Michael A Hall

Phase relationships between millennial‐scale events 64,000–24,000 years ago

A core recovered on the Iberian margin off southern Portugal can be correlated with Greenland ice cores using oxygen isotope variability in planktonic foraminifera which closely matches the ice core records of temperature over Greenland. Our age model identifies the base of every interstadial between 64,000 and 24,000 years ago and uses the Greenland Ice Core Project (GRIP) timescale. The oxygen isotope signal in benthic foraminifera (on this GRIP-based timescale) is quite different from the planktonic record and resembles the temperature record over Antarctica when this is synchronized with Greenland using the record of methane in the atmospheric air in the polar ice cores. We interpret the benthic record as indicating significant fluctuations in ice volume during millennial events, and we suggest that Antarctic temperature changed as a function of ice volume.

Oxygen and carbon isotope record of East Pacific core V19-30: implications for the formation of deep water in the late Pleistocene North Atlantic

Abstract A detailed oxygen and carbon isotope record has been obtained from benthic Foraminifera in core V19-30 from the Carnegie Ridge on the south side of the Panama Basin. Expressing these records and the oxygen and carbon isotope records previously published for Atlantic core M-12392 on a common timescale, it is apparent that the oxygen isotope records are very similar but that the carbon isotope records are quite different. By obtaining the carbon isotope gradient between the two sites as a function of time we show that the production of North Atlantic Deep Water has varied over a wide range during the late Pleistocene, and that the pattern of variation is not simply related to the well known oxygen isotope record. Although the two oxygen isotope records are very similar, changes in the interoceanic gradient are detectable and support the hypothesis that in the glacial mode the North Atlantic was colder, and less oxygenated, than it is today. Shackletons [1] 1977 interpretation whereby the carbon isotope record from the Atlantic core reflects changes in the terrestrial biomass, is an over-simplification. However, the record from the Pacific core V19-30 probably can be explained in these terms since it probably approximates the carbon isotope record of global mean oceanic dissolved CO 2 .

Millennial-scale oceanic climate variability off the Western Iberian margin during the last two glacial periods

High-resolution palaeoclimate records recovered from the Iberian margin in core MD95-2040 exhibit large fluctuations in oceanographic conditions over the last 190 ka. Large-scale cooling of the surface ocean is indicated by the presence of the polar planktonic foraminifer Neogloboquadrina pachyderma (sinistral), and in some instances the occurrence of ice-rafted debris (IRD). Ice-rafting episodes were prevalent in both of the last two glacials with greater intensity in Stages 2 through 4, than in Stage 6. The six youngest Heinrich events are well defined during the last glacial but detrital carbonate is absent from Heinrich layers HL6, HL5 and HL3. Dansgaard–Oeschger stadial-equivalent sub-millennial IRD deposition events have been detected, in particular during Stage 3, allowing a good match with the cooling displayed in the Greenland ice core (GISP2). Sea-surface temperature off Portugal in Stage 6 was in general warmer than during the last glacial, pointing towards a weaker southward influence of polar water masses. Ice rafting occurred mainly in mid-MIS (Marine Isotope Stage) 6 (between 173 and 153 kyr) as a group of poorly differentiated, short-duration quasi-continuous events, mainly marked by the high abundance of sinistral N. pachyderma. Differences exist in IRD composition relative to the last glacial, with a reduced Canadian-derived detrital carbonate component, combined with an important contribution of volcanic particles. The lower magnitude and higher frequency of these events suggest that the higher temperatures would have induced iceberg waning closer to the source areas.

El Niño–Southern Oscillation–like variability during glacial terminations and interlatitudinal teleconnections

Interannual-decadal variability in the equatorial Pacific El Nino–Southern Oscillation (ENSO) induces climate changes at global scale, but its potential influence during past global climate change is not yet well constrained. New high-resolution eastern equatorial Pacific proxy records of thermocline conditions present new evidence of strong orbital control in ENSO-like variability over the last 275,000 years. Recurrent intervals of saltier thermocline waters are associated with the dominance of La Nina–like conditions during glacial terminations, coinciding with periods of low precession and high obliquity. The parallel dominance of δ 13C-depleted waters supports the advection of Antarctic origin waters toward the tropical thermocline. This “oceanic tunneling” is proposed to have reinforced orbitally induced changes in ENSO-like variability, composing a complex high- and low-latitude feedback during glacial terminations.

