Tertiary Minette and Melanephelinite Dikes, Wasatch Plateau, Utah - Records of Mantle Heterogeneities and Changing Tectonics

A swarm of minette and melanephelinite dikes is exposed over 2500 km2 in and near the Wasatch Plateau, central Utah, along the western margin of the Colorado Plateaus in the transition zone with the Basin and Range province. To date, 110 vertical dikes in 25 dike sets have been recognized. Strikes shift from about N80-degrees-W for 24 Ma dikes, to about N60-degrees-W for 18 Ma, to due north for 8-7 m.y. These orientations are consistent with a shift from east-west Oligocene compression associated with subduction to east-west late Miocene crustal extension. Minettes are the most common rock type; mica-rich minette and mica-bearing melanephelinite occurs in 24 Ma dikes, whereas more ordinary minette is found in 8-7 Ma dikes. One melanephelinite dike is 18 Ma. These mafic alkaline rocks are transitional to one another in modal and major element composition but have distinctive trace element patterns and isotopic compositions; they appear to have crystallized from primitive magmas. Major, trace element, and Nd-Sr isotopic data indicate that melanephelinite, which has similarities to ocean island basalt, was derived from small degree melts of mantle with a chondritic Sm/Nd ratio probably located in the asthenosphere, but it is difficult to rule out a lithospheric source. In contrast, mica-bearing rocks (mica melanephelinite and both types of minette) are more potassic and have trace element patterns with strong Nb-Ta depletions and Sr-Nd isotopic compositions caused by involvement with a component from heterogeneously enriched lithospheric mantle with long-term enrichment of Rb or light rare earth elements (REE) (epsilon Nd as low as - 15 in minette). Light REE enrichment must have occurred anciently in the mid-Proterozoic when the lithosphere was formed and is not a result of Cenozoic subduction processes. After about 25 Ma, foundering of the subducting Farallon plate may have triggered upwelling of warm asthenospheric mantle to the base of the lithosphere. Melanephelinite magma may have separated from the asthenosphere and, while rising through the lithosphere, provided heat for lithospheric magma generation. Varying degrees of interaction between melanephelinite and small potassic melt fractions derived from the lithospheric mantle can explain the gradational character of the melanephelinite to minette suite.

Isotope ratios and trace element concentrations of basalts from DSDP Leg 15 sites (Appendix 1)

Formation of the Cretaceous Caribbean plateau, including the komatiites of Gorgona, has been linked to the currently active Galápagos hotspot. We use Hf-Nd isotopes and trace element data to characterise both the Caribbean plateau and the Galápagos hotspot, and to investigate the relationship between them. Four geochemical components are identified in the Galápagos mantle plume: two 'enriched' components with epsilon-Hf and epsilon-Nd similar to enriched components observed in other mantle plumes, one moderately enriched component with high Nb/Y, and a fourth component which most likely represents depleted MORB source mantle. The Caribbean plateau basalt data form a linear array in Hf-Nd isotope space, consistent with mixing between two mantle components. Combined Hf-Nd-Pb-Sr-He isotope and trace element data from this study and the literature suggest that the more enriched Caribbean end member corresponds to one or both of the enriched components identified on Galápagos. Likewise, the depleted end member of the array is geochemically indistinguishable from MORB and corresponds to the depleted component of the Galápagos system. Enriched basalts from Gorgona partially overlap with the Caribbean plateau array in epsilon-Hf vs. epsilon-Nd, whereas depleted basalts, picrites and komatiites from Gorgona have a high epsilon-Hf for a given epsilon-Nd, defining a high-epsilon-Hf depleted end member that is not observed elsewhere within the Caribbean plateau sequences. This component is similar, however, in terms of Hf-Nd-Pb-He isotopes and trace elements to the depleted plume component recognised in basalts from Iceland and along the Reykjanes Ridge. We suggest that the Caribbean plateau represents the initial outpourings of the ancestral Galápagos plume. Absence of a moderately enriched, high Nb/Y component in the older Caribbean plateau (but found today on the island of Floreana) is either due to changing source compositions of the plume over its 90 Ma history, or is an artifact of limited sampling. The high-epsilon-Hf depleted component sampled by the Gorgona komatiites and depleted basalts is unique to Gorgona and is not found in the Caribbean plateau. This may be an indication of the scale of heterogeneity of the Caribbean plateau system; alternatively Gorgona may represent a separate oceanic plateau derived from a completely different Pacific plume, such as the Sala y Gomez.

