Thesis advisor: Jeremy Shakun / Thesis advisor: Tony Wang / To better understand ice sheet and nutrient cycling dynamics in the North Atlantic, three geochemical paleo-proxies have been analyzed in Pleistocene marine sediments: cosmogenic nuclides (10Be and 26Al) in ice-rafted debris (IRD), 40Ar/39Ar in IRD, and foraminifera-bound nitrogen isotopes (FB-δ15N). For Chapter 1, we analyzed 10Be and 26Al concentrations in quartz separates of IRD from last-glacial North Atlantic sediments and used these data to constrain the history of Laurentide Ice Sheet (LIS) cover over Canada during the Pleistocene. While LIS history is well constrained since the Last Glacial Maximum (LGM) (~20,000 years ago), there is little evidence available from earlier times. 26Al/10Be ratios are depressed in these samples, the result of long-term decay under cover, which we suggest is best explained by a persistent LIS over much of the last million years. This finding implies that the LIS did not fully disappear during many Pleistocene interglacials, making the current ice-free Holocene interglacial relatively unique. For Chapter 2, we synthesized 3,762 40Ar/39Ar ages from North Atlantic IRD, including 670 new analyses. 595 of these single-grain analyses come from some of the same sample intervals as Chapter 1. These 40Ar/39Ar ages in IRD, a tracer of IRD provenance, clarify changes in North Atlantic ice sheet extent during the past few glacial cycles. Comparison of 40Ar/39Ar ages with hypothesized ice margins and cosmogenic nuclide data (from Chapter 1) aid in our interpretations. For last-glacial samples, results suggest ice sheets around the basin may have been in a retracted state during Marine Isotope Stage (MIS) 3 (~29-57 ka), an interval of debated ice extent for the LIS. Our synthesis also allows us to present the first complete records 40Ar/39Ar ages in IRD during Heinrich intervals – times when the LIS exhibited iceberg discharge events. These results support the suggestion made by previous work that Heinrich events 3 and 6 are anomalous relative to other intervals. 40Ar/39Ar analyses from earlier glacial periods – the first yet published – highlight that IRD provenance data may be reflecting iceberg survivability in addition to changes in ice extent. Lastly, analyzing cosmogenic nuclides and 40Ar/39Ar ages in IRD from the same sample intervals indicates that both proxies may be used to infer changes in ice sheet provenance; this is the first time cosmogenic nuclides in IRD have been considered in this way. For Chapter 3, we analyzed FB-δ15N at Site U1313 in North Atlantic samples during the Plio-Pleistocene to reconstruct marine nutrient cycling. In the North Atlantic, nutrient cycling is known to play an important role in regulating surface ocean productivity and CO2 drawdown via photosynthesis. We nevertheless lack a complete understanding of nutrient cycling evolution for the Plio-Pleistocene, during which Northern Hemisphere ice sheets and climate exhibited dramatic fluctuations. We find increasing FB-δ15N values at the transition from the Pliocene to the Pleistocene. Additionally, FB-δ15N values are generally higher during glacials compared to interglacials. We suggest these observations can best be explained by an expansion or increase of N2 fixation in the North Atlantic during warmer intervals (interglacials, Pliocene) and a retraction or decrease of N2 fixation during cold intervals (Pleistocene glacials). Considered alongside previously published paleo-proxy data from Site U1313 we suggest these changes in nutrient cycling reflect climate-driven migrations of the North Atlantic Current. In Chapter 4, I reflect upon how I have worked to integrate broader impacts into my Ph.D. work. Using a combined approach, I focused on fostering an inclusive environment within paleoclimate research spaces as well as engaging non-scientists in paleoclimate-and Earth science-related activities. These approaches ranged from K-12 partnership activities to mentoring students to more experimental avenues, such as a collaborative art project. I have evaluated the success of this work using a combination of quantitative metrics and subjective assessments. Participating in these efforts was also crucial for reminding me of the importance of making science accessible to everyone as well as for helping me hone my mentorship and science communication skills. / Thesis (PhD) — Boston College, 2024. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

No description available.

No description available.

