Heat Flux and Hydrography at a Submarine Volcano: Observations
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Heat flux and hydrography at a submarine volcano: Observations and models of the Main Endeavour vent ¯eld in the northeast Paci¯c Scott R. Veirs A dissertation submitted in partial ful¯llment of the requirements for the degree of Doctor of Philosophy University of Washington 2003 Program Authorized to O®er Degree: School of Oceanography University of Washington Graduate School This is to certify that I have examined this copy of a doctoral dissertation by Scott R. Veirs and have found that it is complete and satisfactory in all respects, and that any and all revisions required by the ¯nal examining committee have been made. Chair of Supervisory Committee: Russell McDu® Reading Committee: Russell McDu® William Lavelle Je®rey Parsons Date: In presenting this dissertation in partial ful¯llment of the requirements for the Doctoral degree at the University of Washington, I agree that the Library shall make its copies freely available for inspection. I further agree that extensive copying of this dissertation is allowable only for scholarly purposes, consistent with \fair use" as prescribed in the U.S. Copyright Law. Requests for copying or reproduction of this dissertation may be referred to Bell and Howell Information and Learning, 300 North Zeeb Road, Ann Arbor, MI 48106- 1346, or to the author. Signature Date University of Washington Abstract Heat flux and hydrography at a submarine volcano: Observations and models of the Main Endeavour vent ¯eld in the northeast Paci¯c by Scott R. Veirs Chair of Supervisory Committee: Professor Russell McDu® Oceanography This dissertation discusses hydrothermal plume hydrography and heat flux measure- ment at the Main Endeavour hydrothermal vent ¯eld (MEF) on the Endeavour segment (Juan de Fuca ridge, northeast Paci¯c). Observations are from an underwater vehicle called Autonomous Benthic Explorer (ABE), a lowered CTD, and 2 current meter moorings. Chapter 1 contains motivating questions, terminology, plume theory, and a review of past heat flux measurements at the MEF. A new correction factor is derived relating source heat flux to plume heat flux calculated with isohaline temperature anomalies. Chapter 2 presents setting, methodology, and an analysis of hydrography and currents near the MEF. Hydrography varies on scales as short as 10{50 m and 10{60 min, and fluid is warmer on average than at the segment ends. Oscillatory currents change from mul- tidirectional above the ridge to rectilinear within the axial valley (Thomson et al. 2003). Northward mean flow of 2-5 cm/s within the valley is aligned with the rectilinear oscillations ( 5 cm/s amplitude), while southwestward mean flow of 5-10 cm/s above the ridge is only » intermittently aligned with the multidirectional oscillations ( 5 cm/s amplitude). » Heat flux is estimated within the axial valley (Chapter 3) and above the ridge (Chap- ter 4). In both cases, an advection/di®usion model simulates plume distributions and char- acterizes heat flux statistics. The mean horizontal heat flux within the axial valley through vertical control surfaces 0{100 m above bottom (mab) is 76 MW, based on warming north » relative to south of the MEF and the northward mean flow. The modeled standard deviation of this horizontal flux is 114 MW. The vertical heat flux in plumes rising through a hor- » izontal surface 100 mab (Stahr et al. 2003) is 643 116 MW. This vertical flux, previous » § source flux measurements, and the horizontal flux together imply that heat flux partitioning between focused and di®use sources is 6:1, contradicting the prevalent view that di®use » sources account for 90% of the heat flux at vent ¯elds. The net horizontal heat flux above the ridge through vertical control surfaces extending from 100{400 mab is 442 213 MW, » § consistent with the vertical flux. Past estimates of flux in plumes are higher because they are not net fluxes and likely include contributions from multiple vent ¯elds. TABLE OF CONTENTS List of Figures v List of Tables viii Glossary ix Chapter 1: Measuring Hydrothermal Power 1 1.1 Motivation . 2 1.2 De¯ning heat flux in a hydrothermal context . 5 1.2.1 Heat and flux . 6 1.2.2 The heat flux budget . 8 1.2.3 The source flux . 12 1.2.4 Entrainment . 13 1.2.5 A control volume that assesses dominant processes . 15 1.2.6 Temperature anomaly de¯nition . 