Meteoric smoke concentration in the Vostok ice core estimated from superparamagnetic relaxation and some consequences for estimates of Earth accretion rate

We measured the magnetization of glacial and interglacial ice from the Vostok core to estimate the meteoric smoke concentration in Antarctic ice. We have found that, within the uncertainty of the method, the smoke concentration in ice in Antarctica is equivalent to that previously measured in Greenland ice. The virtually identical smoke concentrations despite the different ice accumulation rates in Greenland and Antarctica suggest that wet deposition is the main deposition mechanism for such ultra-small particles. Given the typical scavenging ratios for atmospheric aerosols, this would imply that previous estimates of accretion rate based on dry deposition are likely to be appreciably overestimated

A sediment trap experiment in the Vema Channel to evaluate the effect of horizontal particle fluxes on measured vertical fluxes

Sediment traps are used to measure fluxes and collect samples for studies in biology, chemistry and geology, yet we have much to learn about factors that influence particle collection rates. Toward this end, we deployed cylindrical sediment traps on five current meter moorings across the Vema Channel to field-test the effect of different horizontal particle fluxes on the collection rate of the traps— instruments intended for the collection of vertically settling particles. The asymmetric flow of Antarctic Bottom Water through the Vema Channel created an excellent natural flume environment in which there were vertical and lateral gradients in the distribution of both horizontal velocity and particle concentration and, therefore, the resulting horizontal flux. Horizontal effects were examined by comparing quantities of collected material (apparent vertical fluxes) with the horizontal fluxes of particles past each trap. We also looked for evidence of hydrodynamic biases by comparing and contrasting the composition of trap material based on particle size and the concentration of Al, Si, Ca, Mg, Mn, Corg and CaCO3. Experimental inverted traps and traps with only side openings were deployed to test a hypothesis of how particles are collected in traps. The vertical flux of surface-water particles should have been relatively uniform over the 45 km region of the mooring locations, so if horizontal transport contributed significantly to collection rates in traps, the calculated trap fluxes should be correlated positively with the horizontal flux. If the horizontal flow caused undertrapping, there should be a negative correlation with velocity or Reynolds number. The gross horizontal flux past different traps varied by a factor of 37, yet the quantity collected by the traps differed by only a factor of 1.4. The calculated horizontal fluxes were 2–4 orders of magnitude larger than the measured apparent vertical fluxes. Mean velocities past the traps ranged from 1–22 cm s−1 (Reynolds numbers of 3,500–43,000 for these traps with a diameter of 30.5 cm and an aspect ratio of ≈3) and showed no statistically significant relationship to the apparent vertical flux. We conclude that at current speeds measured in a very large portion of the world\u27s oceans, vertical fluxes measured with moored, cylindrical traps should exhibit little effect from horizontal currents

On the transport and modification of Antarctic Bottom Water in the Vema Channel

The Verna Channel is a deep passage across the Rio Grande Rise in the South Atlantic through which Antarctic Bottom Water (AABW) must flow on its way northward from the Argentine Basin to the Brazil Basin and eventuafly into the North Atlantic…

Benthic storms, nepheloid layers, and linkage with upper ocean dynamics in the western North Atlantic

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Marine Geology 385 (2017): 304–327, doi:10.1016/j.margeo.2016.12.012.Benthic storms are episodic periods of strong abyssal currents and intense, benthic nepheloid (turbid) layer development. In order to interpret the driving forces that create and sustain these storms, we synthesize measurements of deep ocean currents, nephelometer-based particulate matter (PM) concentrations, and seafloor time-series photographs collected during several science programs that spanned two decades in the western North Atlantic. Benthic storms occurred in areas with high sea-surface eddy kinetic energy, and they most frequently occurred beneath the meandering Gulf Stream or its associated rings, which generate deep cyclones, anticyclones, and/or topographic waves; these create currents with sufficient bed-shear stress to erode and resuspend sediment, thus initiating or enhancing benthic storms. Occasionally, strong currents do not correspond with large increases in PM concentrations, suggesting that easily erodible sediment was previously swept away. Periods of moderate to low currents associated with high PM concentrations are also observed; these are interpreted as advection of PM delivered as storm tails from distal storm events. Outside of areas with high surface and deep eddy kinetic energy, benthic nepheloid layers are weak to non-existent, indicating that benthic storms are necessary to create and maintain strong nepheloid layers. Origins and intensities of benthic storms are best identified using a combination of time-series measurements of bottom currents, PM concentration, and bottom photographs, and these should be coupled with water-column and surface-circulation data to better interpret the specific relations between shallow and deep circulation patterns. Understanding the generation of benthic nepheloid layers is necessary in order to properly interpret PM distribution and its influence on global biogeochemistry.Funding for construction of the Bottom Ocean Monitor was provided by Lamont-Doherty Geological Observatory (now Lamont-Doherty Earth Observatory). BOM and mooring deployments and data analysis were funded by the Office of Naval Research (contracts N00014-75-C-0210 and N00014-80-C-0098 to Biscaye and Gardner at Lamont-Doherty; Contracts N00014-79-C-0071 and N00014-82-C-0019 at Woods Hole Oceanographic Institution and ONR Contracts N00014-75-C-0210 and N00014-80-C-0098 at Lamont-Doherty Geological Observatory to Tucholke), Sandia National Laboratories (contract SL-16-5279 to Gardner), the National Science Foundation (contract OCE 1536565 to Gardner and Richardson), Earl F. Cook Professorship (Gardner), and the Department of Energy (contract DE-FG02-87ER-60555 to Biscaye)