Astronomical calibration age for the Oligocene-Miocene boundary

The stratotype section for the base of the Miocene is at a reversed (below) to normal (above) magnetic transition that is claimed to represent magnetic chron C6Cn.2n (o). Deep Sea Drilling Project (DSDP) Site 522 is the only location we are aware of that unambiguously records the three normal events of C6Cn. We have quantitatively determined the range of the short-lived nannofossil Sphenolithus delphix and the lower limit of S. disbelemnos in DSDP Holes 522 and 522A in order to calibrate their precise relationship to the magnetostratigraphy and to confirm the completeness of the record at this site. Astronomical tuning of Ocean Drilling Program (ODP) Sites 926, 928, and 929 shows that S. disbelemnos appears at 22.67 Ma and that the entire range of S. delphix is from about 22.98 Ma to 23.24 Ma. Using these ages, linear interpolation in DSDP Site 522 suggests that the age of C6Cn.2n (o) and of the Oligocene-Miocene boundary is 22.92 ± 0.04 Ma. Our value, conservatively expressed as 22.9 ± 0.1 Ma, is 0.9 m.y. younger than the currently accepted age of the Oligocene-Miocene boundary and of C6Cn.2n (o), which was assigned an age of 23.8 Ma, based on an estimate of 23.8 ± 1 Ma for the Oligocene-Miocene boundary. The bulk-sediment carbon isotope data from DSDP Site 522 is correlated to the record from benthic foraminifera at ODP Site 929 to refine the calibration of magnetic reversals from C6Cn.1n (o) to C7n.2n (o) at DSDP Site 522 on the astronomical time scale.

Paleoceanographic reconstructions from planktonic foraminifera off the Iberian Margin: Temperature, salinity, and Heinrich events

A quantitative analysis of planktonic foraminifera in a core from the Iberian Margin allows a reconstruction of the evolution of oceanographic parameters during the last glacial cycle with a resolution of ∼1000 years. A principal component analysis performed on 19 species allows the identification of 11 intervals characterized by increased abundances of the subpolar species. The youngest six of these intervals are correlated with the last 6 Heinrich events (HEs). The five cold events older than stage 4 are dated at 81, 90, 110, 129, and 140 ka, respectively. Paleotemperatures reconstructed using the modern analog technique indicate 4°C decreases during all even-numbered isotopic stages and stage 3. During the HEs, temperature decreases reach ∼10°C and seawater δ18O anomalies reach ∼1‰. Temperature and salinity reconstructions indicate that the environment of the Iberian Margin has been under the combined influence of global factors such as the migration of the polar front and iceberg discharge and of regional factors such as the precipitation/evaporation regime on both oceanic and continental area.

Carbon isotope discrimination and grain-yield in barley

Grain yield and grain carbon isotope discrimination (Δ) were measured in yield trials with barley over two seasons at three sites varying in expected seasonal rainfall in northern Syria, and in irrigated and droughted conditions at Cambridge, UK. Discrimination was least (water-use efficiency was greatest) at the lowest-rainfall sites, trial mean Δ ranging from 14.4 × 10−3 at Breda, Syria, to 19.29 × 10−3 in the irrigated at Cambridge. Variation in trial mean Δ was closely associated with trial mean grain-yield, which ranged from 135 to 374 g m−2. Phenotypic correlations between Δ and grain-yield in individual trials were negative (r= −0.53) in the least droughted trials and strongly positive (r=0.84) in the most droughted trials. In nine out of the ten trials, Δ was negatively correlated with days to ear emergence. Correlations between Δ measured in the two seasons and Δ measured in different trials within a season were generally positive and significant. It was concluded that, for barley lines grown in water-limited Mediterranean environments, a large discrimination against carbon-13 may be a useful indication of good yield.

