IODP - ICDP Interactions: Comparing and Contrasting the Drilled Paleoclimate Records on the African Continent With the ODP Records Offshore
The Saharan dust records obtained from ocean drill sites off northwest and northeast Africa stood for years as the icons of African climate. They nicely illustrated a progression in aridification and in the dominant orbital forcing of aridity at precessional frequency prior to 2.8 Ma, then to obliquity forcing from 2.8 to 1.0 Ma, and finally to the eccentricity forcing of the past million years. African climate, in essence, was thought to be responding primarily to the ice sheet dynamics of the Northern Hemisphere. ICDP-sponsored drilling programs on Lake Malawi, East Africa and Lake Bosumtwi, West Africa, have revealed startling new insights into the history of tropical Africa, at least for the past few hundred thousand years. While the conditions of the last glacial maximum were relatively cool and dry, the aridity of the LGM paled in comparison to the megadroughts that occurred in the African tropics prior to 60,000 years ago, with strong precessional (not eccentrical) frequency. Paleotemperature records from East African lakes are a relatively new addition to our knowledge of past African climate, and they are providing surprising, new results as well. They indicate, for example, much cooler temperatures in marine isotope stage IV than during the LGM. High-resolution records derived from XRF scans of the Malawi core indicate strong evidence for D-O - scale variability in the climate of this region, at least in terms of wind and aridity, indicating a tropical role in the "bi-polar see-saw." Promising new records of climate change on the African continent have been coming out of marine sediment cores from the major river deltas of Africa - the Congo, the Nile, in the Gulf of Guinea, and the Zambezi. The results of these studies can nicely complement the lacustrine records, for example by reflecting regions on the African continent where lake sequences are not available. Future drilling by IODP into these marginal settings around the African continent will provide insightful contrasts with the records to be derived from new drilling in the lakes and selected outcrops on the African continent.
Sr-Pb-Os in the Arctic Ocean: Revealing Environmental Cenozoic Changes
The ACEX coring expedition's (2004) goal was to investigate the geological and paleo-environmental evolution of the Arctic basin. As part of this unique operation, a 56 Myr long core was collected from the Lomonosov Ridge that allowed the investigation of striking geological changes including (1) the tectonic shoaling of the Lomonosov Ridge at ca. 44 Myr (leading to a 26 Myr sedimentation hiatus in the sequence), (2) the subsidence of the ridge and (3) the opening of the Fram Strait. This later prompted the ventilation of the basin by North Atlantic water during the Miocene (Jacobson et al. 2007). We have examined these changes in the Lomonosov sedimentary sequence with the use of lead, strontium, and osmium isotopic measurements. These are characterized by different chemical behaviour, input sources and residence times in the ocean (Pb ~ 50 yr, Sr ~2 Myr and Os ~10-30 Kyr). They have the potential to carry complementary information about water masses and detrital input sources when measured on a sedimentary sequence. Os isotopes found in the ACEX core sediments suggest high river input (with radiogenic continental imprint) during the Paleogene lake-stage of the Arctic followed by a thorough ventilation of the basin by North Atlantic waters at 17.5 Myr. Pb isotopes show that this transition from an enclosed lacustrine basin to well established marine condition is accompanied by changes in regional riverine input sources from a dominantly North- American source during the lake-stage (similar to present-day McKenzie River) to a more European sources (Lena River like) after North Atlantic water ventilation. Preliminary Strontium data obtained on leached bulk sediment residues (reflecting detrital input signature) yield fairly radiogenic values for the last 15 Myr (87Sr/88Sr = 0.716-0.718; similar to Haley et al. 2008) and at 45 Myr (~0.715). When Fram strait opens at 17.5 Ma, we observe a sharp decrease in the 87Sr/88Sr ratio: residues having a composition (0.710) close to that of contemporaneous seawater (0.7085). This may reflect a larger input from weathering of oceanic basalts from nearby ridges (Gakkel-Mid Atlantic Ridges). Ongoing Nd isotope work will help refine these hypotheses and enhance our comprehension of the geological history of the Arctic basin.
Stable Isotope Study of the Pleistocene Labrador Sea, northwest North Atlantic (IODP Sites 1302/03 and 1305) with Emphasis on Distinctive Features of Interglacial Thermohaline Circulation Patterns
Sites cored during expeditions 105 and 303 off southern Greenland (IODP 1305, ODP 646) and eastern Canada (IODP 1302/1303) are ideally located for documenting the climatic and glacial history of adjacent lands in addition to providing information on the Atlantic Meridional Overturning Circulation (AMOC), notably with regard to convection in the Labrador Sea (LS) and to the intensity of the Western Boundary UnderCurrent (WBUC) that is tightly controlled by the production of Denmark Strait Overflow Water (DSOW). Early studies at ODP Site 646, nearby IODP 1305, suggested that sea-surface conditions and terrestrial conditions over Greenland differed from one interglacial to the next. New isotopic data from IODP Site 1305 spanning Marine Isotope Stages (MIS) 31 to 1 now provide a means to compare glacial vs. interglacial (G vs. IG) regimes under 40-ka forcing (e.g. MIS 27-31; ca. Jaramillo chron) to those observed under 100-ka forcing, notably during MIS 1-11. In comparison, Site 1302/03 spanning 700 ka, provides direct information on linkages between the NE sector of the Laurentide Ice Sheet (LIS) and the North Atlantic. The MIS 11-1 interval shows large amplitude G vs. IG fluctuations with generally high IG outflow of the WBUC. Closer examination of stable isotope records in planktic and benthic foraminiferal assemblages further illustrate the specificity of each interglacial, and even suggest distinct water masses occupied the inner, northwestern LS (Site 1305), vs. the outer, southeastern LS (site 1302/03), at least during the last interglacial. These new data confirm that the modern AMOC, characterized by Labrador Sea Water formation due to winter cooling and convection in this basin seems mostly exclusive to the present interglacial, and that a lesser rate of DSOW production could have characterized it as well.
