Paul Johnston

Timing of the Last Glacial Maximum from observed sea-level minima

During the Last Glacial Maximum, ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today. But few direct estimates exist of the volume of the ice sheets, or the timing and rates of change during their advance and retreat. Here we analyse four distinct sediment facies in the shallow, tectonically stable Bonaparte Gulf, Australia—each of which is characteristic of a distinct range in sea level—to estimate the maximum volume of land-based ice during the last glaciation and the timing of the initial melting phase. We use faunal assemblages and preservation status of the sediments to distinguish open marine, shallow marine, marginal marine and brackish conditions, and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio-hydro-isostatic modelling. Our results indicate that from at least 22,000 to 19,000 (calendar) years before present, land-based ice volume was at its maximum, exceeding todays grounded ice sheets by 52.5 × 106 km3. A rapid decrease in ice volume by about 10% within a few hundred years terminated the Last Glacial Maximum at 19,000 ± 250 years.

Refining the eustatic sea-level curve since the Last Glacial Maximum using far- and intermediate-field sites

The eustatic component of relative sea-level change provides a measure of the amount of ice transferred between the continents and oceans during glacial cycles. This has been quantified for the period since the Last Glacial Maximum by correcting observed sea-level change for the glacio-hydro-isostatic contributions using realistic ice distribution and earth models. During the Last Glacial Maximum (LGM) the eustatic sea level was 125±5 m lower than the present day, equivalent to a land-based ice volume of (4.6–4.9)×107 km3. Evidence for a non-uniform rise in eustatic sea level from the LGM to the end of the deglaciation is examined. The initial rate of rise from ca. 21 to 17 ka was relatively slow with an average rate of ca. 6 m ka−1, followed by an average rate of ca. 10 m ka−1 for the next 10 ka. Significant departures from these average rates may have occurred at the time of the Younger Dryas and possibly also around 14 ka. Most of the decay of the large ice sheets was completed by 7 ka, but 3–5 m of water has been added to the oceans since that time.

Distribution of the wingless gene product in drosophila embryos: A protein involved in cell-cell communication

wingless, a segment polarity gene required in every segment for the normal development of the Drosophila embryo, encodes a cysteine-rich protein with a signal peptide. A polyclonal antiserum localizes the wingless protein in approximately the same region of the embryo as the wingless mRNA. The pattern of antigen localization changes rapidly during development. In the extended germband stage, stripes of wingless staining are present in the trunk region just anterior to the parasegment boundary; wingless-expressing cells abut engrailed-expressing cells across that boundary. wingless antigen is seen both inside and outside the cell by electron microscopy: inside the cell, in small membrane-bound vesicles and in multivesicular bodies; outside the cell, close to or on the plasma membrane and associated with material in the intercellular space. The multivesicular bodies containing the wingless protein are occasionally found in engrailed-positive cells, suggesting that the wingless protein behaves as a paracrine signal.

Control of Drosophila body pattern by the hunchback morphogen gradient.

Most of the thoracic and abdominal segments of Drosophila are specified early in embryogenesis by the overlapping activities of the hunchback (hb), Krüppel, knirps, and giant gap genes. The orderly expression of these genes depends on two maternal determinants: bicoid, which activates hb transcription anteriorly, and nanos, which blocks translation of hb transcripts posteriorly. Here we provide evidence that the resulting gradient of hb protein dictates where the Krüppel, knirps, and giant genes are expressed by providing a series of concentration thresholds that regulate each gene independently. Thus, hb protein functions as a classical morphogen, triggering several distinct responses as a function of its graded distribution.

Phenocopies induced with antisense RNA identify the wingless gene

Earlier work suggested that the wingless gene of Drosophila is required for cooperation within discrete groups of cells during development. We show that antisense RNA made from a 3 kb transcript produces wingless mutant phenocopies when injected into wild-type eggs, proving that this transcript executes the wingless function. In the accompanying paper, Rijsewijk et al. report cloning of a Drosophila homolog (Dint-1) of the mouse int-1 gene and show that this gene is identical to wingless+. The partial sequence of the wingless cDNA that we have isolated is identical to part of the complete Dint-1 sequence described by Rijsewijk et al.

Sea-level at the Last Glacial Maximum: evidence from northwestern Australia to constrain ice volumes for oxygen isotope stage 2

New sea-level information from the Bonaparte Gulf in northwestern Australia is used to constrain the magnitude and rates of change of ice volumes during the Last Glacial Maximum (LGM ). The region is tectonically stable and far from the former ice-covered regions. The glacio-hydro-isostatic adjustment of the coast is therefore relatively small, and the corrections for this eVect are not sensitive to details of the rebound model. Microfossil analysis and AMS radiocarbon dating of 11 gravity cores taken across the shelf and Bonaparte Gulf demonstrate that: (1) the LGM sea-levels were locally at ’125±4 m; (2) the LGM terminated abruptly at 19 000 cal yr BP with a rapid rise in sea-level of about 15 m over the next 500 years; and (3) the onset of the minimum sea-levels occurred before 22 000 cal yr BP. When corrected for the glacio-hydro-isostatic eVects, the increase of LGM ice volumes over the present-day ice volume is 52.5◊106 km3. The termination of the LGM is marked by a rapid ice discharge of 5.2◊106 km3.

