of the Indian Ocean basin.

A new view of the He–Ar–CO 2 degassing at mid-ocean ridges: Homogeneous composition of magmas from the upper mantle

Deep-sea exploration is rapidly improving our understanding of volatiles geochemistry in mid-ocean-ridge igneous products. It is also placing greater constraints on degassing processes of the Earth’s mantle, with the result that degassing models based on vapour–melt equilibrium are no longer able to explain the increasing number of data. In fact, such models force to postulate an upper mantle strongly heterogeneous at any scale, and cannot account for the widespread carbon supersaturation of the recovered igneous products. Here we review the global He–Ar–CO 2 dataset of fluid inclusions in mid-ocean-ridge glasses using the framework of advanced modelling of multicomponent bubble growth in magmas. We display that non-equilibrium fractionations among He, Ar and CO 2, driven by their different diffusivities in silicate melts, are common in most of the natural conditions of magma decompression and their signature strongly depends on pressure of degassing. Due to the comparable Ar and CO 2 diffusivity, magma degassing at low pressure fractionates both the He/Ar and He/CO 2 ratio by a similar extent, while the slower CO 2 diffusion at high pressure causes early kinetic effects on Ar/CO 2 ratio and dramatically changes the degassing path. On this ground, the very different geochemical signatures among suites of data coming from different ridge segments mainly depend on the depth of the magma chamber where the melt was stored. Besides, the variations inside a single suite highlight variable ascent speed and cooling rate of the emplaced lava. The large variations in both the He/CO 2 and Ar/CO 2 ratios at almost constant He/Ar, displayed in glasses coming from the Mid-Atlantic Ridge 24–30°N segment and the Rodriguez Triple Junction, are therefore interpreted as a high-pressure signature. In contrast, the simultaneous increase in both He/CO 2 and He/Ar of the East Pacific Rise, Pito Seamount and South-East Indian Ridge data sets suggests the dominance of low-pressure fractionation, implying that the shallow magma chambers are at a lower depth than those of the Mid-Atlantic Ridge 24–30°N and Rodriguez Triple Junction. Our conclusions support the presence of a relationship between spreading rate and depth of high-temperature zones below ridges, and are consistent with the depth of magma chambers as suggested from seismic studies. Non-equilibrium degassing explains the volatile systematics of mid-ocean-ridge basalts by starting from a single mantle-derived magma, dispensing with the supposed need for heterogeneities in abundance ratios of volatiles in the mantle below oceanic ridges.