New calibration of Neogene radiolarian events in the North Pacific

New age models for twelve Deep Sea Drilling Project sites in the North Pacific have been produced, based on (in order of importance in our dataset) a recompilation of previously published diatom, calcareous nannofossil and foraminifer first and last occurrences, and magnetostratigraphy. The projected ages of radiolarian first and last occurrences derived from the line of correlation of the age/depth plots have been computed from these sites, and 28 radiolarian events have thereby been newly cross calibrated to North Pacific diatom and other stratigraphy. Several of the North Pacific radiolarian events are older than in previously published equatorial Pacific calibrations, and some may be diachronous within the North Pacific. These patterns may be due to complex latitudinal patterns of clinal variation in morphotypes within lineages, or to migration events from the North Pacific towards the Equator.

Middle Eocene Fe and Ca counts, Ca/Fe ratios, revised splices and astronomical age models form different Sites of IODP Exp342

Accurate age models are a tool of utmost important in paleoclimatology. Constraining the rate and pace of past climate change are at the core of paleoclimate research, as such knowledge is crucial to our understanding of the climate system. Indeed, it allows for the disentanglement of the various drivers of climate change. The scarcity of highly resolved sedimentary records from the middle Eocene (Bartonian - Lutetian Stages; 47.8 - 37.8 Ma) has led to the existence of the "Eocene astronomical time scale gap" and hindered the establishment of a comprehensive astronomical time scale (ATS) for the entire Cenozoic. Sediments from the Newfoundland Ridge drilled during Integrated Ocean Drilling Program (IODP) Expedition 342 span the Eocene gap at an unprecedented stratigraphic resolution with carbonate bearing sediments. Moreover, these sediments exhibit cyclic lithological changes that allow for an astronomical calibration of geologic time. In this study, we use the dominant obliquity imprint in XRF-derived calcium-iron ratio series (Ca/Fe) from three sites drilled during IODP Expedition 342 (U1408, U1409, U1410) to construct a floating astrochronology. We then anchor this chronology to numerical geological time by tuning 173-kyr cycles in the amplitude modulation pattern of obliquity to an astronomical solution. This study is one of the first to use the 173-kyr obliquity amplitude cycle for astrochronologic purposes, as previous studies primarily use the 405-kyr long eccentricity cycle as a tuning target to calibrate the Paleogene geologic time scale. We demonstrate that the 173-kyr cycles in obliquity's amplitude are stable between 40 and 50 Ma, which means that one can use the 173-kyr cycle for astrochronologic calibration in the Eocene. Our tuning provides new age estimates for magnetochron reversals C18n.1n - C21r and a stratigraphic framework for key sites from Expedition 342 for the Eocene. Some disagreements emerge when we compare our tuning for the interval between C19r and C20r with previous tuning attempts from the South Atlantic. We therefore present a revision of the original astronomical interpretations for the latter records, so that the various astrochronologic age models for the middle Eocene in the North- and South-Atlantic are consistent.

Strontium isotopic ages from IODP Exp307

Sr isotope stratigraphy provides a new age model for the first complete section drilled through a deep-water coral mound. The 155-m-long section from Challenger Mound in the Porcupine Sea-bight, southwest of Ireland, is on Miocene siliciclastics and consists entirely of sediments bearing well-preserved cold-water coral Lophelia pertusa. The 87Sr/86Sr values of 28 coral specimens from the mound show an upward-increasing trend, correspond to ages from 2.6 to 0.5 Ma, and identify a significant hiatus from ca. 1.7 to 1.0 Ma at 23.6 m below seafloor. The age of the basal mound sediments coincides with the intensification of Northern Hemisphere glaciations that set up the modern stratification of the northeast Atlantic and enabled coral growth. Mound growth persisted throughout glacial-interglacial fluctuations, reached a maximum rate (24 cm/k.y.) ca. 2.0 Ma, and ceased at 1.7 Ma. Unlike other buried mounds in Porcupine Seabight, Challenger Mound was only partly covered during its growth interruption, and growth restarted ca. 1.0 Ma.

