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Modeling Deep Ocean Water and Sediment Dynamics in the Eastern Pacific Ocean Using Actinium-227 and Other Naturally Occurring Radioisotopes.

紀錄類型:	書目-電子資源 : Monograph/item
正題名/作者:	Modeling Deep Ocean Water and Sediment Dynamics in the Eastern Pacific Ocean Using Actinium-227 and Other Naturally Occurring Radioisotopes./
作者:	Kemnitz, Nathaniel James.
面頁冊數:	1 online resource (251 pages)
附註:	Source: Dissertations Abstracts International, Volume: 84-04, Section: B.
Contained By:	Dissertations Abstracts International84-04B.
標題:	Geology. -
電子資源:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=29394537click for full text (PQDT)
ISBN:	9798351451565

Modeling Deep Ocean Water and Sediment Dynamics in the Eastern Pacific Ocean Using Actinium-227 and Other Naturally Occurring Radioisotopes.
Kemnitz, Nathaniel James.

Modeling Deep Ocean Water and Sediment Dynamics in the Eastern Pacific Ocean Using Actinium-227 and Other Naturally Occurring Radioisotopes. - 1 online resource (251 pages)

Source: Dissertations Abstracts International, Volume: 84-04, Section: B.

Thesis (Ph.D.)--University of Southern California, 2022.

