Geochemical composition of sediments from ODP Site 164-997, Blake Ridge

Twenty-two trace elements in 355 sediment samples from Site 997 on the Blake Ridge were examined by inductively coupled plasma-optical emission spectrometry and inductively coupled plasma-mass spectrometry, for respective fractions of acid-soluble and insoluble compositions. Downhole profiles of these elements exhibit complicated fluctuations throughout late Miocene to Pleistocene, principally due to the variations in the acid-soluble fraction. Noncarbonate composition is given from the acid-insoluble residues, which permits us to recognize secular feature of selected element variance for four intervals. These intervals (I: 0-183 mbsf; II: 183- 440 mbsf; III: 440-618 mbsf; and IV: 618-750 mbsf) are interpreted to have originated from changes in the suite of sediments of particular sources and chemical composition, sedimentation rate, dilution of biogenic carbonate abundance, and possibly the current system that controlled deposition and reworking of the terrigenous materials.

Accumulation rates of sediments and main sedimentary components in ODP Leg 121 holes on Broken Ridge

Broken Ridge, in the eastern Indian Ocean,is overlain by about 1600 m of middle Cretaceous to Pleistocene tuffaceous and carbonate sediments that record the oceanographic history of southern hemisphere mid-to high-latitude regions. Prior to about 42 Ma, Broken Ridge formed the northern part of the broad Kerguelen-Broken Ridge Plateau. During the middle Eocene, this feature was split by the newly forming Southeast Indian Ocean Ridge; since then, Broken Ridge has drifted north from about 55° to 31°S. The lower part of the sedimentary section is characterized by Turonian to Santonian tuffs that contain abundant glauconite and some carbonate. The tuffs record a large but apparently local volcanic input that characterized the central part of Broken Ridge into the early Tertiary. Maestrichtian shallow-water(several hundred to 1000 m depth) limestones and cherts accumulated at some of the highest rates ever documented from the open ocean, 4 to 5 g/cm**2/kyr. A complete (with all biostratigraphic zones) Cretaceous-Tertiary boundary section was recovered from site 752. The first 1.5 m.y. of the Tertiary is characterized by an order-of-magnitude reduction in the flux of biogenic sediments, indicating a period of sharply reduced biological productivity at 55°S, following which the carbonate and silica sedimentation rates almost reach the previous high values of the latest Cretaceous. We recovered a complete section through the Paleocene that contains all major fossil groups and is more than 300 m thick, perhaps the best pelagic Paleocene section encountered in ocean drilling. About 42 Ma, Broken Ridge was uplifted 2500 m in response to the intra-plateau rifting event; subsequent erosion and deposition has resulted in a prominent Eocene angular unconformity atop the ridge. An Oligocene disconformity characterized by a widespread pebble layer probably represents the 30 Ma sea-level fall. The Neogene pelagic ooze on Broken Ridge has been winnowed, and thus its grain size provides a direct physical record of the energy of the southern hemisphere drift current in the Indian Ocean for the past 30 m.y.

The early Eocene global warming events: Implications from ODP Leg 208 Walvis Ridge depth transect