XRF data, magnetic susceptibility, greyscale values and revised depths of sediments from ODP Leg 154 sites

The distinctly cyclic sediments recovered during ODP Leg 154 played an important role in constructing the astronomical time scale and associated astro(bio)chronology for the Miocene, and in deciphering ocean-climate history. The accuracy of the timescale critically depends on the reliability of the shipboard splice used for the tuning and on the tuning itself. New high-resolution colour- and magnetic susceptibility core scanning data supplemented with limited XRF-data allow improvement of the stratigraphy. The revised composite record results in an improved astronomical age model for ODP Site 926 between 5 and 14.4 Ma. The new age model is confirmed by results of complex amplitude demodulation of the precession and obliquity related cycle patterns. Different values for tidal dissipation are applied to improve the fit between the sedimentary cycle patterns and the astronomical solution. Due to the improved stratigraphy and tuning, supported by the results of amplitude demodulation, the revised time scale yields more reliable age estimates for planktic foraminiferal and calcareous nannofossil events. The results of this study highlight the importance of stratigraphy for timescale construction.

Paleoclimate reconstructions from sediment core MD01-2378

On the basis of the radiocarbon (14C) plateau-tuning method a new age model for Timor Sea Core MD01-2378 was established. It revealed a precise centennial-scale phasing of climate events in the ocean, cryo-, and atmosphere during the last deglacial and provides important new insights into causal linkages controlling events of global climate change. At Site MD01-2378 reservoir ages of surface waters dropped from 1600 yr prior to 20 cal ka to 250-500 yr after 18.8 cal ka. This evidence was crucial for generating a high-resolution age model for deglacial events in the Indo-Pacific Warm Pool. Sea-surface temperatures (SST) started to change near 18.8 cal ka, that is ~500 yr after the start of, presumably northern hemispheric, deglacial melt and sea level rise as shown by the benthic foraminiferal oxygen isotope ratio (d18O). However, the SST rise occurred 500-1000 yr prior to the onset of deglacial Antarctic warming and the first major rise in atmospheric carbon dioxide at about 18 ka. The increase in SST may partly reflect reduced seasonal upwelling of cold subsurface waters along the eastern margin of the Indian Ocean, which is reflected by a doubling of the thermal gradient between the sea surface and the thermocline, a halving of chlorin productivity from 19 to 18.5 cal ka, and in particular, by the strong decrease in surface water reservoir ages. Two significant increases in deglacial Timor Sea surface salinities from 19-18.5 and 15.5-14.5 cal ka, may partly reflect the deglacial increase in the distance of local river mouths, partly an inter-hemispheric millennial-scale see-saw in tropical monsoon intensity, possibly linked to a deglacial increase in the dominance of Pacific El Niño regimes over Heinrich stadial 1.

Revised geomagnetic polarity timescale for the late Oligocene to early Miocene of ODP Site 177-1090

At Ocean Drilling Program (ODP) Site 1090 (subantarctic South Atlantic), benthic foraminiferal stable isotope data (from Cibicidoides and Oridorsalis) span the late Oligocene through early Miocene (~24-16 Ma) at a temporal resolution of ~5 ky. Over the same interval, a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity time scale (GPTS), thereby providing direct correlation of the isotope record to the GPTS. In an initial age model, we use the newly derived age of the Oligocene/Miocene (O/M) boundary of 23.0 Ma of Shackleton et al. (2000, doi:10.1130/0091-7613(2000)28<447:ACAFTO>2.0.CO;2), revised to the new astronomical calculation (La2003) of Laskar et al (2004, doi:10.1016/j.icarus.2004.04.005) to recalculate the spline ages of Cande and Kent (1995, doi:10.1029/94JB03098). We then tune the Site 1090 dekta18O record to obliquity using La2003. In this manner, we are able to refine the ages of polarity chrons C7n through C5Cn.1n. The new age model is consistent, within one obliquity cycle, with previously tuned ages for polarity chrons C7n through C6Bn from Shackleton et al. (2000) when rescaled to La2003. The results from Site 1090 provide independent evidence for the revised age of the Oligocene/Miocene boundary of 23.0 Ma. For early Miocene polarity chrons C6AAr through C5Cn, our obliquity-scale age model is the first to allow a direct calibration to the GPTS. The new ages are generally within one obliquity cycle of those obtained by rescaling the Cande and Kent (1995) interpolation using the new age of the O/M boundary (23.0 Ma) and the same middle Miocene control point (14.8 Ma) used by Cande and Kent (1995).