Chemical analyses of ODP Hole 105-647A basalts (Table 2)

Basalt samples recovered from the lowermost 37 m of Leg 105 Hole 647A in the Labrador Sea are fine- to medium grained, have microphenocrysts of clinopyroxene, and show little evidence of alteration. Chemically, these rocks are low potassium (0.01-0.09 wt% K20), olivine- to quartz-normative tholeiites that are also depleted in other incompatible elements. In terms of many of the incompatible trace elements, the Labrador Sea samples are similar both to iV-type midocean ridge basalts (MORBs) and to the terrestrial Paleocene volcanic rocks in the Davis Strait region of Baffin Island and West Greenland. However, significant differences are found in their strontium and neodymium isotope systematics. Hole 647A samples are more depleted in epsilon-Nd (+9.3) and are anomalously rich in 87Sr/86Sr (0.7040) relative to the Davis Strait basalts (epsilon-Nd +2.54 to + 8.97; mean 87Sr/86Sr, 0.7034). We conclude that the Hole 647A and Davis Strait basalts may have been derived from a similar depleted mantle source composition. In addition, the Davis Strait magmas were generated from mantle of more than one composition. We also suggest that there is no geochemical evidence from the Hole 647A samples to support or to refute the existence of foundered continental crust in the Labrador Sea.

Geochemistry and petrology of ODP Site 136-843 basalts

About 13 m of Cretaceous, tholeiitic basalt, ranging from normal (N-MORB) to transitional (T-MORB) mid-ocean-ridge basalts, was recovered at Ocean Drilling Program Site 843 west of the island of Hawaii. These moderately fractionated, aphyric lavas are probably representative of the oceanic basement on which the Hawaiian Islands were built. Whole-rock samples from parts of the cores exhibiting only slight, low-temperature, seawater alteration were analyzed for major element, trace element, and isotopic composition. The basalts are characterized by enrichment in the high field strength elements relative to N-MORB, by a distinct positive Eu anomaly, and by Ba/Nb and La/Nb ratios that are much lower than those of other crustal or mantle-derived rocks, but their isotope ratios are similar to those of present-day N-MORB from the East Pacific Rise. Hole 843A lavas are isotopically indistinguishable from Hole 843B lavas and are probably derived from the same source at a lower degree of partial melting, as indicated by lower Y/Nb and Zr/Nb ratios and by higher concentrations of light and middle rare earth elements and other incompatible elements relative to Hole 843B lavas. Petrographic and trace-element evidence indicates that the Eu anomaly was the result of neither plagioclase assimilation nor seawater alteration. The Eu anomaly and the enrichments in Ta, Nb, and possibly U and K relative to N-MORB apparently are characteristic of the mantle source. Age-corrected Nd and Sr isotopic ratios indicate that the source for the lavas recovered at ODP Site 843 was similar to the source for Southeast Pacific MORB. An enriched component within the Cretaceous mantle source of these basalts is suggested by their initial 208Pb/204Pb-206Pb/204Pb and epsilon-Nd-206Pb/204Pb ratios. The Sr-Pb isotopic trend of Hawaiian post-shield and post-erosional lavas cannot be explained by assimilation of oceanic crust with the isotopic composition of the Site 843 basalts.