19 1.2.7 Relating source and plume anomalies and fluxes in rising plumes . 20 1.2.8 General relationship between isohaline anomaly and source heat flux . 26 1.3 History of MEF heat flux measurement . 28 1.3.1 Heat flux measured in equilibrated MEF plumes . 28 Baker and Massoth 1987 . 28 Rosenberg et al. 1988 . 31 Thomson et al. 1992 . 34 1.3.2 Heat flux measured at MEF sources . 36 Schultz et al. 1992 . 36 Ginster et al. 1994 . 37 i Bemis et al. 1993 . 39 Alternative interpretation of the measurements of Bemis et al. 41 1.3.3 Synopsis . 43 Chapter 2: The Flow Mow Experiment 46 2.1 Setting . 48 2.1.1 Bathymetry . 51 2.1.2 Geology . 51 2.1.3 Hydrothermal activity . 54 2.2 Methodology . 58 2.3 Instrumentation . 59 2.4 Flow . 63 2.4.1 Regional overview . 65 2.4.2 Mean currents . 68 Mean flow above the ridge crests . 68 Vertical shear in the mean flow . 72 Mean flow within the axial valley . 74 2.4.3 Oscillatory currents . 79 2.4.4 Coherence and phase . 85 2.4.5 Synopsis . 88 2.5 Hydrography . 90 2.5.1 Regional overview . 90 2.5.2 Transition to geothermal hydrography . 92 2.5.3 Observations in and near buoyant hydrothermal plumes . 106 Chapter 3: Plumes and Heat Flux in the Valley { Rectilinear Flow 113 3.1 Calculated horizontal heat flux . 114 3.1.1 Observed ¢θ . 115 ABE surveys of north and south surfaces . 116 ii CTD surveys of north and south surfaces . 116 CTD time series north and south of the MEF . 120 Synopsis of spatial and temporal variability . 123 3.1.2 Horizontal heat flux estimation . 127 3.2 Modeled horizontal heat flux . 128 3.2.1 The pu® model . 128 3.2.2 Modeled variance and mean magnitudes . 131 3.2.3 Model implications for observed horizontal flux . 133 3.3 Discussion . 135 3.3.1 Sources of variability in MEF hydrography . 135 3.3.2 Corrections to horizontal and vertical heat flux estimates . 138 3.3.3 Partitioning of power . 139 Chapter 4: Plumes and Heat Flux Above the Ridge { Multidirectional Flow 141 4.1 Observations above the ridge crests . 142 4.1.1 Time series north and south of the MEF: 1800{2200 m . 143 4.1.2 North and south control surfaces: 1800{2200 m . 147 4.1.3 Consecutive surveys of the MEF perimeter: 1800{2070 m . 151 4.2 Calculated horizontal heat flux . 153 4.2.1 Mean net horizontal flux . 156 4.2.2 Quasi-synoptic horizontal flux . 159 4.3 Modeled horizontal heat flux . 161 4.3.1 Modeled variance in flow above ridge crests . 161 4.3.2 Plumes from other vent ¯elds . 165 4.4 Discussion . 167 4.4.1 Sources of hydrographic variability . 167 4.4.2 Comparison of vertical and horizontal heat flux . 169 iii Chapter 5: Conclusions and implications 171 5.1 Main observations . 171 5.1.1 General flow and hydrography . 171 Regional patterns . 171 Axial patterns . 172 Hydrography at the MEF and in rising plumes . 173 5.1.2 Plumes and heat flux within the axial valley . 173 5.1.3 Plumes and heat flux above the ridge . 174 5.2 Central issues and conclusions . 175 5.2.1 What form do plumes take in di®erent types of cross flow? . 175 MEF plume form and dispersal are controlled by advection . 175 Future work on plume formation and dispersal . 180 5.2.2 How can hydrothermal heat flux best be determined? . 181 Accurate estimation of net heat flux through a control volume . 182 Future work on heat flux . 187 5.3 Implications . 189 5.3.1 A heat budget for newly formed oceanic crust . 189 Bibliography 192 Appendix A: List of symbols 203 Appendix B: Conversion table for modi¯ed Julian dates 207 Appendix C: Supplementary material: CD-ROM 208 iv LIST OF FIGURES 1.1 Crustal schematic with energy fluxes . 8 1.2 Generic control volume and heat budget . 10 1.3 A ¯eld-scale control volume . 16 1.4 Modeled plume properties: θ, S, w, ½, and transport pro¯les . 22 1.5 Modeled plume properties: anomalies and fluxes . 24 1.6 Correction factors for heat flux based on temperature anomalies . 25 1.7 Schematic of the Baker and Massoth heat flux measurement . 30 1.8 Schematic of the Rosenberg et al. heat flux measurement . 33 1.9 Schematic of the Bemis et al. experiment . 42 2.1 Endeavour segment bathymetry and hydrothermal sources . 47 2.2 Axial valley relief and current shear relative to plume rise heights . 50 2.3 Bathymetry of the Endeavour segment . 52 2.4 Axis-parallel bathymetric cross-sections and current meter depths . 53 2.5 Bathymetric map of the MEF . 55 2.6 Geologic map of the MEF . 56 2.7 Photographs of CTD and ABE . 60 2.8 Current meter mooring locations and mean vectors . ..