Includes bibliographical references

The disequilibrium that occurs within the U and Th-decay series in marine environments can be exploited to give rates of transport of dynamic processes. To provide the most accurate transport rates, the radiotracer should have the following characteristics: (1) the radiotracer should be easily sampled and measured; (2) the source function and geochemical behavior of the radiotracer should be well understood; (3) the radiotracer's half-life should be on the same order as the process being investigated. This dissertation will utilize the distribution of 227Ac on the basin-scale range in the North- and Southeast Pacific in order to provide rates of transport of other solutes. The half-life of 227Ac (t1/2=21.8 y) is well suited for this scale and its geochemical behavior has been well-studied throughout the last few decades. The GEOTRACES program protocol for sampling and measuring Ac and Ra in the water column has led to multiple 227Ac profiles throughout the world's oceans. This dissertation will utilize U and Th-decay series equilibria to establish 227Ac bottom fluxes that can provide insight into transport rates and circulation pathways in the Northeastern Pacific Ocean.227Ac is directly produced by decay of 231Pa (t1/2=32.8 ky). 231Pa is scavenged from the water column by falling particulates that carry it to sediments, where it decays to its more soluble daughter. A fraction of this 227Ac diffuses out of deep-sea sediments and is transported vertically and horizontally as it decays in the water column. The water column distribution of excess 227Ac (227Acex) was measured along the U.S. GEOTRACES Meridional Transect (GP15) from Alaska to Tahiti in fall 2018. To constrain its benthic input, cores from 5 stations near the northern half of the GP15 transect were collected in the summer of 2017 (C-Disk-IV transect stations 23-50°N). The GP15 transect parallels the C-Disk-IV cruise track in the Northeast Pacific, offset by a few hundred km.Five sediment cores along the C-Disk-IV transect were measured and modeled with the objective of characterizing the behavior of 227Ac, 228Ra, and 226Ra and their fluxes into the overlying water column. Solid phase profiles of these isotopes were measured, and reaction-transport models were applied that incorporate effects of molecular diffusion, bioturbation, sedimentation, distribution coefficient (kd), and fraction of each isotope released to pore water by parent decay (called F). Good fits to the 226Ra profiles showed F values of 57-83% and sedimentation rates of 0.10 - 0.40 cm kyr-1 for C-Disk-IV sediments. The 228Ra profiles were difficult to measure due to high counting uncertainties, but F values obtained from 228Ra profiles were similar to 226Ra values. Most solid phase 226Ra profiles showed a large deficiency compared to 230Th in the upper 15cm of sediments, while the 228Ra profiles showed a modest deficiency relative to its 232Th parent in the top 3cm of sediments. F values for 227Ac had a much larger range than the Ra isotopes, ranging from 5 - 94% for C-Disk-IV sediments. About half of the 227Ac profiles showed a large deficiency relative to 231Pa in the upper few cm of sediments, while the other half showed a very small deficiency. Sediment composition, loss of surficial material, non-steady state behavior, and non-local bioturbation transport of sediments might explain the discrepancy between the two types of 227Ac profiles. It is noteworthy that 230Th budgets indicate significant sediment winnowing at sites with low F values, perhaps indicating that exhumation of formerly buried sediment plays some role.Two independent approaches were used to quantify the source function of 227Ac and 228Ra in the Northeast Pacific: (1) use of solid phase profiles with a reaction-transport model, as well as integrated downcore daughter-parent deficiency; and (2) direct measurement of fluxes based on core incubation. The two independent methods agree within uncertainty, and the average 227Ac and 228Ra sediment fluxes for the Northeast Pacific are 90 ± 20 and 600 ± 200 dpm m-2 yr-1. The 226Ra sediment flux was only determined by the former approach, and the flux calculated in this study is similar to previous work in the North Pacific. The average sediment flux for 226Ra along the C-Disk-IV cruise is 1300 ± 10 dpm m-2-yr-1, which is over 2x higher than the water column inventory of 226Ra in this region (600 dpm m-2-yr-1). 227Ac fluxes for the southern half of the GP15 transect were calculated by estimating F and using 231Pa measurements in the upper few cm of sediments.Profiles of 227Ac and 231Pa were measured and modeled in the water column along the GP15 transect. Along the GP15 transect, 227Ac and 231Pa are typically near equilibrium between 0-3000m depths, and below this horizon, 227Ac is often in excess over its parent. Excess 227Ac (227Acex) generally increases in activity with increasing depth and the highest concentrations of 227Acex are contained within the bottom 1000m. The highest concentrations of 227Acex in the Eastern Pacific are found near the center of the Northeast Pacific Basin (NEPB) and south of 10˚S. These areas are dominated by low sedimentation and high 231Pa activity in sediments. Along the southern leg of the GP15 transect (Sta.19 - 37), some elevated activities of 227Acex are found at mid-depths (~2600m). These areas appear to be influenced by hydrothermal activity from the East Pacific Rise (EPR), due to the proximity of δ3He anomalies and 227Acex activities at those depths, although maxima in the two tracers are not perfectly coincident.Three types of 227Acex profiles were found in the water column along the GP15 transect: (1) an expected, exponential decrease of 227Acex away from the seafloor; (2) a well-mixed 500m thick bottom layer with very little 227Acex above; (3) and lastly, an unexpected, erratic distribution of 227Acex that has local maxima in 227Acex overlying bottom waters of lower concentration. The first type of 227Acex profile is found toward the northern end of the GP15 transect (Sta.6 - 10). These profiles can be generated if water circulation is flowing along a constant depth, where the 227Ac bottom source is constant. If vertical diffusion of 227Acex is constant, it should produce an 227Acex distribution that is exponentially decreasing away from the seafloor due to the combination of diffusion and radioactive decay. This is the most ideal profile and can be used to find apparent vertical eddy diffusivity rates (Kz), if the bottom is flat, or inclined with a constant slope. The apparent vertical diffusivity will be affected by both diapycnal and isopycnal transport, in situations where the isopycnals and/or the bottom topography are inclined.The second type of 227Acex profile found along the GP15 transect shows nearly constant 227Acex activities within the bottom few hundred meters. These profiles are found in the middle of the Northeast Pacific (30˚ - 40˚N) and indicate that the bottom ~500 meters are rapidly mixed, reflecting the density structure. Above this benthic layer, little 227Acex is found, indicating low vertical transport. Profiles in the southern half of the GP15 transect show the 3rd type of 227Acex profile: an erratic distribution of 227Acex in the bottom few hundred meters that correlates with the depth distribution of regional topographic features.

