Franco Marcantonio

Os isotope systematics in ocean island basalts

New ReOs isotopic results for Os-poor basalts from St. Helena, the Comores, Samoa, Pitcairn and Kerguelen dramatically expand the known range of initial 186Os/187Os ratios in OIBs to values as high as 1.7. In contrast to the Os isotopic uniformity of Os-rich basalts from the HIMU islands of Tubuai and Mangaia found by Hauri and Hart [1], our values for St. Helena span most of the known range of Os isotopic variability in oceanic basalts (initial 187Os/186Os ranges from 1.2 to 1.7). Generation of such radiogenic Os in the mantle requires melting of source materials that contain large proportions of recycled oceanic crust. The very low Os concentrations of most of the basalts analyzed here, however, leave them susceptible to modification via interaction with materials containing radiogenic Os in the near-surface environment. Thus the high 186Os/187Os ratios may result from assimilation of radiogenic Os-rich marine sediments, such as Mn oxides, within the volcanic piles traversed by these magmas en route to the surface. Furthermore, the Os isotopic signatures of Os-rich, olivine-laden OIBs may reflect the accumulation of lithospheric olivine, rather than simply their mantle source characteristics. The extent to which these processes alter the view of the mantle obtained via study of ReOs systematics in oceanic basalts is uncertain. These effects must be quantified before ReOs systematics in OIBs can be used with confidence to investigate the nature of mantle heterogeneity and its causes.

Os isotope systematics of La Palma, Canary Islands: Evidence for recycled crust in the mantle source of HIMU ocean islands

Sub-aerial lavas from the single ocean island of La Palma, Canary Islands show as large a variation in 187Os186Os isotope ratios (1.13–1.59) as found across all of French Polynesia [1]. The La Palma lavas, however, display a restricted range of chemical composition and have all been erupted within the last 3.5 Ma. The highest Os isotopic compositions are observed in lavas with low Os concentrations. An uplifted sequence of lavas, that represent the early phase of submarine growth of the island, show extremely heterogeneous 187Os186Os isotope ratios, from 1.21 to 3.53, with the most radiogenic values found in pillow rinds. Assimilation of these pillow rinds by ascending magma can readily account for highly radiogenic ratios (187Os186Os > 1.3) found in lavas with Os concentrations below 30 ppt. n nSamples with Os concentrations too high to be significantly affected by assimilation still display a range in Os isotope ratios from 1.13 to 1.25. We argue that these radiogenic values reflect a HIMU mantle source that contains ancient recycled oceanic crust. Characteristic incompatible trace element ratios suggest further similarities between the mantle beneath La Palma and other HIMU islands. n nWhen potentially contaminated low-Os OIBs are screened from literature data, HIMU islands are found to display the highest Os isotope ratios (up to 1.25). Pbue5f8Os systematics for uncontaminated OIBs do not define a simple two-component mixing relationship between ambient mantle and recycled oceanic crust of a single composition. We suggest that this is due to variable alteration and subduction-induced perturbation of the UPb ratio in the recycled material that forms a component of the HIMU source.

A comparative study of accumulation rates derived by He and Th isotope analysis of marine sediments

Abstract We present a detailed down-core analysis of helium isotope ratios and concentrations for bulk sediments from the central Equatorial Pacific that span the last two glacial-interglacial cycles. Measured 3 He 4 He ratios range from 1.0 × 10−5 to 2.1 × 10−4, or 7.4 to 149 times the atmospheric ratio. The 3He from interplanetary dust particles (IDPs) constitutes virtually all of the 3He measured within the sediment. Because carbonate accumulation rates are high in the Equatorial Pacific, the measured 3He concentrations are lower than have been measured elsewhere, and range from 4.7 × 10−13 to 3.0 × 10−12 cm3STP · g−1. If the cosmic dust 3He-flux is constant with time, sediment mass accumulation rates can be determined from the 3He concentration in sediments. The excess 230Th technique is an entirely independent method for calculating sediment mass accumulation rates because its source is in-situ decay of 234U in seawater. To first order, initial excess 230Th activities correlate with 3He concentrations within this core. Based on the 230Th results, we estimate the 3He-flux to the Earths surface as (9.6 ± 2.0) × 10−16 cm3STP · cm−2 · a−1. If this flux has remained constant over extended periods of time, it can be used to determine sediment accumulation rates beyond the 230Th range (300,000 yr).