Ocean climate variability in the eastern North Atlantic during interglacial marine isotope stage 11: A partial analogue to the Holocene?

Similar orbital geometry and greenhouse gas concentrations during marine isotope stage 11 ( MIS 11) and the Holocene make stage 11 perhaps the best geological analogue period for the natural development of the present interglacial climate. Results of a detailed study of core MD01-2443 from the Iberian margin suggest that sea surface conditions during stage 11 were not significantly different from those observed during the elapsed portion of the Holocene. Peak interglacial conditions during stage 11 lasted nearly 18 kyr, indicating a Holocene unperturbed by human activity might last an additional 6-7 kyr. A comparison of sea surface temperatures ( SST) derived from planktonic foraminifera for all interglacial intervals of the last million years reveals that warm temperatures during peak interglacials MIS 1, 5e, and 11 were higher on the Iberian margin than during substage 7e and most of 9e. The SST results are supported by heavier delta(18)O values, particularly during 7e, indicating colder SSTs and a larger residual ice volume. Benthic delta(13)C results provide evidence of a strong influence of North Atlantic Deep Water at greater depths than present during MIS 11. The progressive ocean climate deterioration into the following glaciation is associated with an increase in local upwelling intensity, interspersed by periodic cold episodes due to ice- rafting events occurring in the North Atlantic.

Associations between grain yield and carbon isotope discrimination in cowpea

Abstract Twenty cowpea ( Vigna unguiculata L. Walp.) genotypes, comprising Kenyan cultivars and Kenyan and introduced breedings lines, were grown in 15 replicated field trials carried out at three locations in semi-arid eastern Kenya over four seasons. Grain and straw yields and the carbon isotope discrimination (Δ) of this material were determined, and days to flowering and maturity observed. None of the seasons in which the trials were conducted was appreciably drier than the long term average for the sites, and some trials received excessive rain. Averaging the data for the 15 trials, one genotype yielded much less grain (77 g m −2 ) than the 19 others, whose yield ranged from 125 to 177 g m −2 (average 148 g m −2 ). Trial mean yield trials had a high straw Δ ( r = +0.567) and had received more rain between flowering and maturity ( r = +0.428) than lower yielding ones. Among genotypes, averaging over trials, the correlation coefficient ( r ) between grain yield and grain Δ was +0.394, and between grain yield and straw Δ, +0.460. Early genotypes had the highest grain Δ and straw Δ. The correlation among genotypes between date of flowering and grain Δ was −0.632 and with straw Δ, −0.502. When comparisons were made among trials, there was no clear relationship between the strength of the correlations among genotypes between grain yield and straw Δ or grain Δ and the degree of stress experienced by a trial (as indicated by the rainfall it had received). It appeared that this was because the variances of grain Δ and straw Δ were greater in the most droughted trials, whereas the variance of yield was greatest in the least droughted trials. It is concluded that the genetic correlation between grain yield and either grain Δ or straw Δ is unlikely to be strong enough to make either Δ a useful surrogate or adjunct in selecting for high yield in the segregating generations of a breeding programme for semi-arid eastern Kenya, but that Δ may be of value in the selection of parent genotypes.

Carbon isotope discrimination as a means of evaluating drought resistance in barley, rice and cowpeas

SummaryIn all the crops studied there was variation among genotypes in carbon isotope discrimination (Δ). In trials of barley genotypes there were strong positive correlations between grain yield and Δ, though in trials which were irrigated or received abundant rainfall, this correlation was non-significant or negative. Some of the variation in yield and Δ was related to date of heading, as early varieties in stressed trials had highest yields and had high Δ.In an experiment with 8 early and 10 late flowering rice genotypes there were consistent positive correlations between yield and Δ in the early group, but in the late group the correlations tended to be negative. Among the early group the early genotypes were highest yielding, and may be presumed to have avoided the worst effects of drought and hence had higher Δ. In the late group, there was no correlation between yield and date of flowering.In one experiment with cowpeas, a weak positive correlation between grain yield and Δ was observed, but in a s...