Heinrich Events (HEs) define intervals of major ice rafting from the Laurentide Ice Sheet (LIS) into the North Atlantic during that last glacial period. The discovery of potential European-sourced precursors to HEs suggest that the smaller, but climactically sensitive, European ice sheets (EIS) may have played a role in the triggering of HEs and their impact on global climates. Environmental magnetism has proved itself to be a useful, rapid and non-destructive tool in the identification and quantification of provenance in sediments from various depositional environments. In this work, environmental magnetic analyses are applied to marine sediment records from the European margin of the NE Atlantic and known to contain ice-rafted debris (IRD) from both LIS and EIS sources. The primary aim in the work of this thesis is to evaluate the methodology as a means of distinguishing IRD provenance. From the data obtained here it is possible to identify several magnetic events that correspond to the HEs and other layers of detrital material and which correlate well to previous standard petrological analyses performed on the same core materials. Magnetic signatures differ within the HEs, suggesting a changing balance of input from multiple sources as opposed to a single LIS source. The data suggest a phasing of these compositional differences through individual HEs. The potential of using environmental magnetic techniques in the identification of IRD provenance within marine sediments is discussed, as is the significance of the observed provenance variations within the cores studied.

No description available.

Thesis (M.A. in National Security Affairs)--Naval Postgraduate School, June 2003. / Thesis advisor(s): Donald Abenheim, Daniel J. Moran. Includes bibliographical references (p. 83-92). Also available online.

The western North Atlantic population of the northern right whale (Eubalaena glacialis) was severely depleted by whaling over the last century. Despite over fifty years of protection, fewer than four hundred individuals are believed to exist in the North Atlantic. Relatively little is known about the diving behavior, respiratory physiology, or diel activity of these whales, although such information could be useful in predicting population abundance and distribution in relation to food or environmental variables. Dive behavior data can be collected visually, but collection is limited to daylight hours, calm weather, and fortuitous encounters with study animals. Satellite-monitored radio tags offer the opportunity to collect data on individual whales' diving behavior continuously, over long distances and periods of time, and at relatively low cost. Thirteen North Atlantic right whales were tagged and monitored in the Gulf of Maine between 1989 and 1991. One male was tagged in 1989; two adult males, six adult females (two with calves) and one juvenile were tagged in 1990; one adult female (with a calf) and two juveniles were tagged in 1991. The duration of monitoring for the whales varied from <1 day to 43 days. Characteristics of the radio tags were different over each year in order to evaluate different attachment mechanisms and methods of collecting and summarizing data for dive duration, dive frequency, and time submerged. These data were then used to describe the dive behavior and to predict aspects of the respiratory physiology for these whales. The number of dives, their duration, and the time submerged varied considerably among individual whales and between years. Over all, the whales spent 79% of their time underwater. However, most dives were short (i.e., 95% were <14 min; the mean dive duration was 92.3 sec), although dives of 30-40 min duration were observed for several individuals. In general, the number of dives a right whale made was inversely related to the average duration of its dives within a time interval. Furthermore, over a given time interval, the number of times a whale dove was a better predictor of the percent time it was submerged than was the average duration of its dives. There was no evidence of diel variation in dive behavior (i.e., number of dives, average dive duration, or percent time submerged) among these whales. Age, sex, and reproductive status may have affected dive behavior, although these trends were not statistically significant due to the small number of study animals and individual behavioral variability. Males tended to dive more often and averaged shorter dives than females. Females with calves dove more often and averaged shorter dives than females without calves. Juvenile females dove less often but averaged longer dives than adult males or females with calves. It was predicted that the aerobic dive limit of an "average" right whale was approximately 14 min. Ninety-five percent of the dives recorded for the 11 monitored right whales were < 14 min. Furthermore, there was no evidence of recuperative periods (i.e., prolonged periods at the surface) after long dives. These observations were consistent with the idea that the North Atlantic right whales dove within their aerobic scope. They further suggest that physiological parameters alone probably have little influence on dive characteristics, except to set an upper limit on the duration of a dive. Satellite telemetry provides an opportunity to monitor the movements and behavior of free-ranging animals, and overcome many of the short-comings associated with traditional, human-based visual techniques for tracking and studying such animals. Although the tags used in this study were prototypes and varied in their design from year to year, several right whales were monitored simultaneously and were tracked over thousands of kilometers. Advances in tag miniaturization, attachment, and software will likely extend the time tags stay attached and the detail of the behavioral and environmental variables that can be monitored. As testimony to the power of this technology, this application of satellite telemetry to monitor great whales yielded one of the most extensive, long-term, continuously-monitored data sets yet recorded on the diving behavior of a baleen whale. / Graduation date: 1993

