James E T Channell

North Atlantic palaeointensity stack since 75ka (NAPIS–75) and the duration of the Laschamp event

Six relative palaeointensity records from the north Atlantic Ocean were stacked together to produce a new record for the last 75 kyr (NAPIS–75). Five of these records have been previously correlated at millennial scale and placed on the GISP2 age scale, the sixth record was tied to the others using magnetic susceptibility. From 75 ka the field strength exhibits some oscillations, with a first minimum ca.65 ka, followed by a progressive increase to a broad maximum centred at ca.48 ka. There is then a well–marked low at 40 ka, corresponding to the directional anomaly of the Laschamp event. Another intensity low, observed at ca.34 ka, corresponds in age to the Mono Lake event. After a high at 33 ka and two lows at 30 and 24 ka with a broad maximum between, the field strength seems to slowly increase to the upper limit of the studied interval. In the 10–20 kyr interval some differences exist between individual records, and fine–scale details are not always resolved. In the 20–75 kyr interval, on the other hand, well–resolved millennial–scale features are superimposed to the broader trends. The duration of the Laschamp event, which is recorded directionally in five cores, appears to be about 1500 years, consistent with a recent suggestion on the origin of geomagnetic excursions.

A Cenozoic record of the equatorial Pacific carbonate compensation depth

Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0–3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.

Pliocene-Pleistocene ice rafting history and cyclicity in the Nordic Seas during the last 3.5 Myr

A continuous 3.5 Myr IRD record was produced from Ocean Drilling Program (ODP) Site 907. A timescale based on magnetic polarity chrons, oxygen isotope stratigraphy (for the last 1Myr) and orbital tuning was developed. The record documents a stepwise inception of large-scale glacial cycles in the Nordic Seas region, the first being a marked expansion of the Greenland ice sheet at 3.3 Ma. A second step occurred at 2.74 Ma by an expansion of large scale ice sheets in the Northern Hemisphere. Ice sheet variability around the Nordic Seas was tightly coupled to global ice volume over the past 3.3 Myr. Between 3 and 1 Ma, most of the variance of the IRD signal is in the 41 kyr band, whereas the last 1 Myr is characterized by stronger 100 kyr variance. The Gamma Ray Porosity Evaluator (GRAPE) density record is closely linked with IRD variations and documents sub orbital variability resembling the late Quaternary Heinrich/Bond cycles.

Relative geomagnetic paleointensity and δ18O at ODP Site 983 (Gardar Drift, North Atlantic) since 350 ka

Abstract At ODP Site 983, relative geomagnetic paleointensity and planktic and benthic δ18O records have been acquired for the last 350 kyr. The mean sedimentation rate in this interval is 11.3 cm/kyr. Magnetic properties and hysteresis ratios indicate that pseudo-single domain magnetite is the remanence carrier. Volume susceptibility (k), anhysteretic (ARM) and isothermal (IRM) remanence values vary by a factor of 3–4, well within the criteria usually cited for paleointensity studies. Natural remanent magnetization (NRM) is normalized by ARM and IRM to acquire the paleointensity proxy. Arithmetic means of NRM/ARM and NRM/IRM, calculated for five demagnetization steps in the 25–45 mT range, constitute the relative paleointensity estimates. Some paleointensity lows (particularly those at ∼40, ∼120 and ∼188 ka) are associated with directional excursions of the field, especially the event at ∼188 ka (referred to here as the Iceland Basin Event) that constitutes a short-lived polarity reversal. For the last 200 kyr, the records can be correlated with other high-resolution paleointensity records such as those from the Labrador Sea, Mediterranean/Somali Basin and Sulu Sea, implying that the millennial scale features are globally synchronous. A labeling system for paleointensity features is proposed that ties prominent highs and lows to oxygen isotope stages.