The New Tahiti Sea-Level Record (IODP Expedition 310) sets the Age of Melt Water Pulse 1A as Synchronous with the Bolling Warming
So far, the most complete and accurate sea-level record that encompassed the period between the Last Glacial Maximum and the present day is based on cores drilled offshore the Barbados coral reef (1,2). This record suggests a non-monotonous sea-level rise punctuated by dramatic accelerations, the so-called Melt Water Pulse events, that correspond to massive inputs of continental ice. The most extreme of these events, the MWP1-A, initially identified in the coral-based sea level record from the Barbados island, suggests a sea- level rise of ~20 meters between 14.1 and 13.6 ka (3,4). However, this event remains enigmatic and controversial. Several records are consistent with its occurrence (5,6), but no broad agreement emerges about its timing. The temporal relationship between the MWP1-A and the abrupt climatic events that punctuated the last deglaciation is a subject of controversial debates (7,8). Furthermore, the ice source responsible for such a step in sea-level rise is still elusive (9,10). Consequently, it remains a key issue to fully confirm the existence and amplitude of the MWP-1A by a precise coral reef record. The recent IODP Expedition 310 Tahiti Sea Level offers a unique opportunity to extend the existing Tahiti sea-level curve that documents the deglacial sea level rise for the last 13.8 ka (5). Located at a considerable distance from the major ice sheets and characterized by slow and regular subsidence rates, the Tahiti coral reefs provide an ideal setting to constrain MWP events that are thought to have punctuated the last deglaciation. The offshore coring operations carried out during Expedition 310 recovered more than 400 m of post-glacial reef material, ranging from 122 to 40 m below modern sea level (11). Post-glacial coral material was selected using strict mineralogical and isotopic screening criteria in order to preclude any post-mortem diagenetic alteration. More than 60 U-Th ages were obtained on shallow to deeper corals that extend the previous Tahiti record to 16 ka and allow to document the sea-level rise during the key period of the MWP-1A. Our results confirm the occurrence of an acceleration of the sea-level rise during that period. However, the timing and duration of this event differ significantly from observations from Barbados (3,4). These new results indicate that the MWP-1A occurred at about 14.6 ka BP, synchronously with the Bolling onset. This allows us to revisit the relationship between the MWP-1A and the climate history of the last deglaciation. (1) Fairbanks, 1989, Nature 342, 637. (2) Bard et al., 1990, Nature 346, 456. (3) Fairbanks et al., 2005, QSR 24, 1781. (4) Peltier and Fairbanks, 2006, QSR 25, 3322. (5) Bard et al., 1996, Nature 382, 241. (6) Hanebuth et al., 2000, Science 288, 1033-1035. (7) Weaver et al., 2003, Science 299, 1709-1713. (8) Stanford et al., 2006, Paleoceanography 21. (9) Clark et al., 1996, Paleoceanography 11, 563-577. (10) Clark et al.; 2002. Science 295, 2438-2441. (11) Camoin et al., 2007, Proc. IODP, 310
Pliocene Stratigraphy and Palaeoceanography of the Eastern North Atlantic Based on Dinoflagellate Cysts and Geochemistry
An independently calibrated record of dinoflagellate cyst and acritarch events is presented for the Early Pliocene through Middle Pleistocene (ca. 4.0 to 0.5 Ma) of eastern North Atlantic DSDP Site 610. A new age model is established for this hole and tied to marine isotope stratigraphy and magnetostratigraphy back to 3.6 Ma, augmented with new data on calcareous nannofossils. Biostratigraphic events of 19 dinoflagellate cyst and seven acritarch species in DSDP Hole 610A are calibrated to the latest astronomically-tuned Neogene time scale (ATNTS 2004) via our new age model. Comparing records across the North Atlantic and Mediterranean reveals several near-synchronous bio-events. The precise stratigraphy for Hole 610A allows us to evaluate the impact of palaeoceanographic and climatic events on the dinoflagellate cyst record. Climatic and oceanographic reorganizations associated with the onset of Northern Hemisphere glaciation appear responsible for the disappearance of many species between 2.8 and 2.6 Ma. Palynology and foraminiferal Mg/Ca and δ18O of Globigerina bulloides were combined to elucidate eastern North Atlantic palaeoceanography, and in particular North Atlantic Drift (NAD) variability between 3.40 and 3.20 Ma (mid- Pliocene). This time interval is of particular interest since a global cooling event (Marine Isotope Stage M2, ca. 3.30 Ma) interrupted global warmth and strong North Atlantic thermohaline circulation during the mid-Pliocene. Our sea-surface temperature data from DSDP Site 610 and IODP Site 1308 support an intense NAD at both sites immediately before and after MIS M2, and a cooling of ca. 2 to 3°C during MIS M2. The dinoflagellate cyst record demonstrates that the NAD did not influence the eastern North Atlantic during MIS M2. The reduced northward heat transport possibly allowed ice sheet growth on Greenland, and might explain the earliest record of IRD at 53°N (Site 610). The NAD returned to its previous pathway in less than 6000 years with a return to warmer Pliocene conditions. The temporary slowdown and shift of the NAD could be related to a short re-opening of the Panamanian Gateway around that time.