REE and istotopic composition of ODP Site 127-797 clays

X-ray diffraction analyses have been carried out on 128 samples of Miocene to Quaternary sediments from ODP Sites 794, 795 and 797. Some clay fractions of samples from Site 797 have also been studied for rare earth elements and by Nd isotopic analyses. These three sites display similar lithological and clay assemblages (with dominant chlorite, illite and smectite) showing that the sedimentation was homogeneous throughout the whole Japan Sea Basin. Three mineralogical zones are recognized. The first zone (Lower Miocene sandy clay of Sites 794 and 797) is mainly composed of chlorite resulting from hydrothermal transformation of arc-derived smectite, due to sill injections during the initial oceanic spreading stage. The second zone (Lower Miocene to Lower Pliocene siliceous claystone and diatomaceous silty clay) is dominated by arc-derived smectite; the abundance of this mineral decreases upwards while illite and chlorite increase. This trend reflects a change of detrital source, from an eastern arc-derived source (epsilon -Nd**t>-3.3); variable LREE enrichment) to a western continental crust source (epsilon-Nd**t<-9.4; shale-like REE patterns); climatic modifications in the current dynamics are proposed as a cause for this change. The third zone (Upper Pliocene to Recent silty clay with minor diatom oozes) is characterized at Site 797 by increasing amounts of illite and chlorite. This reflects a more and more important western supply which is assumed to be related to tectonic rejuvenations of the Asian margin or climatic modifications affecting the alteration conditions or the current dynamics. At Sites 794 and 795, the more or less sharp supply of chlorite seems to be driven by the incipient subduction zone on the eastern margin of the Japan Sea.

Isotope geochemistry of Ninetyeast Ridge basalts

The Ninetyeast Ridge lavas have Sr and Nd isotopic ratios intermediate between those of Indian Ocean MORBs and those of the very enriched Kerguelen hot spot. In an Nd-Sr isotope diagram, they also plot close to the fields of St. Paul Island lavas and of the early magmatism on Kerguelen Archipelago. The Ninetyeast Ridge lavas were generated by mixing among at least three components: a depleted, MORB-type component, such as the one erupted today on the Southeast Indian Ridge; a very enriched, high- Sr/ Sr, low-epsilon-Nd, OIB-type component (the Kerguelen hot spot); and an OIB-type component comparable to that sampled from the St. Paul (and Amsterdam) lavas. The Ninetyeast Ridge lavas show a typical Dupal anomaly signature and Pb, Sr, and Nd isotopic systematics indicate that the Kerguelen hot spot was involved in the ridge's formation as the Indian plate moved northward. The different sites cored during ODP Leg 121 show a trend in their isotopic compositions, from less radiogenic Pb/ Pb ratios and intermediate 87Sr/86Sr and 143Nd/**Nd ratios in the oldest lavas (Site 758) toward more radiogenic 206Pb/204Pb, higher epsilon-Nd, and lower 87Sr/86Sr values in the youngest lavas (Site 756). The lavas from Site 757 have 206Pb/204Pb ratios intermediate between those of the lavas from Sites 756 and 758 and higher 87Sr/86Sr and lower epsilon-Nd values. The relative proportions of the hot spot(s) and MORB component have evolved with time, reflecting differences of tectonic setting: the relative proportion of the Kerguelen hot spot component appears lower in the younger Site 756 lavas than in the older lavas from Sites 757 and 758. Site 756 coincides with the beginning of rifting at the Southeast Indian Ridge, about 43 Ma ago. The formation of the early Kerguelen Archipelago lavas may have drained most of the plume-derived material toward the Antarctic plate. Alternatively, the proximity of the spreading-ridge axis may account for the isotopic similarity of the Site 756 lavas to young lavas erupted on the Southeast Indian Ridge, from 33? to 37?S. The older lavas of Ninetyeast Ridge may have formed when the hot spot and ridge axis did not exactly coincide. The involvement of the third component, a St. Paul hot spot, in the genesis of the Ninetyeast Ridge lavas, especially for the Site 756 lavas, is clearly indicated by Sr, Pb, and Nd isotope systematics and also by trace element ratios. These data, together with those from the Kerguelen Plateau, indicate that the Kerguelen hot spot has been active more or less continuously in the South Indian Ocean for at least 115 Ma. This could indicate that the plume, and by inference the Dupal anomaly, is deep seated in origin.