Five sections drilled in multiple holes over a depth transect of more than 2200 m at the Walvis Ridge (SE Atlantic) during Ocean Drilling Program (ODP) Leg 208 resulted in the first complete early Paleogene deep-sea record. Here we present high-resolution stratigraphic records spanning a ~4.3 million yearlong interval of the late Paleocene to early Eocene. This interval includes the Paleocene-Eocene thermal maximum (PETM) as well as the Eocene thermal maximum (ETM) 2 event. A detailed chronology was developed with nondestructive X-ray fluorescence (XRF) core scanning records and shipboard color data. These records were used to refine the shipboard-derived spliced composite depth for each site and with a record from ODP Site 1051 were then used to establish a continuous time series over this interval. Extensive spectral analysis reveals that the early Paleogene sedimentary cyclicity is dominated by precession modulated by the short (100 kyr) and long (405 kyr) eccentricity cycles. Counting of precession-related cycles at multiple sites results in revised estimates for the duration of magnetochrons C24r and C25n. Direct comparison between the amplitude modulation of the precession component derived from XRF data and recent models of Earth's orbital eccentricity suggests that the onset of the PETM and ETM2 are related to a 100-kyr eccentricity maximum. Both events are approximately a quarter of a period offset from a maximum in the 405-kyr eccentricity cycle, with the major difference that the PETM is lagging and ETM2 is leading a 405-kyr eccentricity maximum. Absolute age estimates for the PETM, ETM2, and the magnetochron boundaries that are consistent with recalibrated radiometric ages and recent models of Earth's orbital eccentricity cannot be precisely determined at present because of too large uncertainties in these methods. Nevertheless, we provide two possible tuning options, which demonstrate the potential for the development of a cyclostratigraphic framework based on the stable 405-kyr eccentricity cycle for the entire Paleogene.

Geochemistry of rare earth elements in basalts from Walvis Ridge

Selected basalts from a suite of dredged and drilled samples (IPOD sites 525, 527, 528 and 530) from the Walvis Ridge have been analysed to determine their rare earth element (REE) contents in order to investigate the origin and evolution of this major structural feature in the South Atlantic Ocean. All of the samples show a high degree of light rare earth element (LREE) enrichment, quite unlike the flat or depleted patterns normally observed for normal mid-ocean ridge basalts (MORBs). Basalts from Sites 527, 528 and 530 show REE patterns characterised by an arcuate shape and relatively low (Ce/Yb)N ratios (1.46-5.22), and the ratios show a positive linear relationship to Nb content. A different trend is exhibited by the dredged basalts and the basalts from Site 525, and their REE patterns have a fairly constant slope, and higher (Ce/Yb)N ratios (4.31-8.50). These differences are further reflected in the ratios of incompatible trace elements, which also indicate considerable variations within the groups. Mixing hyperbolae for these ratios suggest that simple magma mixing between a 'hot spot' type of magma, similar to present-day volcanics of Tristan da Cunha, and a depleted source, possibly similar to that for magmas being erupted at the Mid-Atlantic Ridge, was an important process in the origin of parts of the Walvis Ridge, as exemplified by Sites 527, 528 and 530. Site 525 and dredged basalts cannot be explained by this mixing process, and their incompatible element ratios suggest either a mantle source of a different composition or some complexity to the mixing process. In addition, the occurrence of different types of basalt at the same location suggests there is vertical zonation within the volcanic pile, with the later erupted basalts becoming more alkaline arid more enriched in incompatible elements. The model proposed for the origin and evolution of the Walvis Ridge involves an initial stage of eruption in which the magma was essentially a mixture of enriched and depleted end-member sources, with the N-MORB component being small. The dredged basalts and Site 525, which represent either later-stage eruptives or those close to the hot spot plume, probably result from mixing of the enriched mantle source with variable amounts and variable low degrees of partial melting of the depleted mantle source. As the volcano leaves the hot spot, these late-stage eruptives continue for some time. The change from tholeiitic to alkalic volcanism is probably related either to evolution in the plumbing system and magma chamber of the individual volcano, or to changes in the depth of origin of the enriched mantle source melt, similar to processes in Hawaiian volcanoes.