Geomagnetic paleointensity and environmental record from Labrador Sea core MD95-2024: global marine sediment and ice core chronostratigraphy for the last 110 kyr

Piston core MD95-2024 from the Labrador Rise provides a continuous record of rapidly deposited detrital layers denoting Laurentide ice sheet (LIS) instability. The core also provides a high-resolution record of geomagnetic paleointensity, that is consistent with, but at higher temporal resolution than previous Labrador Sea records. Correlation to the Greenland Summit ice cores (GRIP/GISP2) is achieved by assuming that Labrador Sea detrital layers correspond to cold stadials in the ice cores. This allows a GISP2 official chronology to be placed on MD95-2024 which is consistent with the inverse correlation between paleointensity and the flux of cosmogenic isotopes ( 10 Be and 36 Cl) in Greenland ice cores. Synchronous millennial scale variability observed from the MD95-2024 paleointensity

Reconciling astrochronological and 40Ar/39Ar ages for the Matuyama-Brunhes boundary and late Matuyama Chron

When five Matuyama-Brunhes (M-B) boundary records from the North Atlantic are placed on isotope age models, produced by correlation of the δ18O record directly or indirectly to an ice volume model, the M-B boundary lies consistently at the young end of marine isotope stage 19 with a mean age for the midpoint of the reversal of 773.1 ka (standard deviation = 0.4 kyr), ∼7 kyr younger than the presently accepted astrochronological age for this polarity reversal (780–781 ka). Two recently proposed revisions of the age of the 40Ar/39Ar Fish Canyon sanidine (FCs) standard to 28.201 ± 0.046 Ma and 28.305 ± 0.036 Ma would adjust 40Ar/39Ar ages applicable to the M-B boundary (and other reversals and excursions back to 1.2 Ma) to ages older than the new astrochronological ages by 8–24 kyr. The variables used to construct the ice volume models cannot account for the discrepancy. The FCs standard age that best fits the astrochronological ages is 27.93 Ma, which is within the uncertainty associated with the commonly used value of 28.02 (±0.16) Ma but younger than the recently proposed FCs ages. The EDC2 and EDC3 age models in the Dome C (Antarctic) ice core yield ages of 771.7 ka and 766.4 ka, respectively, for the 10Be flux peak that denotes the paleointensity minimum at the reversal boundary, implying that the EDC2 (rather than EDC3) age model is consistent with the observations from marine sediments, at least close to the M-B boundary.

Geomagnetic palaeointensities and astrochronological ages for the Matuyama{Brunhes boundary and the boundaries of the Jaramillo Subchron: palaeomagnetic and oxygen isotope records from ODP Site 983

We have measured relative geomagnetic palaeointensity proxies, palaeomagnetic directions, and δ18O for the 700–1100 ka interval from ODP Site 983 (Gardar Drift, North Atlantic), where mean sedimentation rates are ca.13 cm kyr−1. The age model was generated by matching the benthic δ18O data to the Ice Volume Model and confirmed by tuning the precessional components of both signals. For the Matuyama–Brunhes boundary (MBB) and the boundaries of the Jaramillo Subchronozone, the duration of the polarity reversal process, defined by virtual geomagnetic polar latitudes of less than 45°, is ca.5 kyr. Whereas the generally accepted astrochronological estimates for the boundaries of the Jaramillo Subchronozone lie within the polarity transitions as recorded at Site 983, the astrochronological age for the Matuyama–Brunhes polarity transition (780 ka) is ca.5 kyr older than the onset of this transition at Site 983 (775 ka). The polarity reversals lie within palaeointensity lows, with abrupt recovery of palaeointensity post reversal. There is no progressive (‘sawtooth’) decrease in palaeointensity within the Jaramillo Subchronozone or between the top of the Jaramillo and the MBB, but rather, within polarity chrons, several short intervals of low palaeointensity which sometimes coincide with high–amplitude secular variation. Orbital (100 and 41 kyr) periods are present in the palaeointensity record. As they are not obviously attributable to climate/lithology in these records, they may be a feature of the geomagnetic field itself.

South Atlantic and North Atlantic geomagnetic paleointensity stacks (0–80 ka): implications for inter-hemispheric correlation