Geochemistry of ODP Sites 121-758 and 121-757

Nd isotopes are useful tracers for paleoceanography due to the short Nd residence time in seawater and the large differences between the isotopic signatures of various geological reservoirs. Therefore, ?Nd variations reflect the geological history of individual oceanic basins. Using a differential dissolution technique, which extracts Nd isotopes of seawater trapped in MnO2 coatings and carbonates in marine sediment, we measured almost two hundred samples from ODP Sites 758 and 757 in the Northern Bay of Bengal covering the last 4 Ma. For the first time, we have shown a covariation between epsilon-Nd and d18O over at least the last 800 ka. We also show that from 4 Ma to 2.6 Ma, epsilon-Nd is almost constant and starts to fluctuate at 2.6 Ma when northern glaciations increased. From 2.6 Ma to 1 Ma the fluctuation period is close to 40 ka while from 1 Ma to present it is dominantly 100 ka. We attribute these findings to mixing between Himalayan river water (that ultimately originates as Indian summer monsoon rain) and normal Bay of Bengal seawater. Previous studies on seawater, using epsilon-Nd, d18O analyzed on planktonic foraminifera and sedimentary data, can be integrated into this model. A simple quantitative binary mixing model suggests that the summer monsoon rain was more intense during interglacial than glacial periods. During last glacial episode, the monsoon trajectory was deviated to the east. At a large scale, the Indian monsoon is fully controlled by the variations in Northern Hemisphere climate but with a complex response function to this forcing. Our study clearly establishes the large potential of Nd isotope data to evaluate the hydrological river regime during the Quaternary and its relationship with climate fluctuations, particularly when the sediment archive is sampled close to sediment sources.

Neodymium isotopic data, radiocarbon dates and calibrated ages of sediment samples off the La Plata Estuary, southeastern South America

The continental margin off the La Plata Estuary (SE South America) is characterized by high fluvial sediment supply and strong ocean currents. High-resolution sediment-acoustic data combined with sedimentary facies analysis, AMS-14C ages, and neodymium isotopic data allowed us to reconstruct late Quaternary sedimentary dynamics in relation to the two major sediment sources, the La Plata Estuary and the Argentine margin. Sediments from these two provinces show completely different dispersal patterns. We show that the northward-trending La Plata paleo-valley was the sole transit path for the huge volumes of fluvial material during lower sea levels. In contrast, material from the Argentine margin sector was transported northwards by the strong current system. Despite the large sediment volumes supplied by both sources, wide parts of the shelf were characterized by either persistent non-deposition or local short-term depocenter formation. The location and formation history of these depocenters were primarily controlled by the interplay of sea level with current strength and local morphology. The high sediment supply was of secondary importance to the stratigraphic construction, though locally resulting in high sedimentation rates. Thus, the shelf system off the La Plata Estuary can be considered as a hydrodynamic-controlled end-member.