Upper Cretaceous sediments of ODP Hole 122-762C

Well-developed Campanian to Maestrichtian pelagic cyclic sediments were recovered from Hole 762C on the Exmouth Plateau, off northwest Australia, during Ocean Drilling Program Leg 122. The cycles consist of nannofossil chalk (light beds) and clayey nannofossil chalk (dark beds). Both light and dark beds are strongly to moderately bioturbated, alternate on a decimeter scale, and exhibit gradual boundaries. Bioturbation introduces materials from a bed of one color into an underlying bed of another color, indicating that diagenesis is not responsible for the cyclicity. Differences in composition between the light and dark beds, revealed by calcium carbonate measurement and X-ray diffraction analysis, together with trace fossil evidence, indicate that the cycles in the sediments are a depositional feature. Diagenetic processes may have intensified the appearance of the cycles. Spectral analysis was applied to the upper Campanian to lower Maestrichtian cyclic sediments to examine the regularity of the cycles. Power spectra were calculated from time series using Walsh spectral analysis. The most predominant wavelengths of the color cycles are 34-41 cm and 71-84 cm. With an average sedimentation rate of 1.82 cm/k.y. in this interval, we found the time durations of the cycles to be around 41 k.y. and 21 k.y., respectively, comparable to the obliquity and precession periods of the Earth's rotation, which strongly suggests an orbital origin for the cycles. On the basis of sedimentological evidence and plate tectonic reconstruction, we propose the following mechanism for the formation of the cyclic sediments from Hole 762C. During the Late Cretaceous, when there was no large-scale continental glaciation, the cyclic variations in insolation, in response to cyclic orbital changes, controlled the alternation of two prevailing climates in the area. During the wetter, equable, and warmer climatic phases under high insolation, more clay minerals and other terrestrial materials were produced on land and supplied by higher runoff to a low bioproductivity ocean, and the dark clayey beds were deposited. During the drier and colder climatic phases under low insolation, fewer clay minerals were produced and put into the ocean, where bioproductivity was increased and the light beds were deposited.

Cyclostratigraphy and eccentricity tuning of the early Oligocene through early Miocene record from Walvis Ridge

Few astronomically calibrated high-resolution (<=5 kyr) climate records exist that span the Oligocene-Miocene time interval. Notably, available proxy records show responses varying in amplitude at frequencies related to astronomical forcing, and the main pacemakers of global change on astronomical time-scales remain debated. Here we present newly generated X-ray fluorescence core scanning and benthic foraminiferal stable oxygen and carbon isotope records from Ocean Drilling Program Site 1264 (Walvis Ridge, southeastern Atlantic Ocean). Complemented by data from nearby Site 1265, the Site 1264 benthic stable isotope records span a continuous ~13-Myr interval of the Oligo-Miocene (30.1-17.1 Ma) at high resolution (~3.0 kyr). Spectral analyses in the stratigraphic depth domain indicate that the largest amplitude variability of all proxy records is associated with periods of ~3.4 m and ~0.9 m, which correspond to 405- and ~110-kyr eccentricity, using a magnetobiostratigraphic age model. Maxima in CaCO3 content, d18O and d13C are interpreted to coincide with ~110 kyr eccentricity minima. The strong expression of these cycles in combination with the weakness of the precession- and obliquity-related signals allow construction of an astronomical age model that is solely based on tuning the CaCO3 content to the nominal (La2011_ecc3L) eccentricity solution. Very long-period eccentricity maxima (~2.4-Myr) are marked by recurrent episodes of high-amplitude ~110-kyr d18O cycles at Walvis Ridge, indicating greater sensitivity of the climate/cryosphere system to short eccentricity modulation of climatic precession. In contrast, the responses of the global (high-latitude) climate system, cryosphere, and carbon cycle to the 405-kyr cycle, as expressed in benthic d18O and especially d13C signals, are more pronounced during ~2.4-Myr minima. The relationship between the recurrent episodes of high-amplitude ~110-kyr d18O cycles and the ~1.2-Myr amplitude modulation of obliquity is not consistent through the Oligo-Miocene. Identification of these recurrent episodes at Walvis Ridge, and their pacing by the ~2.4-Myr eccentricity cycle, revises the current understanding of the main climate events of the Oligo-Miocene.

