ALBERT

Description: Fourteen research flights were conducted with the National Center for Atmospheric Research (NCAR) C-130 near Christmas Island (2° N, 157° W) during the summer of 2007 as part of the Pacific Atmospheric Sulfur Experiment (PASE). In order to tightly constrain the scalar budget of DMS, vertical eddy fluxes were measured at various levels in the marine boundary layer (MBL) from ~30 m to the top of the mixed layer (~500 m) providing improved accuracy of the flux divergence calculation in the DMS budget. The observed mean mole fraction of DMS in the MBL exhibited the well-known diurnal cycle, ranging from 50–95 pptv in the daytime to 90–110 pptv at night. Contributions from horizontal advection are included using a multivariate regression of all DMS flight data within the MBL to estimate the mean gradients and trends. With this technique we can use the residual term in the DMS budget as an estimate of overall photochemical oxidation. Error analysis of the various terms in the DMS budget indicate that chemical losses acting on time scales of up to 110 h can be inferred with this technique. On average, photochemistry accounted for ~7.4 ppt hr −1 loss rate for the seven daytime flights, with an estimated error of 0.6 ppt hr−1. The loss rate due to expected OH oxidation is sufficient to explain the net DMS destruction without invoking the action of additional oxidants (e.g., reactive halogens.) The observed ocean flux of DMS averaged 3.1 (±1.5) μmol m−2 d−1, and generally decreased throughout the sunlit hours. Over the entire mission, the horizontal advection contribution to the overall budget was merely -0.1 ppt hr−1, indicating a mean atmospheric DMS gradient nearly perpendicular to the east-southeasterly trade winds and the chlorophyll gradient in the equatorial upwelling ocean. Nonetheless, horizontal advection was a significant term in the budget of any given flight, ranging from −1.2 to 2.5 ppt hr−1 , indicating a patchy and variable surface seawater DMS distribution, and thus needs to be accounted for in budget studies.

Description: The stratigraphic record in the James and Hudson Bay Lowlands indicates that the sequence of glacial events at the geographical center of the 12.6 × 106 km2 Laurentide Ice Sheet may have been more complex than hitherto imagined. Isoleucine epimerization ratios of in situ and transported shells recovered from till and associated marine and fluvial sediments cluster into at least 4 discrete groups. Two alternative explanations of the data are offered, of which we strongly favor the first. Hypothesis 1: Setting the age of the “last interglacial” marine incursion, the Bell Sea, at 130,000 yr B.P. results in a long-term average diagenetic temperature for the lowlands of +0.6°C. Using this temperature enables us to predict the age of shells intermediate in age between the “last interglaciation” and the incursion of the Tyrrell Sea 8000 yr ago. Between these two interglacial marine inundations, Hudson Bay is predicted to have been free of ice along its southern shore about 35,000, 75,000, and 105,000 yr ago based on amino acid ratios from shells occurring as erratics in several superimposed tills and fluvial sediments. These results suggest (1) that traditional concepts of ice-sheet build-up and decay must be reexamined; (2) that “high” sea levels may have occurred during the Wisconsin Glaciation; and (3) that a critical reappraisal is required of the open ocean δ18O record as a simple indicator of global ice volume. An alternative, Hypothesis 2, is also examined. It is based on the assumption that the 35,000-yr-old deposits calculated on the basis of Hypothesis 1 date from the “last interglaciation”; this, in effect, indicates that the Missinaibi Formation, commonly accepted as sediments of the “last interglaciation,” are about 500,000 yr old and that the effective diagenetic temperature in the lowlands during approximately the last 130,000 yr has been close to −6°C. We argue for rejection of this alternative hypothesis.