Geochemistry of North Atlantic Igneous Province basalts

New Sr-Nd-Pb-Hf data require the existence of at least four mantle components in the genesis of basalts from the the North Atlantic Igneous Province (NAIP): (1) one (or more likely a small range of) enriched component(s) within the Iceland plume, (2) a depleted component within the Iceland plume (distinct from the shallow N-MORB source), (3) a depleted sheath surrounding the plume and (4) shallow N-MORB source mantle. These components have been available since the major phase of igneous activity associated with plume head impact during Paleogene times. In Hf-Nd isotope space, samples from Iceland, DSDP Leg 49 (Sites 407, 408 and 409), ODP Legs 152 and 163 (southeast Greenland margin), the Reykjanes Ridge, Kolbeinsey Ridge and DSDP Leg 38 (Site 348) define fields that are oblique to the main ocean island basalt array and extend toward a component with higher 176Hf/177Hf than the N-MORB source available prior to arrival of the plume, as indicated by the compositions of Cretaceous basalts from Goban Spur (~95 Ma). Aside from Goban Spur, only basalts from Hatton Bank on the oceanward side of the Rockall Plateau (DSDP Leg 81) lie consistently within the field of N-MORB, which indicates that the compositional influence of the plume did not reach this far south and east ~55 Ma ago. Thus, Hf-Nd isotope systematics are consistent with previous studies which indicate that shallow MORB-source mantle does not represent the depleted component within the Iceland plume (Thirlwall, J. Geol. Soc. London 152 (1995) 991-996; Hards et al., J. Geol. Soc. London 152 (1995) 1003-1009; Fitton et al., 1997 doi:10.1016/S0012-821X(97)00170-2). They also indicate that the depleted component is a long-lived and intrinsic feature of the Iceland plume, generated during an ancient melting event in which a mineral (such as garnet) with a high Lu/Hf was a residual phase. Collectively, these data suggest a model for the Iceland plume in which a heterogeneous core, derived from the lower mantle, consists of 'enriched' streaks or blobs dispersed in a more depleted matrix. A distinguishing feature of both the enriched and depleted components is high Nb/Y for a given Zr/Y (i.e. positive DeltaNb), but the enriched component has higher Sr and Pb isotope ratios, combined with lower epsilon-Nd and epsilon-Hf. This heterogeneous core is surrounded by a sheath of depleted material, similar to the depleted component of the Iceland plume in its epsilon-Nd and epsilon-Hf, but with lower 87Sr/86Sr, 208Pb/204Pb and negative DeltaNb; this material was probably entrained from near the 670 km discontinuity when the plume stalled at the boundary between the upper and lower mantle. The plume sheath displaced more normal MORB asthenosphere (distinguished by its lower epsilon-Hf for a given epsilon-Nd or Zr/Nb ratio), which existed in the North Atlantic prior to plume impact. Preliminary data on MORBs from near the Azores plume suggest that much of the North Atlantic may be 'polluted' not only by enriched plume material but also by depleted material similar to the Iceland plume sheath. If this hypothesis is correct, it may provide a general explanation for some of the compositional diversity and variations in inferred depth of melting (Klein and Langmuir, 1987 doi:10.1029/JB092iB08p08089) along the MAR in the North Atlantic.

SHRIMP zircon geochronology and geochemistry of Phanerozoic granitoids in southeastern Korea

Phanerozoic granitoids are widespread in the Korean Peninsula and form a part of the East Asian Cordilleran-type granitoid belt extending from southeastern China to Far East Russia. Here we present SHRIMP zircon U-Pb ages and geochemical and Nd isotopic compositions of Late Paleozoic to Early Jurassic granitoid plutons in the northern Gyeongsang basin, southeastern Korea; namely the Jangsari, Yeongdeok, Yeonghae, and Satkatbong plutons. The granite and associated gabbroic rocks from the Jangsari pluton were coeval and respectively dated at 257.3 ± 2.0 Ma and 255.7 ± 1.4 Ma. This result represents the first finding of a Late Paleozoic pluton in South Korea. Three granite samples from the Yeongdeok pluton yielded a slightly younger age span ranging from 252.9 ± 2.5 Ma to 246.7 ± 2.1 Ma. Two diorite samples from the Yeonghae pluton gave much younger ages of 195.1 ± 1.9 Ma and 196.3 ± 1.6 Ma. An Early Jurassic age of 192.4 ± 1.6 Ma was also obtained from a diorite sample from the Satkatbong pluton. The mineral assemblage and Al2O3/(Na2O + K2O) versus Al2O3/(CaO + Na2O + K2O) relationship indicate that all the analyzed plutons are subduction zone granitoids. Enrichments in large-ion-lithophile-elements and depletions in high-field-strength-elements of these plutons are also concordant with geochemical characteristics typical for the subduction zone magma. The presence of Late Permian to Early Triassic arc system is in contrast with the conventional idea that the arc magmatism along the continental margin of the Korean Peninsula has commenced from Early Jurassic after the termination of Triassic collisional orogenesis. The epsilon-Nd(t) values of the granitoid plutons are consistently positive (2.4-4.6), suggesting that crustal residence time of the basement beneath the Gyeongsang basin is relatively short. Moreover, the reevaluation of previously-published data reveals that geochemical compositions of the Yeongdeok pluton are compatible with those of high-silica adakites; La/Yb = 37.5-114.6, Sr/Y = 138.2-214.0, SiO2 = 62.9-72.0 wt. %, Al2O3 = 15.5-17.0 wt. %, Sr = 562-1173 ppm, MgO = 0.4-1.6 wt. %, Y = 3-6 ppm, Yb = 0.18-0.45 ppm, and Eu/Eu* = 0.92-1.31. The occurrence of adakites in southeastern Korea, and presumably in the Hida belt of central-western Japan, is indicative of a hot subduction regime developing at least partly along the East Asian continental margin during the Permian-Triassic transition period.

