Graham P. Weedon

Intensified deep Pacific inflow and ventilation in Pleistocene glacial times.

The production of cold, deep waters in the Southern Ocean is an important factor in the Earths heat budget. The supply of deep water to the Pacific Ocean is presently dominated by a single source, the deep western boundary current east of New Zealand. Here we use sediment records deposited under the influence of this deep western boundary current to reconstruct deep-water properties and speed changes during the Pleistocene epoch. In physical and isotope proxies we find evidence for intensified deep Pacific Ocean inflow and ventilation during the glacial periods of the past 1.2u2009million years. The changes in throughflow may be directly related to an increased production of Antarctic Bottom Water during glacial times. Possible causes for such an increased bottom-water production include increasing wind strengths in the Southern Ocean or an increase in annual sea-ice formation, leaving dense water after brine rejection and thereby enhancing deep convection. We infer also that the global thermohaline circulation was perturbed significantly during the mid-Pleistocene climate transition between 0.86 and 0.45u2009million years ago.

Astronomical calibration of the Jurassic time-scale from cyclostratigraphy in British mudrock formations

Three British Jurassic mudrock formations have been investigated, via time–series analysis, for evidence of sedimentary cyclicity related to orbital–climatic (Milankovitch) cyclicity: the Blue Lias, the Belemnite Marls and the Kimmeridge Clay Formation. Magnetic–susceptibility measurements through the Blue Lias (uppermost Triassic to Sinemurian) were used to generate high–resolution time–series. The data indicate the presence of a regular sedimentary cycle that gradually varies in wavelength according to sedimentation rate. Tuning of this cycle to the 38ka Jurassic obliquity cycle produces spectral evidence for two additional regular cycles of small amplitude. These correspond to the 95 ka component of orbital eccentricity and the 20 ka orbital–precession cycles. Cycle counting allowed the minimum duration of four ammonite zones to be estimated and the duration of the Hettangian stage is estimated to be at least 1.29 Ma. Calcium carbonate measurements through the Belemnite Marls (lower Pliensbachian) are characterized by two scales of cyclicity that can be firmly linked to orbital–precession (20 ka) and the 123 ka component of eccentricity. A time–scale has been developed from the precession–related sedimentary cycles, with cycle counts used to constrain the duration of two ammonite zones. In the Kimmeridge Clay Formation (Kimmeridgian–Tithonian), magnetic–susceptibility measurements made on exposures, core material and down boreholes can be correlated at the decimetre scale. Only measurements of magnetic susceptibility made below the Yellow Ledge Stone Band (midway through the formation) are suitable for analysis of the bedding–scale cyclicity. A large–amplitude sedimentary cycle detected in the lower part of the formation is probably related to the orbital–obliquity cycle (38 ka). In certain stratigraphic intervals, there is evidence for small–amplitude cycles related to orbital precession (20 ka). This study of the British Jurassic shows that, in the Rhaetian–Sinemurian, the dominant cyclicity was related to obliquity. In the Pliensbachian this had shifted to dominantly precession, and in the Kimmeridgian obliquity again dominated. These shifts in cycle dominance presumably reflect changing local or global palaeoclimatic and/or palaeoceanographic conditions.

Cyclostratigraphy, orbital tuning and inferred productivity for the type Kimmeridge Clay (Late Jurassic), Southern England