Description: Moraines deposited by the outermost ice advance across Judge Daly Promontory, northeastern Ellesmere Island, reflect thin, topographically controlled ice lobes extending to sea-level. The termini of two ice lobes were investigated and both produced ice shelves where they flowed into isostatically depressed embayments along western Kennedy Channel. Morphological evidence for these ice shelves occurs at the entrance to these valleys where steeply descending lateral moraines become abruptly horizontal for 2 km. In addition, both the horizontal moraines and associated pro-glacial terraces are fossiliferous down-valley from the apparent grounding line. Based on the differences in elevation between the horizontal moraines and the valley bottoms, the two ice shelves had estimated thicknesses ofc. 110 and 150 m. A proglacial outwash terrace at 175 m a.s.l. is considered to represent the approximate relative sea-level during the formation and break-up of the ice shelves. This relative sea-level is consistent with the water depths required to float the calculated ice thicknesses in both valleys. Associated with these ice margins are finite14C dates of 28 000-30 000 B.P. and amino-acid age estimates of 〉35 000 B.P. The importance and likelihood of additional past ice shelves in the Canadian High Arctic is discussed.

Description: Moraines deposited by the outermost ice advance across Judge Daly Promontory, northeastern Ellesmere Island, reflect thin, topographically controlled ice lobes extending to sea-level. The termini of two ice lobes were investigated and both produced ice shelves where they flowed into isostatically depressed embayments along western Kennedy Channel. Morphological evidence for these ice shelves occurs at the entrance to these valleys where steeply descending lateral moraines become abruptly horizontal for 2 km. In addition, both the horizontal moraines and associated pro-glacial terraces are fossiliferous down-valley from the apparent grounding line. Based on the differences in elevation between the horizontal moraines and the valley bottoms, the two ice shelves had estimated thicknesses of c. 110 and 150 m. A proglacial outwash terrace at 175 m a.s.l. is considered to represent the approximate relative sea-level during the formation and break-up of the ice shelves. This relative sea-level is consistent with the water depths required to float the calculated ice thicknesses in both valleys. Associated with these ice margins are finite 14C dates of 28 000-30 000 B.P. and amino-acid age estimates of 〉35 000 B.P. The importance and likelihood of additional past ice shelves in the Canadian High Arctic is discussed.

Description: All of Australia's largest mammalian vertebrates became extinct 50 to 45 ka (thousand years ago), shortly after human colonization. Between 60 and 40 ka Australian climate was similar to present and not changing rapidly. Consequently, attention has turned toward plausible human mechanisms for the extinction, with proponents for over-hunting, ecosystem change, and introduced disease. To differentiate between these options we utilize isotopic tracers of diet preserved in eggshells of two large, flightless birds to track the status of ecosystems before and after human colonization. δ13C preserved in their eggshells monitor a bird's dietary intake in the weeks to months before egg-laying. More than 500 dated eggshells from central Australia of the Australian emu (Dromaius novaehollandiae), an opportunistic, dominantly herbivorous feeder, provide a continuous 140 kyr dietary δ 13C reconstruction. More than 350 dated eggshells from the same region of the heavier, extinct, giant bird Genyornis newtoni define its dietary intake from 140 ka until its extinction about 50 ka. Additional dietary records for both species were developed from two distant regions. Dromaius eggshell dietary δ13C reveals an unprecedented reduction in the bird's food resources about 50 ka, coeval in all three regions, suggesting conversion at that time of a tree/shrub savannah with occasionally rich grasslands to the modern desert scrub. We speculate that ecosystem collapse across the arid and semi-arid zones is a consequence of systematic burning by early humans. Genyornis diet everywhere is more restricted than in co-existing Dromaius, implying a more specialized feeding strategy. These data suggest that generalist feeders, such as Dromaius, were able to adapt to a changed vegetation regime, whereas more specialized feeders, such as Genyornis, became extinct. The altered vegetation may have also impacted Australian climate. Changes in the strength of climate feedbacks linked to vegetation and soil type (moisture recycling, surface roughness, albedo) may have weakened the penetration of monsoon moisture into the continental interior under the new ecosystem. Climate modeling suggests such a shift may have reduced monsoon rain in the interior by as much as 50%.