Neodymium isotope composition of planktonic foraminifera from ODP Hole 121-758A

This study presents neodymium isotope and elemental data for cleaned planktonic foraminifera from ODP site 758 in the southernmost reaches of the Bay of Bengal in the north-east Indian Ocean. Cleaning experiments using oxidative-reductive techniques suggest that diagenetic Fe-Mn oxyhydroxide coatings can be effectively removed, and that the measured Nd isotope composition reflects the composition of seawater from which the foraminiferal calcium carbonate was precipitated. Modern core-top Pulleniatina obliquiloculata and Globorotalia menardii give epsilon-Nd values of 310.12 +/- 0.16 and 310.28 +/- 0.16, respectively, indistinguishable from recent direct measurements of surface seawater in this area. A high-resolution Nd isotope record obtained from G. menardii for the past 150 kyr shows systematic variations (Delta epsilon-Nd = 3) on glacial-interglacial timescales. The timing of those variations shows a remarkable correspondence with the global oxygen isotope record, which suggests a process controlling the Nd isotope composition that responds in phase with global climate cycles. Palaeoclimate reconstruction indicates that during the last glacial maximum changes in monsoon circulation resulted in a reduction in rainfall over the Indian subcontinent, and a decrease in the flux of river water delivered to the Bay of Bengal. Thus, changes in the riverine input of Nd, a change in either flux or composition, most likely caused the isotope variations, although changes in dust source or local ocean circulation may have also played a role. These results clearly establish a link between climate change and variations in radiogenic isotopes in the oceans, and illustrate the potential of Nd isotopes in foraminifera for highresolution palaeoceanographic reconstruction.

Neodymium isotopes from fossil fish teeth and detrital sediment, and oxygen isotopes from benthic forams from ODP Hole 119-738 B

Ephemeral polar glaciations during the middle-to-late Eocene (48-34 Ma) have been proposed based on far-field ice volume proxy records and near-field glacigenic sediments, although the scale, timing, and duration of these events are poorly constrained. Here we confirm the existence of a transient cool event within a new high-resolution benthic foraminiferal d18O record at Ocean Drilling Program (ODP) Site 738 (Kerguelen Plateau; Southern Ocean). This event, named the Priabonian oxygen isotope maximum (PrOM) Event, lasted ~140 kyr and is tentatively placed within magnetochron C17n.1n (~37.3 Ma) based on the correlation to ODP Site 689 (Maud Rise, Southern Ocean). A contemporaneous change in the provenance of sediments delivered to the Kerguelen Plateau occurs at the study site, determined from the <63 µm fraction of decarbonated and reductively leached sediment samples. Changes in the mixture of bottom waters, based on fossil fish tooth epsilon-Nd, were less pronounced and slower relative to the benthic d18O and terrigenous epsilon-Nd changes. Terrigenous sediment epsilon-Nd values rapidly shifted to less radiogenic signatures at the onset of the PrOM Event, indicating an abrupt change in provenance favoring ancient sources such as the Paleoproterozoic East Antarctic craton. Bottom water epsilon-Nd reached a minimum value during the PrOM Event, although the shift begins much earlier than the terrigenous epsilon-Nd excursion. The origin of the abrupt change in terrigenous sediment provenance is compatible with a change in Antarctic terrigenous sediment flux and/or source as opposed to a reorganization of ocean currents. A change in terrigenous flux and/or source of Antarctic sediments during the oxygen isotope maximum suggests a combination of cooling and ice growth in East Antarctica during the early late Eocene.