Laguna Potrok Aike, Southern Patagonia, Argentina: Achievements and Resulting Potential for the ICDP Project PASADO
Observations on the Fidelity of the Paleomagnetic Record Preserved in Deep-Sea Sediments Acquired Through IODP Drilling
In theory, the paleomagnetic record preserved in sediments allows observations that truthfully reflect the behavior of the geomagnetic field, providing a range of stratigraphic opportunities at a variety of temporal and spatial scales. In practice, our understanding of the magnetic acquisition process is such that the details remain poorly constrained. The drilling of North Atlantic sediments during Integrated Ocean Drilling Program (IODP) Expedition 303 provided a unique opportunity to test the fidelity of the deep-sea sediment paleomagnetic record. Shipboard and u-channel paleomagnetic measurements from the upper 12.5 mcd (ca. 25 ka) of IODP Site U1305 (Lat: 57.28.5 N, Long. 48.31.8 W, water depth 3459 m) on the Eirik Drift show that the highly variable sedimentation during the glacial through early Holocene give way to the continuous deposition of homogenous silty clays over the upper 4.5 mcd or last ca. 8.1 kyr. These uniform high-resolution (mean 65 cm /kyr) sediments preserve a strong, stable, well-defined and reproducible magnetization. Comparison with regional paleomagnetic secular variation (PSV) records derived from well-dated, ultra-high resolution (> 1 m/kyr) sediments demonstrate that Site U1305 preserves a magnetization that captures the full amplitude of the North Atlantic regions geomagnetic field behavior. Age estimates, however, suggest that Site U1305 PSV record is older than indicated by either regional PSV template. Implying that deep-sea sediments may preserve a high fidelity and accurate geomagnetic record that may be several hundred years or more younger than the age sediment it is preserved within. Implications of these observations will be discussed.
Imaging Gas Hydrates off Vancouver Island Using Deep-towed Multichannel Seismic Data and Ocean Bottom Seismometer Data
Gas hydrate with a prominent bottom-simulating-reflector (BSR) is widespread on the mid-slope offshore Vancouver Island, where it was the primary target of the 2005 Integrated Ocean Drilling Program (IODP) at Sites U1327 and U1328. Besides conventional sea-surface vertical-incidence reflection data, sediment structures in the region were imaged using two unconventional methodologies: the multichannel Deep-Towed Acoustics and Geophysics System (DTAGS) and a profile of 10 ocean bottom seismometers (OBS) with an instrument spacing of only 100 m. Towed about 300 m above the seafloor, the high-frequency (220-820 Hz) DTAGS system provides a high vertical and lateral resolution image. New processing routines were developed to accommodate the very high accuracy positioning requirements for source and receivers. These resulted in high quality images, resolving many near-vertical zones of low seismic amplitude, indicative of vents. At Bullseye vent, the DTAGS data show that the upper part of a 40-m-thick near-surface massive hydrate layer, drilled at U1328, is a high-reflectivity zone that likely corresponds to a fracture zone. The top of this feature, and thus hydrate, extends over an area of about 500 m by 500 m, and is anticlinal in shape with a northeast- southwest axis dipping at a shallow angle to the southwest. The OBS data, collected in the region of U1327, provide excellent constraints for standard 2D traveltime inversion. Velocity models indicate a fairly uniform increase of velocity with depth along a 5-km-long profile centred on the drillholes, with average hydrate concentrations of 10-15% in the 100-m-thick interval above the BSR. Remarkable structural images were also obtained from the OBS data, by applying prestack depth migration to the water-layer-multiples that form the downgoing wavefield above the seafloor. This multiple-migration (or mirror-imaging) method produces an image with a much broader illumination of the subsurface than is possible for conventional imaging using the primaries, especially for the very shallow reflections. Mirror imaging, which has comparable quality as results from a single-channel surface-towed streamer, fits well a dense-shot and sparse-receiver geometry because it enables greater tolerance to large intervals between the receivers.