Electronic reproduction.
Ann Arbor, Mich. :
ProQuest,
2023

Mode of access: World Wide Web

ISBN: 9798351451565Subjects--Topical Terms:

516570
Geology.
Subjects--Index Terms:

210PbIndex Terms--Genre/Form:

542853
Electronic books.

Modeling Deep Ocean Water and Sediment Dynamics in the Eastern Pacific Ocean Using Actinium-227 and Other Naturally Occurring Radioisotopes.
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245 1 0 $a Modeling Deep Ocean Water and Sediment Dynamics in the Eastern Pacific Ocean Using Actinium-227 and Other Naturally Occurring Radioisotopes.
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500 $a Source: Dissertations Abstracts International, Volume: 84-04, Section: B.
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502 $a Thesis (Ph.D.)--University of Southern California, 2022.
504 $a Includes bibliographical references
520 $a The disequilibrium that occurs within the U and Th-decay series in marine environments can be exploited to give rates of transport of dynamic processes. To provide the most accurate transport rates, the radiotracer should have the following characteristics: (1) the radiotracer should be easily sampled and measured; (2) the source function and geochemical behavior of the radiotracer should be well understood; (3) the radiotracer's half-life should be on the same order as the process being investigated. This dissertation will utilize the distribution of 227Ac on the basin-scale range in the North- and Southeast Pacific in order to provide rates of transport of other solutes. The half-life of 227Ac (t1/2=21.8 y) is well suited for this scale and its geochemical behavior has been well-studied throughout the last few decades. The GEOTRACES program protocol for sampling and measuring Ac and Ra in the water column has led to multiple 227Ac profiles throughout the world's oceans. This dissertation will utilize U and Th-decay series equilibria to establish 227Ac bottom fluxes that can provide insight into transport rates and circulation pathways in the Northeastern Pacific Ocean.227Ac is directly produced by decay of 231Pa (t1/2=32.8 ky). 231Pa is scavenged from the water column by falling particulates that carry it to sediments, where it decays to its more soluble daughter. A fraction of this 227Ac diffuses out of deep-sea sediments and is transported vertically and horizontally as it decays in the water column. The water column distribution of excess 227Ac (227Acex) was measured along the U.S. GEOTRACES Meridional Transect (GP15) from Alaska to Tahiti in fall 2018. To constrain its benthic input, cores from 5 stations near the northern half of the GP15 transect were collected in the summer of 2017 (C-Disk-IV transect stations 23-50°N). The GP15 transect parallels the C-Disk-IV cruise track in the Northeast Pacific, offset by a few hundred km.Five sediment cores along the C-Disk-IV transect were measured and modeled with the objective of characterizing the behavior of 227Ac, 228Ra, and 226Ra and their fluxes into the overlying water column. Solid phase profiles of these isotopes were measured, and reaction-transport models were applied that incorporate effects of molecular diffusion, bioturbation, sedimentation, distribution coefficient (kd), and fraction of each isotope released to pore water by parent decay (called F). Good fits to the 226Ra profiles showed F values of 57-83% and sedimentation rates of 0.10 - 0.40 cm kyr-1 for C-Disk-IV sediments. The 228Ra profiles were difficult to measure due to high counting uncertainties, but F values obtained from 228Ra profiles were similar to 226Ra values. Most solid phase 226Ra profiles showed a large deficiency compared to 230Th in the upper 15cm of sediments, while the 228Ra profiles showed a modest deficiency relative to its 232Th parent in the top 3cm of sediments. F values for 227Ac had a much larger range than the Ra isotopes, ranging from 5 - 94% for C-Disk-IV sediments. About half of the 227Ac profiles showed a large deficiency relative to 231Pa in the upper few cm of sediments, while the other half showed a very small deficiency. Sediment composition, loss of surficial material, non-steady state behavior, and non-local bioturbation transport of sediments might explain the discrepancy between the two types of 227Ac profiles. It is noteworthy that 230Th budgets indicate significant sediment winnowing at sites with low F values, perhaps indicating that exhumation of formerly buried sediment plays some role.Two independent approaches were used to quantify the source function of 227Ac and 228Ra in the Northeast Pacific: (1) use of solid phase profiles with a reaction-transport model, as well as integrated downcore daughter-parent deficiency; and (2) direct measurement of fluxes based on core incubation. The two independent methods agree within uncertainty, and the average 227Ac and 228Ra sediment fluxes for the Northeast Pacific are 90 ± 20 and 600 ± 200 dpm m-2 yr-1. The 226Ra sediment flux was only determined by the former approach, and the flux calculated in this study is similar to previous work in the North Pacific. The average sediment flux for 226Ra along the C-Disk-IV cruise is 1300 ± 10 dpm m-2-yr-1, which is over 2x higher than the water column inventory of 226Ra in this region (600 dpm m-2-yr-1). 