Neodymium isotope ratios of fish debris from late Cretaceous black shales

Neodymium isotopes of fish debris from two sites on Demerara Rise, spanning ~4.5 m.y. of deposition from the early Cenomanian to just before ocean anoxic event 2 (OAE2) (Cenomanian-Turonian transition), suggest a circulation-controlled nutrient trap in intermediate waters of the western tropical North Atlantic that could explain continuous deposition of organic-rich black shales for as many as ~15 m.y. (Cenomanian-early Santonian). Unusually low Nd isotopic data (epsilon-Nd(t) ~-11 to ~-16) on Demerara Rise during the Cenomanian are confirmed, but the shallower site generally exhibits higher and more variable values. A scenario in which southwest-flowing Tethyan and/or North Atlantic waters overrode warm, saline Demerara bottom water explains the isotopic differences between sites and could create a dynamic nutrient trap controlled by circulation patterns in the absence of topographic barriers. Nutrient trapping, in turn, would explain the ~15 m.y. deposition of black shales through positive feedbacks between low oxygen and nutrient-rich bottom waters, efficient phosphate recycling, transport of nutrients to the surface, high productivity, and organic carbon export to the seafloor. This nutrient trap and the correlation seen previously between high Nd and organic carbon isotopic values during OAE2 on Demerara Rise suggest that physical oceanographic changes could be components of OAE2, one of the largest perturbations to the global carbon cycle in the past 150 m.y.

Neodymium isotope record of late Paleocene to eraly Eocene fish teeth

High-resolution, fish tooth Nd isotopic records for eight Deep Sea Drilling Project and Ocean Drilling Program sites were used to reconstruct the nature of late Paleocene-early Eocene deep-water circulation. The goal of this reconstruction was to test the hypothesis that a change in thermohaline circulation patterns caused the abrupt 4-5°C warming of deep and bottom waters at the Paleocene/Eocene boundary - the Paleocene-Eocene thermal maximum (PETM) event. The combined set of records indicates a deep-water mass common to the North and South Atlantic, Southern and Indian oceans characterized by mean epsilon-Nd values of ~-8.7, and different water masses found in the central Pacific Ocean (epsilon-Nd ~-4.3) and Caribbean Sea (epsilon-Nd ~1.2). The geographic pattern of Nd isotopic values before and during the PETM suggests a Southern Ocean deep-water formation site for deep and bottom waters in the Atlantic and Indian ocean basins. The Nd data do not contain evidence for a change in the composition of deep waters prior to the onset of the PETM. This finding is consistent with the pattern of warming established by recently published stable isotope records, suggesting that deep- and bottom-water warming during the PETM was gradual and the consequence of surface-water warming in regions of downwelling.

Geochemistry of Mid-Atlantic Ridge sediments

Geochemical compositions and Sr and Nd isotopes were measured in two cores collected ~2 and 5 km from the Rainbow hydrothermal vent site on the Mid-Atlantic Ridge. Overall, the cores record enrichments in Fe and other metals from hydrothermal fallout, but sequential dissolution of the sediments allows discrimination between a leach phase (easily leachable) and a residue phase (refractory). The oxy-anion and transition metal distribution combined with rare earth element (REE) patterns suggest that (1) the leach fraction is a mixture of biogenic carbonate and hydrothermal Fe-Mn oxy-hydroxide with no significant contribution from detrital material and (2) >99.5% of the REE content of the leach fraction is of seawater origin. In addition, the leach fraction has an average 87Sr/86Sr ratio indistinguishable from modern seawater at 0.70916. Although we lack the epsilon-Nd value of present-day deep water at the Rainbow vent site, we believe that the REE budget of the leach fraction is predominantly of seawater origin. We suggest therefore that the leach fraction provides a record of local seawater epsilon-Nd values. Nd isotope data from these cores span the period of 4-14 ka (14C ages) and yield epsilon-Nd values for North East Atlantic Deep Water (NEADW) that are higher (-9.3 to -11.1) than those observed in the nearby Madeira Abyssal Plain from the same depth (-12.4 ± 0.9). This observation suggests that either the Iceland-Scotland Overflow Water (ISOW) and Lower Deep Water contributions to the formation of NEADW are higher along the Mid-Atlantic Ridge than in the surrounding basins or that the relative proportion of ISOW was higher during this period than is observed today. This study indicates that hydrothermal sediments have the potential to provide a higher-resolution record of deep water epsilon-Nd values, and hence deepwater circulation patterns in the oceans, than is possible from other types of sediments.