Isothermal remanent magnetization of Greenland ice: Preliminary results

The magnetic mineral content of wind“transported dust should reflect atmospheric transport dynamics and conditions in its source areas, and could thus be used as an environmental proxy. To test the feasibility of determining the magnetic mineral content in polar ice, isothermal remanent magnetization (IRM) was measured on a small suite of Greenland ice samples of Holocene (interglacial) and Last Glacial Maximum (LGM) age. Although the extremely low dust concentrations limit weak field (susceptibility) measurements, all samples contained an easily measurable concentration of magnetic minerals that can be estimated using IRM intensity provided that special precautions are used. IRM experiments at liquid nitrogen temperatures indicate ice magnetic properties which are consistent with that expected from varying concentrations of magnetite or maghemite. Interestingly, the Holocene ice samples tend to have higher magnetic concentrations, despite having much lower total polar dust contents, than the few LGM ice samples tested thus far

Magnetization of polar ice: a measurement of terrestrial dust and extraterrestrial fallout

Laboratory-induced remanent magnetization of polar ice constitutes a measurement of the magnetization carried by the ferromagnetic dust particles in the ice. This non-destructive technique provides a novel kind of information on the dust deposited on the surface of polar ice sheets. Measurements made on ice samples from Greenland (North GRIP ice core) and Antarctica (Vostok and EPICA-Dome C ice cores) allowed the recognition of a fraction of magnetic minerals in ice whose concentration and magnetic properties are directly related to that of insoluble dust. The source of this fraction of magnetic minerals thus appears closely related to terrestrial dust transport and deposition and its magnetic properties are informative of the dust provenance areas. The rock-magnetic properties of the dust may reflect distinct changes of dust source areas from glacial to interglacial periods in agreement with and adding further information to the isotopic (87Sr/86Sr and 143Nd/144Nd) analyses. A second magnetic fraction consists of particles of nanometric size, which are superparamagnetic at freezer temperature and whose concentration is independent of the mass of aerosol dust found in the ice. The source of these nanometric-sized magnetic particles is ascribed to fallout of “meteoric smoke” and their concentration in ice was found to be compatible with the extraterrestrial fallout inferred from Ir concentrations. The diameter of the smoke particles as inferred from magnetic measurements is in the range of about 7–20 nm

Public policy and the promise of digital credit for financial inclusion

Digital credit products are characterized by a lending process that is instantaneous, automated, and remote. While digital credit has the potential to reach less collateralized, less mobile, and more remote cohorts of borrowers, there are also risks in relying on digital credit for financial inclusion. This paper investigates the digital credit policy environment and the extent to which it may support pro-poor digital credit market development using two types of documents: a set of 23 regulatory documents specifically mentioning either digital or online credit or lending, and another set of 298 informal documents relevant to digital credit based on a systematic web search. After reviewing the literature on the effects of credit expansion and automated credit scoring, we summarize the characteristics of the current digital credit regulatory environment in low- and middle-income countries. Our findings suggest that few regulations specifically target digital credit markets, and that the current regulatory environment may not support the full potential of digital credit to reach historically underserved credit consumers. Most countries do not explicitly target financial inclusion as part of their digital credit policies. However, we do find evidence that informal web documents consider financial inclusion to a greater extent than formal regulatory documents

Calendar-year dating of the Greenland Ice Sheet Project 2 (GISP2) ice core from the early sixth century using historical, ion, and particulate data

We use the occurrence of unusual or out-of-season dust storms and dissolved ion data as proxies for dust to propose a calendar-year chronology for a portion of the Greenland Ice Sheet Project 2 (GISP2) ice core during the early sixth century A.D. Our new time scale moves a small sulfate peak to early 537 A.D., which is more consistent with recent findings of a 6 mo to 18 mo time lag between volcanic eruptions and atmospheric fallout of their sulfate aerosols. Our new time scale is consistent with a small volcanic input to the A.D. 536–537 climate downturn. We use the time range of Ni-rich fragments and cosmic spherules to provide an independent test of the chronology. The time range of Ni-rich fragments and cosmic spherules matches historical observations of "dancing stars" starting in the summer of A.D. 533 and lasting until A.D. 539 or 540. These dancing stars have been previously attributed to cosmogenic dust loading of Earth's atmosphere. The time scale cannot be shifted to be either younger or older by 1 yr without destroying the match to historical accounts of dancing stars

What caused terrestrial dust loading and climate downturns between A.D. 533 and 540?

Sn-rich particles, Ni-rich particles, and cosmic spherules are found together at four discrete stratigraphic levels within the 362-360 m depth interval of the Greenland Ice Sheet Project 2 (GISP2) ice core (72.6°N, 38.5°W, elevation: 3203 m). Using a previously derived calendar-year time scale, these particles span a time of increased dust loading of Earth's atmosphere between A.D. 533 and 540. The Sn-rich and Ni-rich particles contain an average of 10–11 wt% C. Their high C contents coupled with local enrichments in the volatile elements I, Zn, Cu, and Xe suggest a cometary source for the dust. The late spring timing of extraterrestrial input best matches the Eta Aquarid meteor shower associated with comet 1P/Halley. An increased flux of cometary dust might explain a modest climate downturn in A.D. 533. Both cometary dust and volcanic sulfate probably contributed to the profound global dimming during A.D. 536 and 537 but may be insufficient sources of fine aerosols. We found tropical marine microfossils and aerosol-sized CaCO3 particles at the end A.D. 535–start A.D. 536 level that we attribute to a low-latitude explosion in the ocean. This additional source of dust is probably needed to explain the solar dimming during A.D. 536 and 537. Although there has been no extinction documented at A.D. 536, our results are relevant because mass extinctions may also have multiple drivers. Detailed examinations of fine particles at and near extinction horizons can help to determine the relative contributions of cosmic and volcanic drivers to mass extinctions