Storm-induced open-ocean convective mixing is one of the primary processes controlling the supply of nitrate to the sunlit layer of the oligotrophic North Atlantic Subtropical Gyre (NASG). Yet, the magnitude and timing of nitrate fluxes during winter convection is poorly understood due to an absence of targeted process studies. In the northwest NASG, multiple quasi-Lagrangian studies were conducted during the boreal winters of 2004 and 2005 in an effort to sample strong winter convection. During each of the time-series studies, inventories of vertically fluxed nitrate were quantified approximately every twelve hours using the distribution of helium isotopes ([delta]�He) and nitrate in the water column. This method is known as the Helium Flux Gauge Technique (HFGT). Large variability in surface forcing and density structure of the upper ocean was observed between the two years; however, only winter 2005 experienced convective mixing to depths greater than 150 m. In winter 2004, mild atmospheric conditions coincided with a positive phase in the winter North Atlantic Oscillation (NAO), consistent with the dominant regime experienced during the previous decade. On average 36 � 9 mmol m[-2] of fluxed nitrate was inferred by excess �He in the mixed layer of the ocean during the winter 2004 study period. This inventory of physically transported nitrate is attributed to the sampling of waters laterally advected from nearby eddy features. The sampling of multiple water masses is likely due to the inability of the drogue to persistently follow water masses efficiently. Although physical evidence indicates spatial variability within the time-series data, the length scales of convective mixing appear to be greater than those associated with spatial aliasing as a result of drogue performance. This observation provides us with increased confidence that the objectives for the present study are not compromised by spatial variability in the data. In contrast, winter 2005 experienced a negative NAO, strong physical forcing and convective mixing to depths > 250 m. Two convectively modified water masses, most likely resulting from a single storm event, were sampled at different stages of development. These two water masses exhibit large variability in the magnitude of nitrate entrained in the convective layer from the thermocline. An average inventory of 247 � 56 mmol NO₃[-]m[-2] was entrained in the rapidly expanding convective layer of the first water mass in the first few days following the storm approach. In contrast, ongoing entrainment of nitrate was absent from the second water mass, sampled two weeks later when the depth of the surface mixed layer was consistently ~ 300 m. These results indicate that surrounding fluid is entrained into the convective layer when it is actively expanding in the vertical. On the other hand, significant fluid entrainment does not occur at the base of the plume once sinking waters have reached a level of neutral buoyancy. The persistence of elevated nitrate stocks (~ 100 mmol m[-2]) in the convective layer two to three weeks after the inferred injection event, suggests sub-optimal nitrate uptake by resident phytoplankton. Phytoplankton growth was most likely resource limited by light or a micronutrient such as iron. Despite the implied biolimitation, changes in chlorophyll-a, a proxy for phytoplankton biomass, indicate net production within the convective layer. On average, the convective layer was observed to support an inventory of 62 � 6mg chlorophyll-a m[-2], increasing at an average rate of 3.4mg m[-2] d[-1]. This inventory indicates a slow build-up of phytoplankton biomass to near bloom levels, ahead of the main spring bloom that typically follows formation of the seasonal thermocline near Bermuda. Net production in the convective layer was likely due to transient periods of increased (weak) surface stability that were observed to support high phytoplankton biomass, following the cessation of thermocline fluid entrainment. When nitrate and excess �He in samples collected from the thermocline were regressed for the purpose of quantifying nitrate fluxes, the results showed that between 1.6 - 2.0 [mu]mol kg[-1] of dissolved nitrate was present during formation of the water mass. This suggests the source of this excess (above Redfield ratios) nitrate in the thermocline of the NASG is not local, and has ramifications for local nitrogen fixation budgets determined using geochemical approaches. Thesis supervisors: William J. Jenkins, Senior Scientist, WHOI (United States of America); Philip W. Boyd, Senior Scientist, NIWA (New Zealand); Michael W. Lomas, Senior Scientist, BIOS (Bermuda)