Description: The moisture balance across northern and central Australia is dominated by changes in the strength of the Australian Summer Monsoon. Lake-level records that record changes in monsoon strength on orbital timescales are most consistent with a Northern Hemisphere insolation control on monsoon strength, a result consistent with recent modeling studies. A weak Holocene monsoon relative to monsoon strength 65–60 ka, despite stronger forcing, suggests a changed monsoon regime after 60 ka. Shortly after 60 ka humans colonized Australia and all of Australia's largest mammals became extinct. Between 60 and 40 ka Australian climate was similar to present and not changing rapidly. Consequently, attention has turned toward plausible human mechanisms for the extinction, with proponents for over-hunting, ecosystem change, and introduced disease. To differentiate between these options we utilize isotopic tracers of diet preserved in eggshells of two large, flightless birds to track the status of ecosystems before and after human colonization. More than 800 dated eggshells of the Australian emu (Dromaius novaehollandiae), an opportunistic, dominantly herbivorous feeder, provide a 140-kyr dietary reconstruction that reveals unprecedented reduction in the bird's food resources about 50 ka, coeval in three distant regions. These data suggest a tree/shrub savannah with occasionally rich grasslands was converted abruptly to the modern desert scrub. The diet of the heavier, extinct Genyornis newtoni, derived from 〉550 dated eggshells, was more restricted than in co-existing Dromaius, implying a more specialized feeding strategy. We suggest that generalist feeders, such as Dromaius, were able to adapt to a changed vegetation regime, whereas more specialized feeders, such as Genyornis, became extinct. We speculate that ecosystem collapse across arid and semi-arid zones was a consequence of systematic burning by early humans. We also suggest that altered climate feedbacks linked to changes in vegetation may have weakened the penetration of monsoon moisture into the continental interior, explaining the failure of the Holocene monsoon. Climate modeling suggests a vegetation shift may reduce monsoon rain in the interior by as much as 50%.

Description: Fourteen research flights were conducted with the National Center for Atmospheric Research (NCAR) C-130 near Christmas Island (2° N, 157° W) during the summer of 2007 as part of the Pacific Atmospheric Sulfur Experiment (PASE). In order to tightly constrain the scalar budget of DMS, fluxes were measured at various levels in the marine boundary layer (MBL) from near the surface (30 m) to the top of the mixed layer (500 m) providing greater accuracy of the flux divergence calculation in the DMS budget. The observed mean mole fraction of DMS in the MBL exhibited the well known diurnal cycle, ranging from 50 pptv in the daytime to 110 pptv at night. Contributions from horizontal advection are included using a multivariate regression of all DMS flight data from within the MBL to estimate the mean gradients and trends. With this technique we consider the residual term in the DMS budget as an estimate of overall photochemical oxidation. Error analysis of the various terms in the DMS budget indicate that chemical losses acting on time scales of up to 110 h can be inferred with this technique. On average, photochemistry accounted for 7.3 ppt hr−1 loss rate for the seven daytime flights, with an estimated error of 0.6 ppt/hr. The loss rate due to expected OH oxidation is sufficient to explain the net DMS destruction without invoking the action of additional oxidants (e.g. reactive halogens.) The observed ocean flux of DMS averaged 3.1 (±1.5)μmol m−2 d−1, and generally decreased throughout the sunlit hours. The average entrainment flux at the top of the MBL was 2.5 μmol m−2 d−1; therefore the flux divergence term in the budget equation only contributed an average increase of 1.3 ppt hr−1 to the mean MBL mole fraction. Over the entire mission, the horizontal advection contribution to the overall budget was 0.2 ppt hr−1, indicating a mean atmospheric DMS gradient nearly perpendicular to the east-southeasterly trade winds and the chlorophyll gradient in the equatorial upwelling ocean. Nonetheless, horizontal advection was a significant term in the budget of any given flight, ranging from −1.5 to 2.3 ppt hr−1, indicating a patchy and random seawater DMS distribution, and thus needs to be accounted for in budget studies.