PIES time series in the North Atlantic at the Mid-Atlantic Ridge (2006-2010)

In the western North Atlantic, warm and saline water is brought by the North Atlantic Current (NAC) from the subtropics into the subpolar gyre. Four inverted echo sounders with high precision pressure sensors (PIES) were moored between 47°40' N and 52°30' N to study the main pathways of the NAC from the western into the eastern basin. The array configuration that forms three segments (northern, central, and southern) allows partitioning of the NAC and some assessment of NAC flow paths through the different Mid-Atlantic Ridge fracture zones. We exploit the correlation between the NAC transport measured between 2006 and 2010 and the geostrophic velocity from altimeter data to extend the time series of NAC transports to the period from 1992 to 2013. The mean NAC transport over the entire 21 years is 27 ± 5 Sv, consisting of 60% warm water of subtropical origin and 40% subpolar water. We did not find a significant trend in the total transport time series, but individual segments had opposing trends, leading to a more focused NAC in the central subsection and decreasing transports in the southern and northern segments. The spectral analysis exhibits several significant peaks. The two most prominent are around 120 days, identified as the time scale of meanders and eddies, and at 4-9 years, most likely related to the NAO. Transport composites for the years of highest and lowest NAO indices showed a significantly higher transport (+2.9 Sv) during strong NAO years, mainly in the southern segment.

Rare earth element concentrations and geochemistry of carbonate minerals from the Logatchev Hydrothermal Field and the Gakkel Ridge

The ultramafic-hosted Logatchev Hydrothermal Field (LHF) at 15°N on the Mid-Atlantic Ridge and the Arctic Gakkel Ridge (GR) feature carbonate precipitates (aragonite, calcite, and dolomite) in voids and fractures within different types of host rocks. We present chemical and Sr isotopic compositions of these different carbonates to examine the conditions that led to their formation. Our data reveal that different processes have led to the precipitation of carbonates in the various settings. Seawater-like 87Sr/86Sr ratios for aragonite in serpentinites (0.70909 to 0.70917) from the LHF are similar to those of aragonite from the GR (0.70912 to 0.70917) and indicate aragonite precipitation from seawater at ambient conditions at both sites. Aragonite veins in sulfide breccias from LHF also have seawater-like Sr isotope compositions (0.70909 to 0.70915), however, their rare earth element (REE) patterns show a clear positive europium (Eu) anomaly indicative of a small (< 1%) hydrothermal contribution. In contrast to aragonite, dolomite from the LHF has precipitated at much higher temperatures (~100 °C), and yet its 87Sr/86Sr ratios (0.70896 to 0.70907) are only slightly lower than those of aragonite. Even higher temperatures are calculated for the precipitation of deformed calcite veins in serpentine-talc fault schists form north of the LHF. These calcites show unradiogenic 87Sr/86Sr ratios (0.70460 to 0.70499) indicative of precipitation from evolved hydrothermal fluids. A simple mixing model based on Sr mass balance and enthalpy conservation indicates strongly variable conditions of fluid mixing and heat transfers involved in carbonate formation. Dolomite precipitated from a mixture of 97% seawater and 3% hydrothermal fluid that should have had a temperature of approximately 14 °C assuming that no heat was transferred. The much higher apparent precipitation temperatures based on oxygen isotopes (~ 100 °C) may be indicative of conductive heating, probably of seawater prior to mixing. The hydrothermal calcite in the fault schist has precipitated from a mixture of 67% hydrothermal fluid and 33% seawater, which should have had an isenthalpic mixing temperature of ~ 250 °C. The significantly lower temperatures calculated from oxygen isotopes are likely due to conductive cooling of hydrothermal fluid discharging along faults. Rare earth element patterns corroborate the results of the mixing model, since the hydrothermal calcite, which formed from waters with the greatest hydrothermal contribution, has REE patterns that closely resemble those of vent fluids from the LHF. Our results demonstrate, for the first time, that (1) precipitation from pure seawater, (2) conductive heating of seawater, and (3) conductive cooling of hydrothermal fluids in the sub-seafloor all can lead to carbonate precipitation within a single ultramafic-hosted hydrothermal system.