Storm-induced open-ocean convective mixing is one of the primary processes controlling the supply of nitrate to the sunlit layer of the oligotrophic North Atlantic Subtropical Gyre (NASG). Yet, the magnitude and timing of nitrate fluxes during winter convection is poorly understood due to an absence of targeted process studies. In the northwest NASG, multiple quasi-Lagrangian studies were conducted during the boreal winters of 2004 and 2005 in an effort to sample strong winter convection. During each of the time-series studies, inventories of vertically fluxed nitrate were quantified approximately every twelve hours using the distribution of helium isotopes ([delta]�He) and nitrate in the water column. This method is known as the Helium Flux Gauge Technique (HFGT). Large variability in surface forcing and density structure of the upper ocean was observed between the two years; however, only winter 2005 experienced convective mixing to depths greater than 150 m. In winter 2004, mild atmospheric conditions coincided with a positive phase in the winter North Atlantic Oscillation (NAO), consistent with the dominant regime experienced during the previous decade. On average 36 � 9 mmol m[-2] of fluxed nitrate was inferred by excess �He in the mixed layer of the ocean during the winter 2004 study period. This inventory of physically transported nitrate is attributed to the sampling of waters laterally advected from nearby eddy features. The sampling of multiple water masses is likely due to the inability of the drogue to persistently follow water masses efficiently. Although physical evidence indicates spatial variability within the time-series data, the length scales of convective mixing appear to be greater than those associated with spatial aliasing as a result of drogue performance. This observation provides us with increased confidence that the objectives for the present study are not compromised by spatial variability in the data. In contrast, winter 2005 experienced a negative NAO, strong physical forcing and convective mixing to depths > 250 m. Two convectively modified water masses, most likely resulting from a single storm event, were sampled at different stages of development. These two water masses exhibit large variability in the magnitude of nitrate entrained in the convective layer from the thermocline. An average inventory of 247 � 56 mmol NO₃[-]m[-2] was entrained in the rapidly expanding convective layer of the first water mass in the first few days following the storm approach. In contrast, ongoing entrainment of nitrate was absent from the second water mass, sampled two weeks later when the depth of the surface mixed layer was consistently ~ 300 m. These results indicate that surrounding fluid is entrained into the convective layer when it is actively expanding in the vertical. On the other hand, significant fluid entrainment does not occur at the base of the plume once sinking waters have reached a level of neutral buoyancy. The persistence of elevated nitrate stocks (~ 100 mmol m[-2]) in the convective layer two to three weeks after the inferred injection event, suggests sub-optimal nitrate uptake by resident phytoplankton. Phytoplankton growth was most likely resource limited by light or a micronutrient such as iron. Despite the implied biolimitation, changes in chlorophyll-a, a proxy for phytoplankton biomass, indicate net production within the convective layer. On average, the convective layer was observed to support an inventory of 62 � 6mg chlorophyll-a m[-2], increasing at an average rate of 3.4mg m[-2] d[-1]. This inventory indicates a slow build-up of phytoplankton biomass to near bloom levels, ahead of the main spring bloom that typically follows formation of the seasonal thermocline near Bermuda. Net production in the convective layer was likely due to transient periods of increased (weak) surface stability that were observed to support high phytoplankton biomass, following the cessation of thermocline fluid entrainment. When nitrate and excess �He in samples collected from the thermocline were regressed for the purpose of quantifying nitrate fluxes, the results showed that between 1.6 - 2.0 [mu]mol kg[-1] of dissolved nitrate was present during formation of the water mass. This suggests the source of this excess (above Redfield ratios) nitrate in the thermocline of the NASG is not local, and has ramifications for local nitrogen fixation budgets determined using geochemical approaches. Thesis supervisors: William J. Jenkins, Senior Scientist, WHOI (United States of America); Philip W. Boyd, Senior Scientist, NIWA (New Zealand); Michael W. Lomas, Senior Scientist, BIOS (Bermuda)

Thesis (M.A.)--Carleton University, 2001. / Includes bibliographical references (p. 134-158). Also available in electronic format on the Internet.