Silicic tephras in ODP Legs 151, 152, and 162

A 6-m.y.-long composite marine record of explosive silicic volcanism from five Ocean Drilling Program sites in the subpolar North Atlantic was compared with several marine records of global and local paleoclimate proxies (benthic d18O and ice-rafted debris records). Coarsening and high frequency of occurrence of Icelandic tephras were recorded in 3.6-3 Ma sediments, suggesting that these tephras were dispersed farther from the source by enhanced westerly winds over the subpolar North Atlantic. The 40Ar/39Ar ages were determined by laser probe on K-feldspar and biotite phenocrysts of tephras that were erupted from the Jan Mayen volcanic system. Compared to the tuned paleomagnetic age model, the 40Ar/39Ar dating (0.618+/-0.007 Ma to 4.90+/-0.05 Ma) yields a new age model that postdates by 155 k.y. the inception of ice rafting on the Iceland Plateau during the cold marine isotope stage M2 (i.e., 3.3-3.14 Ma).

Chemical composition of basalts from coarse debris of the Kara Sea bottom sediments

Study of basaltic debris from the Kara Sea bottom has shown its similarity to traps of the Eastern Siberia in mineralogy, structures and chemical composition. In comparison with oceanic tholeiites, the source of traps and Kara Sea basin basaltic melts was enriched in REE and some other incompatible elements. K-Ar dating of two samples of supposed autochtonous location from the eastern part of the Kara Sea basin has shown 209 and 218 Ma - younger than traps (247-248 Ma). Origin of Siberian traps used to connect with action of the mantle plume (Iceland plume, according to geodinamic reconstruction). Our new age data may be interpreted as an evidence of the Siberian plate moving over the head of plume.

Amino acid racemization of Neogloboquadrina pachyderma from the Arctic Ocean

The long-term rate of racemization for amino acids preserved in planktonic foraminifera was determined by using independently dated sediment cores from the Arctic Ocean. The racemization rates for aspartic acid (Asp) and glutamic acid (Glu) in the common taxon, Neogloboquadrina pachyderma, were calibrated for the last 150 ka using 14C ages and the emerging Quaternary chronostratigraphy of Arctic Ocean sediments. An analysis of errors indicates realistic age uncertainties of about ±12% for Asp and ±17% for Glu. Fifty individual tests are sufficient to analyze multiple subsamples, identify outliers, and derive robust sample mean values. The new age equation can be applied to verify and refine age models for sediment cores elsewhere in the Arctic Ocean, a critical region for understanding the dynamics of global climate change.

(Table 3) Foraminiferal 87Sr/86Sr ratios and ages of ODP Hole 162-986D sediments

Site 986 was drilled to 965 meters below seafloor (mbsf) on the western Svalbard margin to record the onset of glaciations and to date and document the glacial evolution in the Svalbard-Barents Sea region during the Pliocene-Pleistocene. In this paper, results of sedimentological analyses are discussed in light of seismic stratigraphy and new age determinations. The latter were difficult to obtain in the glacial deposits, and datums are sparse. Through combined paleomagnetic data, biostratigraphy, and Sr isotopes, however, an overall chronology for the main evolutionary steps is suggested. The cored sequence at Site 986 is younger than 2.6 Ma, and the lower 60 m of the section contains no evidence of a major glacial influence. An initial glaciation is interpreted to have occurred at ~2.3 Ma, resulting in increased sand deposition from debris flows at Site 986 and forming a prominent seismic reflector, R7. However, glaciers probably did not reach the shelf break until ~1.6-1.7 Ma (Reflector R6), after which the depositional environment was dominated by diamictic debris flows. A gradual change in source area from the Barents Sea to Svalbard is recorded primarily by changes in carbonate and smectite content, ~355 mbsf (Reflector R5), at an interpolated age of 1.4-1.5 Ma. During the last ~1 m.y., Site 986 has undergone more distal deposition as the main depocenters have shifted laterally. This has resulted in less frequent debris flows and more turbidites and hemipelagic deposits, with a slight fining upward of the cored sediments.