A Pb isotope record of mid-Atlantic US atmospheric Pb emissions in Chesapeake Bay sediments

We have analyzed sediments from three sites in the mesohaline portion of Chesapeake Bay (CB) for Pb isotopes. The well-preserved and well-dated sediments provide an excellent opportunity to compare the anthropogenic Pb isotope record in CB to other Pb isotope records of US industrial atmospheric emissions. Over the past century, there is excellent temporal agreement between anthropogenic CB 206Pb/207Pb isotope ratios and those determined in a dated coral from Bermuda [Earth and Planetary Science Letters, 82 (1987) 289] almost 2000 km away. We use this correlation to argue that CB sediments contain a regional, industrial atmospheric Pb isotope signal that is representative of the mid-Atlantic region of the US. Anthropogenic Pb is found in sediments deposited as early as approximately 1800. From about 1800 to 1930, the Pb signal in the CB sediments is probably derived from the burning of coal. After this period, and up until about the 1980s, the signal is overwhelmed by the Pb derived from the combustion of gasoline.

Variations in productivity and eolian fluxes in the northeastern Arabian Sea during the past 110 ka

Abstract High-resolution (one to two samples/ka) radionuclide proxy records from core 93KL in the northeastern Arabian Sea provide evidence for millennial climate variability over the past 110 ka. We interpret 230Th-normalized 232Th fluxes as a proxy for eolian input, and authigenic uranium concentrations as a proxy for past productivity. We attribute orbital and suborbital variations in both proxies to changes in the intensity of the southwest Indian Ocean monsoon. The highest 230Th-normalized 232Th fluxes occur at times that are consistent with the timing of the Younger Dryas, Heinrich events 1–7 and cold Dansgaard–Oeschger stadial events recorded in the GISP2 ice core. Such high dust fluxes may be due to a weakened southwest monsoon in conjunction with strengthened northwesterlies from the Arabian Peninsula and Mesopotamia. Authigenic uranium concentrations, on the other hand, are highest during warm Dansgaard–Oeschger interstadials when the southwest monsoon is intensified relative to the northwesterly winds. Our results also indicate that on orbital timescales maximum average eolian fluxes coincide with the timing of marine isotopic stage (MIS) 2 and 4, while minimum fluxes occur during MIS 1, 3 and 5. Although the forcing mechanism(s) controlling suborbital variabilities in monsoonal intensity is still debated, our findings suggest an atmospheric teleconnection between the low-latitude southwest monsoon and North Atlantic climate.

The accretion rate of extraterrestrial 3He based on oceanic 230Th flux and the relation to Os isotope variation over the past 200,000 years in an Indian Ocean core

Abstract In the eastern equatorial Indian Ocean, the flux of extraterrestrial 3 He, a proxy of interplanetary dust particles (IDPs), has been relatively constant over the past 200 ka. The flux is equal to (1.1±0.4)×10 −12 cm 3 STP cm −2 ka −1 , a value obtained using the xs 230 Th profiling method. Variations in mass accumulation rates (MARs) derived assuming a constant extraterrestrial 3 He flux have a 40-ka periodicity similar to that observed in the δ 18 O-derived MARs. This frequency is similar to that of the Earths obliquity. Measured 187 Os/ 188 Os ratios are less radiogenic than present-day seawater (0.49–0.98), reflecting the mixing of Os derived from extraterrestrial, terrigenous and hydrogenous sources. When coupled with He data measured on the same samples, Os isotope data yield important information about the terrigenous component supplied to the eastern equatorial Indian Ocean. The amount of Os in the sample derived from the extraterrestrial component can be deduced with the help of the helium systematics. Once corrected for the extraterrestrial component of Os, Os isotope signatures, in conjunction with the 4 He concentrations, suggest a supply of terrigenous material from Indonesian ultramafic and Himalayan crustal sources that clearly varies through time.

Abrupt intensification of the SW Indian Ocean monsoon during the last deglaciation: constraints from Th, Pa, and He isotopes

Abstract Sediments from western Arabian Sea core 74KL representing the last 23 ka were analyzed for helium, thorium, and protactinium isotopes. Assuming global average fluxes of extraterrestrial 3He and 230Th, the average 3He-derived sediment mass accumulation rate (MAR) is a factor of 1.8 higher than the average 230Th-derived MAR. 3He- and 230Th-derived MARs converge, however, during the Younger Dryas (YD) and during the peak of the early Holocene humid interval. These features, not seen anywhere else in the world, probably reflect a combination of climate-driven changes in the flux of 230Th and 3He. Ratios of xs231Pa/xs230Th, proxies of paleoproductivity, are lowest during the last glacial maximum (LGM), and increase abruptly during the Bolling–Allerod. Later, following a sudden decrease to near-LGM values during the YD, they rise abruptly to maximum values for the entire record in the early Holocene. We hypothesize that low xs231Pa/xs230Th ratios reflect low productivity due to the decreased intensity of the SW monsoon, whereas the opposite is true for high ratios. The correlation between Arabian Sea productivity and monsoonal upwelling, on the one hand, and North Atlantic climate variability, on the other, suggests a linkage between high- and low-latitude climates caused by changing patterns of atmospheric circulation.