227Ac fluxes for the southern half of the GP15 transect were calculated by estimating F and using 231Pa measurements in the upper few cm of sediments.Profiles of 227Ac and 231Pa were measured and modeled in the water column along the GP15 transect. Along the GP15 transect, 227Ac and 231Pa are typically near equilibrium between 0-3000m depths, and below this horizon, 227Ac is often in excess over its parent. Excess 227Ac (227Acex) generally increases in activity with increasing depth and the highest concentrations of 227Acex are contained within the bottom 1000m. The highest concentrations of 227Acex in the Eastern Pacific are found near the center of the Northeast Pacific Basin (NEPB) and south of 10˚S. These areas are dominated by low sedimentation and high 231Pa activity in sediments. Along the southern leg of the GP15 transect (Sta.19 - 37), some elevated activities of 227Acex are found at mid-depths (~2600m). These areas appear to be influenced by hydrothermal activity from the East Pacific Rise (EPR), due to the proximity of δ3He anomalies and 227Acex activities at those depths, although maxima in the two tracers are not perfectly coincident.Three types of 227Acex profiles were found in the water column along the GP15 transect: (1) an expected, exponential decrease of 227Acex away from the seafloor; (2) a well-mixed 500m thick bottom layer with very little 227Acex above; (3) and lastly, an unexpected, erratic distribution of 227Acex that has local maxima in 227Acex overlying bottom waters of lower concentration. The first type of 227Acex profile is found toward the northern end of the GP15 transect (Sta.6 - 10). These profiles can be generated if water circulation is flowing along a constant depth, where the 227Ac bottom source is constant. If vertical diffusion of 227Acex is constant, it should produce an 227Acex distribution that is exponentially decreasing away from the seafloor due to the combination of diffusion and radioactive decay. This is the most ideal profile and can be used to find apparent vertical eddy diffusivity rates (Kz), if the bottom is flat, or inclined with a constant slope. The apparent vertical diffusivity will be affected by both diapycnal and isopycnal transport, in situations where the isopycnals and/or the bottom topography are inclined.The second type of 227Acex profile found along the GP15 transect shows nearly constant 227Acex activities within the bottom few hundred meters. These profiles are found in the middle of the Northeast Pacific (30˚ - 40˚N) and indicate that the bottom ~500 meters are rapidly mixed, reflecting the density structure. Above this benthic layer, little 227Acex is found, indicating low vertical transport. Profiles in the southern half of the GP15 transect show the 3rd type of 227Acex profile: an erratic distribution of 227Acex in the bottom few hundred meters that correlates with the depth distribution of regional topographic features.
520 $a The irregularity appears to reflect high density stratification, coupled with inputs from irregular topography in the Southeast Pacific that produces localized maxima in the 227Acex source function at multiple depths, which then travels horizontally along isopycnals, mimicking the complex source function.Horizontal advection of 227Acex is significant in some parts of the transect, as shown by comparing the integrated decay of 227Acex in the water column to the benthic source: between 40˚ - 30˚N and 10˚ - 0˚N, water column decay is smaller than benthic input and south of 10˚S, it is larger than benthic input. Areas where horizontal advection does not produce a significant effect are: north of 40°N, between 30˚ - 10˚N, and near the equator. In these areas, water column decay is comparable to benthic input. This pattern is consistent with predictions from an inverse model in the Northeast Pacific (Hautala, 2018) that indicates deep-water advection is strongest between 40˚ - 30˚N and circulation is moving in a west-east direction (along 150˚W). North of 40˚N, the model suggests that circulation is moving in a north-south direction.The last chapter of thesis focuses on sedimentary dynamics in the Santa Monica Basin (SMB) during the last 250 years, with an emphasis on the last 40 years. Mass accumulation rates (MAR) for the deepest and lowest oxygen-containing parts of the SMB basin have been remarkably consistent during the past century, averaging 17.1 ± 0.6 mg cm-2 yr-1. However, MAR were slower prior to ~ 1900 CE (~10.5 mg cm-2 yr-1). The increase in sedimentation rate towards the recent occurs at about the time previous studies predicted an increase in siltation and the demise of a shelly shelf benthic fauna on the SMB shelf. The post-1900 CE constancy of sedimentation through a period of massive urbanization in Los Angeles is surprising.
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