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

Natural Disasters and Livelihoods: Evidence from Pests, Floods, and Disease in African Countries

Natural disasters and plagues have marked the course of human history, and remain among the greatest challenges and threats to economies and society. This dissertation uses a number of publicly-available datasets and natural experiments to explore how exposure to natural disasters affect households and societies in low- and middle-income African countries, with a particular focus on effects over time in agricultural communities. The disasters I study include desert locust swarms, floods, and the COVID-19 pandemic. In Chapter 1, I study the long-term effects of transitory agricultural shocks on the risk of violent conflict across Africa and the Arabian Peninsula using local variation in the areas exposed to desert locust swarms. In Chapter 2, I explore issues of measurement in determining what communities have been exposed to floods and how the use of different flooding definitions affects conclusions about the medium-term impacts of flood exposure on the incomes and activities of households in agricultural communities in Nigeria. In Chapter 3, I use a natural experiment during the COVID-19 pandemic to analyze how changes in household childcare burdens affect adults' labor supply in Kenya.My first chapter studies how transitory agricultural shocks affect the local risk of violent conflict over time. I answer this question using data on conflict events desert locust swarms across 0.25 degree grid cells in Africa and the Arabian peninsula from 1997-2018. Using modern difference-in-differences and event study approaches, I compare areas exposed to a desert locusts swarm--a localized agricultural disaster--at some point during this period to nearby areas with similar underlying risk of exposure. Local variation in exposure is caused by swarm flight patterns along their migratory paths during major outbreaks, characterized by daily downwind flights of over 100km. I find that having been exposed to a locust swarm significantly increases the average annual probability of violent conflict in a cell by 0.8 percentage points (43%). This is a very large effect, equivalent to the effect of a 1.6 degree C higher temperature in the same year. Effects persist for at least 14 years and are driven by swarms arriving in crop cells during the main growing season.I interpret the results using a model of occupational choice to explore income-related mechanisms. Persistent increases in conflict following a localized disaster suggest a decrease in the opportunity cost of fighting in affected areas. In line with this, I find that swarm exposure significantly reduces cereal yields in subsequent years, building on previous studies finding persistent effects of locust exposure on measures of household well-being and agricultural profits. Such effects indicate a permanent income mechanism for this severe transitory shock. This mechanism is not sufficient to explain the pattern of long-term impacts on violent conflict risk: the feasibility of engaging in violent conflict also matters. Increases in conflict risk are concentrated in years with active fighting groups in neighboring areas who may recruit or coerce individuals to join in violent conflict. Conflict therefore increases only when the reduced opportunity cost of fighting is combined with opportunities to fight. Patterns of long-term impacts on violent conflict are similar for severe droughts, indicating the mechanisms are not specific to locust shocks. Long-term impacts of transitory economic shocks on conflict risk add further motivation for policies mitigating the risk of such shocks and promoting household resilience and long-term recovery.My second chapter explores how extreme weather events affect livelihood decisions of agricultural households over time, focusing on the devastating 2012 floods in Nigeria. I use nationally-representative panel household survey data together with satellite imagery to analyze how exposure to the 2012 floods affects household labor supply and income in subsequent years. I first show that identification of flooding exposure varies depending on whether survey reports or MODIS satellite imagery are used. Differences in characteristics of communities identified as flooded by only one definition imply specific measurement issues for each. These results indicate that more attention is needed to considering how flooding is defined and measured, and whether there are different impacts of exposure to different types of floods.As in Chapter 1, I use a difference-in-differences design comparing changes in outcomes over time for households in communities exposed to flooding in 2012 against households in non-exposed communities that had a similar risk of flooding.I find no effects of flood exposure on engagement in wage employment or on total household income but a small significant increase in the probability of experiencing food insecurity using both definitions. The value of crop production falls, driven by a decrease in the value of commercial crops, while staple crop production either increases or remains stable. Using a MODIS-based flooding definition, I find that households exposed to floods significantly increase non-farm enterprise income relative to non-exposed households, as they reallocate labor from crop production to existing businesses. Using a survey-based definition, I find that flooding causes some farm households to become engaged in non-farm enterprise but without significantly increasing enterprise income while some non-farm households begin production of subsistence crops, with limited returns. The results emphasize that survey and satellite measures capture different types of flooding-related events which accordingly have different effects on exposed households. Studies of the impacts of floods should carefully consider their choice of flooding measure and the type of flooding they are most interested in analyzing.My third chapter, coauthored with Dennis Egger and Utz Pape, identifies the impact of a shock to childcare and child labor on adult labor supply and intra-household allocation of productive activities in the context of COVID-19-related school closures in Kenya. Using nationally-representative bi-monthly panel data, we compare changes in labor supply after schools partially reopened in October 2020 for adults with children in a grade eligible to return against adults with children in adjacent grades. We find that a child returning to school increases adults' weekly work hours by 29% in the short run, concentrated among the most flexible margins of adjustment, particularly household agriculture.Contrary to evidence from high-income settings, overall effects are not gendered. However, equal average labor supply responses for women and men are driven by different mechanisms particular to low- and middle-income settings. Women free up more time than men when childcare burdens fall but specialize more in childcare when returning students were net caregivers to younger siblings during school closures. Women also shoulder more of the reduction in child agricultural labor when students return to school, and shift from non-agricultural work into household agriculture (more easily combined with care of younger children). A back-of-the-envelope calculation suggests that school closures account for at least 40% of the overall drop in labor supply during the pandemic in Kenya, and a fall in GDP of 2.6%. Our results suggest policies increasing childcare access could substantially increase adult labor supply in low- and middle-income countries.Climate change is increasing not only global temperatures but also the frequency and severity of extreme weather events. Globalization has made societies around the world more connected but also more vulnerable to the spread of pests and disease. Yet there is little evidence on how such disasters affect households in low-income countries over time, and how they affect labor and livelihood decisions of poor farm households in particular. The three chapters of my dissertation combine publicly-available data, economic theory, and rigorous identification based on modern econometric methods to contribute to policy discussions around mitigation and adaptation to climate change, structural transformation, and civil conflict, among the greatest concerns for policymakers in Africa and around the world