Three independently measured variables (magnetic susceptibility, photoelectric factor and total gamma-ray) obtained from throughout the type Kimmeridge Clay Fm in Dorset (Southern England) were used to identify regular metre-scale, sedimentary cycles. Spectral analysis demonstrates that for long stratigraphical intervals the cycles are expressed as large-amplitude cycles of 1.87–4.05 m wavelength and smaller-amplitude cycles of around half that wavelength. These cycles are interpreted to record orbital obliquity and precession, respectively. The much larger amplitude of the inferred obliquity cycles compared with the precession cycles may indicate a high-latitude climatic forcing transferred to lower latitudes via sea-level variations. Orbital tuning indicates that the Early Kimmeridgian (sensu anglico) lasted at least 3.6 Ma (95 longer-wavelength cycles) and the Late Kimmeridgian at least 3.9 Ma (103 longer-wavelength cycles). The first detailed productivity estimates for the Kimmeridge Clay Fm, on a cycle-by-cycle calculation, indicate that average productivity of the type Kimmeridge Clay (220 g m−2 a−1) was less than the average productivity on modern continental shelves. The high average organic carbon content of the type Kimmeridge Clay (3.8% total organic carbon) cannot be attributed to high average productivity. However, the average organic carbon content is consistent with low siliciclastic mineral dilution of organic matter and/or elevated preservation linked to reduced bottom-water oxygenation.

Cyclostratigraphy and the Early Jurassic timescale: Data from the Belemnite Marls, Dorset, southern England

Time series based on determinations (wt%) of calcium carbonate and total organic carbon have been generated for the entire Lower Jurassic hemipelagic Belemnite Marls, Dorset, southern Britain. This formation was deposited during early Pliensbachian time, at a paleolatitude of about 35°N, in an epicontinental sea that was largely enclosed by the supercontinent of Pangea. The sequence contains compositionally diverse light and dark marl bedding couplets, the thicknesses of which are notably reduced in the upper third of the section. The regularity of the couplets in segments of the sequence, combined with a regular amplitude modulation, indicates an origin that is related to the orbital-precession cycle (i.e., one of the Milankovitch parameters). A timescale is developed by assigning a duration of 20 k.y. per couplet, and this suggests that the entire formation represents at least 1.78 m.y. Using the new timescale, bundles of bedding couplets are shown to have periods consistent with the 123 k.y. component of the orbital-eccentricity variations. The amplitude of the couplets varies at the same frequencies as the bundle cycles, in accordance with the interpretation that the couplets record precession and the bundles record eccentricity. However, despite having the same frequency of variation, there is no consistent relationship (coherence) between the variations in amplitude of the couplet cycle and the bundle cycles as would be predicted by the eccentricity-precession relationship. This mismatch can be explained in terms of nonlinear behavior of a climatic system characterized by a varying response time or nonlinear response of the sedimentary system itself. The data contain no evidence for orbital-obliquity cycles. Because the obliquity cycle affects insolation principally at high latitudes, the climatic factors that indirectly controlled the sedimentary cyclicity must have arisen at relatively low latitudes. The Belemnite Marls timescale indicates highly variable minimum durations for ammonite zones and subzones. True durations cannot be determined because it is possible that the succession is incomplete as a result of undetected erosion and/or nondeposition. A combination of the new results with cyclostratigraphic data from Yorkshire, northeast England, and the Southern Alps, Switzerland, indicates, based on cycle counts, that the Pliensbachian Stage lasted at least 4.82 m.y. Marine Sr-isotope ratios appear to have decreased linearly from the start of Jurassic time until the end of Pliensbachian time. The rate of decrease in 87 Sr/ 86 Sr established using the Belemnite Marls timescale was 0.000042/m.y. (or less if the main part of the formation is incomplete). Using this rate, with the observed changes in Sr-isotope ratios, gives minimum durations of 2.86 m.y. for Hettangian time, 7.62 m.y. for Sinemurian time, and 6.67 m.y. for Pliensbachian time.

Regular and irregular climatic cycles and the Belemnite Marls (Pliensbachian, Lower Jurassic, Wessex Basin)

Time series of carbonate and organic-carbon values are reported for 10.5 m of the Belemnite Marls. The light and dark marl beds form decimetre-scale couplets and metre-scale bundles. Light marls are carbon-poor and carbonate-rich whereas dark marls and laminated shales are carbon-rich and carbonate-poor. Sediment composition appears to have been indirectly controlled by climate; the regular couplets may record orbital-precession cycles (20 ka). However, the bundles cannot be linked to changes in eccentricity; rather they apparently record irregular, large-amplitude climatic variations with periods of a few hundred thousand years. It should not be assumed that 100–400 ka cyclicity in climate-related sedimentary sequences can be explained only by orbital forcing.