Recent coring of high-deposition-rate Atlantic sediments has led to the development of North Atlantic (NAPIS) and South Atlantic (SAPIS) geomagnetic paleointensity stacks. The SAPIS stack comprises five records from the sub-Antarctic South Atlantic (41–471S, 6–101E). Four are from piston cores collected during the Ocean Drilling Program (ODP) Leg 177 site survey cruise (Earth Planet. Sci. Lett. 175 (2000) 145). The other is ODP Site 1089 (Leg 177) (J. Geophys. Res., 2001, submitted for publication). All but one (4-PC03) of the SAPIS cores have oxygen isotope records which are used in conjunction with lithologic and geomagnetic variability to derive an optimized correlation to Site 1089. The Site 1089 age model is derived by mapping the benthic and planktic isotope data to those from nearby core RC11-83 which has 14 calibrated radiocarbon ages in the 11–41 ka interval. Below this level, the chronology is derived by matching the Site 1089 benthic oxygen isotope data to the SPECMAP stack. The NAPIS stack comprises six records from a wide area of the North Atlantic (Philos. Trans. R. Soc. Ser. A 358 (2000) 1009). Correlation between cores was based on marine isotopic stage boundaries, augmented by millennial-scale features (Heinrich events and fluctuations in concentration-dependent magnetic parameters). NAPIS was placed on the GISP2 chronology, using the marine to ice-core oxygen isotope correlation proposed by Voelker et al. (Radiocarbon 40 (1998) 517) for one of the NAPIS cores. In the resulting age model, the LaschampEvent, recorded in five of six cores, falls in a very narrow ( o1 kyr) age range and coincides with the 10 Be and 36 Cl peak measured in the ice cores. Comparison of the two stacks, placed on their own independent age models, indicates that common millennial scale paleointensity features are preserved. Although more work is needed to define the ‘‘true’’ global content of the paleointensity record as well as the precise age of many features, it is readily apparent that paleointensity can provide a global correlation tool at a resolution unattainable from isotope data alone. r 2002 Published by Elsevier Science Ltd.

Geomagnetic paleointensity for the last 100 kyr from the sub-antarctic South Atlantic: a tool for inter-hemispheric correlation

Abstract We report relative paleointensity proxy records from four piston cores collected near the Agulhas Ridge and Meteor Rise (South Atlantic). The mean sedimentation rate of the cores varies from 24 cm/kyr to 11 cm/kyr. The two cores with mean sedimentation rates over 20 cm/kyr record positive remanence inclinations at 40–41 ka coeval with the Laschamp Event. Age models are based on oxygen isotope data from three of the cores, augmented by radiocarbon ages from nearby Core RC11-83, and by correlation of paleointensity records for the one core with no oxygen isotope data. The relative paleointensity proxy records are the first from the South Atlantic and from the high to mid-latitude southern hemisphere. Prominent paleointensity lows at ∼40 ka and ∼65 ka, as well as many other features, can be correlated to paleointensity records of comparable resolution from the northern hemisphere. The records are attributable, in large part, to the global-scale field, and therefore have potential for inter-hemispheric correlation at a resolution difficult to achieve with isotope data alone.

Late Oligocene to early Miocene geochronology and paleoceanography from the subantarctic South Atlantic

Mi2 as well as several other distinctive isotope events. Our data suggest that the d 18 O maximum commonly associated with the Oligocene/Miocene (O/M) boundary falls within C6Cn.2r (23.86 Ma). The d 13 C maximum coincides, within the temporal resolution of our record, with C6Cn.2n/r boundary and hence to the O/M boundary. Comparison of the stable isotope record from Site 1090 to the orbitally tuned stable isotope record from ODP Site 929 across the O/M boundary shows that variability in the two records is very similar and can be correlated at and below the O/M boundary. Site 1090 stable isotope records also provide the first deep Southern Ocean end-member for reconstructions of circulation patterns and late Oligocene to early Miocene climate change. Comparison to previously published records suggests that basin to basin carbon isotope gradients were small or nonexistent and are inconclusive with respect to the direction of deep water flow. Oxygen isotope gradients between sites suggest that the deep Southern Ocean was cold in comparison to the North Atlantic, Indian, and the Pacific Oceans. Dominance of cold Southern Component Deep Water at Site 1090, at least until 17 Ma, suggests that relatively cold circumpolar climatic conditions prevailed during the late Oligocene and early Miocene. We believe that a relatively cold Southern Ocean reflects unrestricted circumpolar flow through the Drake Passage in agreement with bathymetric reconstructions. INDEX TERMS: 4267 Oceanography: General: Paleoceanography; 1035 Geochemistry: Geochronology; 4870 Oceanography: Biological and Chemical: Stable isotopes; KEYWORDS: Paleoceanography; 1035 geochronology, stable isotopes