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…

Nephelometer and current observations at the STIE site, Panama Basin

The LDGO-Thorndike film recording nephelometer was used in three modes (profiling, short-term tethered and long-term moored) to measure changes in particle concentrations on time scales of minutes to weeks and space scales of meters to 25 km while measurements were being made on production and settling rates of particles. Although the nepheloid layer had no large near-bottom increase suggestive of local resuspension, there was an unusually thick nepheloid layer due to resuspension and advection of sediment from the basin walls. The concentration of particles increased by a factor of 3 between 900 m and the seafloor at 3840 m, while the vertical flux of particles measured in traps increased by only a factor of 1.7 over that distance. The horizontal flux of particles past traps at all depths is estimated to have varied by less than 20% and, therefore, does not appear to influence the flux measured with sediment traps. Changes with time in small-particle concentrations measured by the moored nephelometer were less than 30%, but the concentration of large particles changed by 100%

Cross-shelf exchange in the northwestern Black Sea

The transports of water, heat, and salt between the northwestern shelf and deep interior of the Black Sea are investigated using a high-resolution three-dimensional primitive equation model. From April to August 2005, both onshore and offshore cross-shelf break transports in the top 20 m were 0.24 Sv on average, which is equivalent to the replacement of 60% of the volume of surface shelf waters (0–20 m) per month. Two main exchange mechanisms are studied: Ekman transport, and transport by mesoscale eddies and associated meanders of the Rim Current. The Ekman drift causes nearly uniform onshore or offshore flow over a large section of the shelf break, but it is confined to the upper layers. In contrast, eddies and meanders penetrate deep down to the bottom, but they are restricted laterally. During the strong wind events of 15–22 April and 1–4 July, some 0.66 × 1012 and 0.44 × 1012 m3 of water were removed from the northwestern shelf, respectively. In comparison, the single long-lived Sevastopol Eddy generated a much larger offshore transfer of 2.84 × 1012 m3 over the period 23 April to 30 June, which is equivalent to 102% of the volume of northwestern shelf waters. Over the study period, salt exchanges increased the average density of the shelf waters by 0.67 kg m−3 and reduced the density contrast between the shelf and deep sea, while lateral heat exchanges reduced the density of the shelf waters by 0.16 kg m−3 and sharpened the shelf break front

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

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)

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