A 160-k.y.-old record of El Niño–Southern Oscillation in marine production and coastal runoff from Santa Barbara Basin, California, USA

A 160-k.y.-old laminated sediment record from the Santa Barbara Basin, California, analyzed using scanning electron microscope techniques, provides a history of interannual variability of marine production and coastal runoff. We used backscatter electron imagery to measure the components of the varve; these include a terrigenous lamina formed by seasonal runoff from winter rains together with a diatomaceous lamina that records marine production during the spring and early summer. Spectral analysis of terrigenous and diatomaceous laminae thickness reveals significant periodicities of 3.1 and 8.4 yr in the terrigenous series; these are indistinguishable, within the frequency resolution of the spectra, from significant periodicities of 3.5 and 7.6 yr in the diatomaceous series. The 3.1 and 3.5 yr periodicities record El Nino modulation of coastal runoff and marine production; the 8.4 and 7.6 yr periodicities are consistent with modulation by strong to very strong El Nino events. This is supported by the results of cross-spectral analysis of the terrigenous and diatomaceous records, which reveal inverse or antiphase relationships at 3.5 and 7.6 yr. Our work adds to a body of evidence that suggests that El Nino has been a persistent feature of late Quaternary climate variability.

Neogene palaeoceanography of Chatham Rise (Southwest Pacific) based on sediment geochemistry

Approximately one thousand sediment samples from ODP Site 1123 on the Chatham Rise, east of New Zealand, have been examined for inorganic elemental concentrations. ODP 1123 provides a record of sediment drift deposition under the Deep Western Boundary Current, the main inflow of deep water to the Pacific Ocean since the Early Oligocene, though a major hiatus spans the late Early Oligocene to the Early Miocene. Normalisation of the elemental concentrations by aluminium was used to allow for the effects of variable carbonate dilution. The elemental ratios were used as proxies for sediment composition and as palaeoceanographic indices. The samples were collected at a resolution designed to sample adequately any variation in elemental ratios at the scale of the Milankovitch orbital cycles. The sampled intervals span the Early Oligocene, Early Miocene, mid-Miocene and Late Pleistocene to Recent. Anomalous Si/Al, K/Al, Ti/Al values in the upper Pleistocene section, often associated with horizons of low carbonate, are attributed to tephras derived from North Island. Not all of the tephras detected geochemically had been detected visually in the cores. A total of 37 tephra events between 1.17 Ma BP and the present are recognised based on this and the shipboard investigations. The tephra events cluster at intervals of approximately 326u2008000 years (326 ka) perhaps due to variations in eruption frequency on North Island and/or to variations in the regional palaeowind intensity and direction. In the Late Pleistocene to Recent P/Al (inferred nutrient availability), percent calcium carbonate (%CaCO3) and Ba/Al (inferred productivity) varied regularly at a period of 40u2008000 years with these factors lagging minimum global ice volumes (interglacials). During the mid-Miocene %CaCO3, Ba/Al, P/Al and Si/Al all gradually increased with %CaCO3 and P/Al showing regular 138u2008000-yr cyclicity and Ba/Al showing 44-ka cyclicity. Inferred productivity (Ba/Al) may have been rising in association with increasing nutrient availability (P/Al) at the same time as increased vigour of the Deep Western Boundary Current that was connected to a period of rapid ice-sheet growth in Antarctica. In the Early Miocene P/Al and Si/Al were much higher than subsequently and both %CaCO3 and P/Al exhibited 131u2008000-yr cycles. By far the highest nutrient levels and inferred productivity at this site apparently occurred during the Early Oligocene as revealed by long-term changes in P/Al and Si/Al. A progressive rise in K/Al, but stable Ti/Al from the Early Oligocene to the Recent probably indicates increased proportions of illite in the clay mineral fraction of the drift sediments caused by increased flux of debris from the Southern Alps.