TERRIGENOUS HELIUM IN DEEP-SEA SEDIMENTS

Abstract We have measured the isotope ratios of helium contained in various terrigenous materials that contribute to deep-sea sediments. These materials include ice-rafted debris from the North Atlantic, Chinese Loess, and sediment collected at or near the mouths of three large rivers: the Amazon, the Ganges, and the Yangtze. We observe terrigenous 3He/4He ratios that vary from 1.8 × 10−9 to 4.6 × 10−7, i.e., values that are higher than the theoretically-derived range of 10−9 to 10−8. Atlantic and Pacific deep-sea sediment 3He/4He ratios can be explained by mixing of helium from similar extraterrestrial but different terrigenous sources. Terrigenous sources for North Atlantic and North Pacific sediments are characterized by 3He and 4He contents that are higher, and 3He/4He ratios that are lower, than those for central and eastern equatorial Pacific sediments. This is consistent with the supply to the North Atlantic and North Pacific oceans of older cratonic continental material that contains high amounts of in situ-produced nucleogenic and radiogenic helium. Terrigenous material transported to central and eastern equatorial Pacific sediments contains lower amounts of 3He and 4He and higher 3He/4He ratios, indicative of supply from a more juvenile Andean source. In the equatorial Atlantic (core V31-135), we have used previously-published 230Th data to determine an extraterrestrial 3He flux of (1.16 ± 0.15) × 10−12 cm3STP · cm−2 · ka−1, within error of that previously determined in sediments from the equatorial Pacific Ocean ([0.78 ± 0.29] × 10−13 cm3STP · cm−2 · ka−1; Marcantonio et al., 1996 ).

Sediment focusing creates 100-ka cycles in interplanetary dust accumulation on the Ontong Java Plateau

The accumulation of extraterrestrial 3 He, a tracer for interplanetary dust particles (IDPs), in sediments from the Ontong Java Plateau (OJP; western equatorial Pacific Ocean) has been shown previously to exhibit a regular cyclicity during the late Pleistocene, with a period of V100 ka. Those results have been interpreted to reflect periodic variability in the global accretion of IDPs that, in turn, has been linked to changes in the inclination of Earth’s orbit with respect to the invariable plane of the solar system. Here we show that the accumulation in OJP sediments of authigenic 230 Th, produced by radioactive decay of 234 U in seawater, exhibits a 100-ka cyclicity similar in phase and amplitude to that evident in the 3 He record. We interpret the similar patterns of 230 Th and 3 He accumulation to reflect a common origin within the ocean^climate system. Comparing spatial and temporal patterns of sediment accumulation against regional patterns of biological productivity and against the well-established pattern of CaCO3 dissolution in the deep Pacific Ocean leads to the further conclusion that a common 100-ka cycle in accumulation of biogenic, authigenic and extraterrestrial constituents in OJP sediments reflects the influence of climate-related changes in sediment focusing, rather than changes in the rate of production or supply of sedimentary constituents. 7 2002 Elsevier Science B.V. All rights reserved.

Re-Os isotopic systematics in chromitites from the Stillwater Complex, Montana, USA

Abstract New Re-Os isotopic data on chromitites of the Stillwater Complex demonstrate isotopic equilibrium between cumulate chromite and whole rock. Initial osmium isotopic ratios for the chromitites, chosen for their freshness, are consistent with derivation from a mantle-derived magma that suffered little or no interaction with the continental crust prior to crystallization. Molybdenite, separated from a sample of the G-chromitite, yields a Re-Os age of 2740 Ma, indistinguishable from the age of the intrusion. The presence of molybdenite documents rhenium, and probably osmium, mobilization by hydrothermal fluids that permeated the intrusion shortly after crystallization. Initial osmium isotopic variability observed in chromitites and other rocks from the Stillwater Complex could result from interaction with these fluids. In this context, there is no compelling reason to call on assimilation of crust by mantle-derived magma to explain the osmium or neodymium isotopic variability. Although osmium isotopic systematics have been affected by hydrothermal processes, Re-Os results demonstrate that more than 95% of the osmium, and by inference other PGEs in the Stillwater Complex, derive from the mantle.