The muscle pattern of a segment of Drosophila may be determined by neurons and not by contributing myoblasts

Each segment of Drosophila has a characteristic pattern of muscles. Like the segments of the cuticle and the central nervous system, the muscle pattern is ultimately dependent on the deployment of selector genes such as elements of the bithorax complex. We use nuclear transplantation to make genetic mosaics in which the donor, but not the host, is mutant for part of the bithorax complex. Making use of a muscle pattern that is found only in the male, we ask which cells have to be mutant in order to obtain mutant muscles and find that these crucial cells do not contribute to the muscles themselves. The evidence implicates neurons that innervate the muscles. Our hypothesis is that the sex and segmental identity of the motor or neurosecretory neurons determine the development of muscle pattern.

Water-load definition in the glacio-hydro-isostatic sea-level equation

Abstract Models of glacio-hydro-isostatic rebound and the concomitant sea-level change have been progressively improved over the past three decades. Recently, the procedures used by the group at the Australian National University (ANU) for the hydro-isostatic component of the theory have been questioned (Quat. Sci. Rev. 21 (2002) 409) although the details of the ANU groups procedures have not been published because they are mainly computational in nature rather than representing significant conceptual advances. Because of this criticism, we set out here in detail the procedures that have been used for the treatment of the migration of shorelines as sea levels rise and fall, the effect of retreat and advancing grounded ice on shelves and shallow seas, and the transitions from grounded to floating ice (and vice versa). We conclude that there is no basis for the criticism, that these formulations and their implementation provide a high resolution and complete description of both sea-level change and of the estimates of volumes of ice exchanged with the oceans. The results from this formulation are confirmed by the entirely independent analyses of Milne et al. (Quat. Sci. Rev. 21 (2002) 361) and Mitrovica and Milne (Geophys. J. Int. (2002), submitted for publication) who conclude that our formulation is significantly more accurate than the procedure advocated by Peltier (Science 265 (1994) 195; Rev. Geophys. 36 (1998a) 603, Geophys. Res. Lett. 25 (1998b) 3955).

Shore-line reconstruction around Australia during the Last Glacial Maximum and Late Glacial Stage

Abstract Being both relatively tectonically stable and far from formerly glaciated regions, continental Australia is particularly well-suited for the study of sea-level evolution since the Last Glacial Maximum (LGM) and the Late Glacial Stage. Sea-level data from such regions are valuable indicators of ice-volume equivalent sea level since the effects of glacio-hydro-isostatic terms is less significant for far-field sites though still not negligible. In this study, we review published sea-level observations and results of recently developed numerical models for several areas of coastal Australia and produce corresponding palaeogeographic maps of the results of our numerical models. The results suggest that the Australian continental shelf was largely exposed during the LGM and provide detailed predictions of the timing of inundation as a function of position. Temporal and spatial variation of sea level provide important constraints that can in turn be used to improve the accuracy of numerical models, and better determine important rheological response parameters for the mantle and environmental conditions during the Late Glacial. The palaeoshoreline data from the modeling performed in this study indicate several key locations for future palaeo-environmental studies of sea-level change.

Geological causes of recent (~100 yr) vertical land movement in the Netherlands.

Abstract Geodetic levelling data record differential vertical movements of the top of the Pleistocene sands of up to 1.5 mm/year in the Netherlands over the last century. We compare these movements to (a) mean tectonic, isostatic and compaction movements at time scales of millions of years obtained by backstripping of Cenozoic stratigraphy, and (b) estimates of recent (∼100 yr) movements from process modelling of isostasy (glacio- and hydro-isostasy) and compaction. The process rates at time scales of millions of years are insufficient to account for the geodetic observations by an order of magnitude. The isostasy and compaction rates inferred for the last century are also insufficient; they explain less than half of the observed movements. This suggests that the residual — observed rates less isostasy and compaction estimates — which is interpreted to represent tectonic crustal deformation, constitutes an important contribution to present-day movements. The surprisingly high rates of short-term tectonic vertical movements in a region which is relatively inactive, seismically, indicate that correction of tide-gauge records for the glacio-isostatic signal alone does not yield an appropriate measure of eustatic sea-level rise along the Dutch coast.