Caption title; Includes bibliographical references
VII. SUMMARY A. Important results from the 1989 and 1990 field seasons The Panel is impressed by the effort made by Collier, Dymond and their coworkers to determine if hydrothermal inputs are present in Crater Lake. The data has been primarily descriptive and geochemical in nature, reflecting the expertise of the researchers and limitations of submersible and ROV research. Their work supports the following conclusions: 1. The Panel agrees that a slightly warmer and more saline water (SHEF) is entering the lake bottom. 2. The SHEF strongly influence the major element geochemistry and environmental isotope compositions of the lake waters. 3. The SHEF fluid "venting" is associated with interesting features. The bacterial mats are particularly unusual and fascinating. Descriptive evidence indicates that these mats are dynamic features. The presence of pools of relatively high density enhanced salinity water on the lake bottom is also an interesting observation. Descriptive evidence indicates that these pools form from the sinking and transport of fluids from vents or seep zones, some of which find their immediate source under rocks. 4. The work described in the 1990 report helps establish that SHEF includes a "magmatic" component, and that the He isotope composition is dominated by a mantle or magmatic source leaking into the lake. 5. The Panel notes the discovery of water with temperature in excess of 15 °C, which is appreciably higher than regional meteoric water, that is entering the lake near the bottom. It should also be noted that a reasonable geotherm within a Holocene volcanic edifice could reach these temperatures at a depth of 0.5 km. 6. The discovery of the Palisades Point features is important because it demonstrates the SHEF are not restricted to the Chaski Slide portion of the Detailed Study Area. However, water column measurements indicate that the greatest influx of fluids occurs in the South Basin, which tends to support the decision to utilize most of the submersible vehicle observation time within that basin. 7. The discovery of the siliceous spires at Skell Head indicates that influx of higher temperature buoyant fluids has occurred on the lake bottom sometime 35 in the past. The time when this influx occurred cannot be established from observations available up to the present. 8. There is evidence that the SHEF fluids are mixed in the bottom layer and that advection into the East Basin occurs at the depth of the sill between the South and East Basins. 9. The freon data reported provide the most sensitive indicators currently available of the time scale of deep water ventilation and establish the deep water renewal time to be about two years, assuming a steady-state vertical mixing process. This finding is a critical new result which helps constrain the magnitude of chemical fluxes from SHEF fluids into the deep waters of the lake, averaged over the mean vertical mixing time. 10. The 2"Rn activities observed in samples of deep water clearly establish the depth and general location at which the SHEF fluids are delivered to the deep waters of the lake. The distribution of this tracer in the lake water provides unequivocal evidence of influx of high "^Rn fluids to the deep waters of the lake at the time of sampling during August 1989. B. Other comments While the above results and conclusions are both interesting and important there remain some uncertainties about the origin and characteristics of the SHEF inputs to the deep waters of Crater Lake. These uncertainties include: 1. The role of the SHEF fluids in the mixing of Crater Lake cannot be defined at the present time. Mixing dynamics are not well established from the available data, and improved sampling is recommended as part of any future monitoring program. Nitrate levels suggest that the lake is not completely mixed to its deepest level. 2. The nature of the system that supplies SHEF fluids to the lake bottom is \/ery poorly defined. The size of the reservoir and the maximum temperatures in the reservoir are not constrained by the present data. The possibility that Crater Lake is underlain by a large high-temperature system still remains to be proved or refuted. Oxygen isotope work would be very helpful, as well as more reliable heat flow data, in resolving this issue. One Panel member believes that the work reported has established that the SHEF fluids form from reactions of lake or spring water with hydrothermally-altered volcanic rocks. This alteration may have occurred during earlier volcanic episodes. He argues 36 that the SHEF water chemistry is not consistent with being derived from a moderate or high-temperature hydrothermal system. 3. Arguments that the siliceous spires at Skell Head (which strongly indicate high temperature fluid input) are "recent" features are not conclusively supported by existing evidence. The submersible did not make a thorough study of the features, and the video tape shows the features to appear broken up. Without better knowledge of water movement, the lack of sediments is not conclusive evidence of recent activity. 4. The nature of the bacterial mats is still unknown despite recommendations from the previous Panel. The Panel recognized the investigators' efforts to secure volunteer help in this area; still, very little quantitative information about mat growth or metabolic rates has been obtained. 5. A geological model of the hydrothermal system cannot be made. The hypothesis that SHEF fluids enter the lake along the ring fractures that bound the caldera remains largely untested. We appreciate the limitations of submersible or ROV observations in collecting structural information. Even obtaining the strike of features is very difficult. However, until such data is obtained, the geological context of the SHEF fluids remains unknown. 37 EXECUTIVE SUMMARY The Panel has discussed in great detail exactly what warm, slightly-saline inflows to Crater Lake should be called, and settled on "Salinity- and Heat-Enriched Fluids", or SHEF. We considered using "hydrothermal water", the American Geological Institute's definition of which is "subsurface water whose temperature is high enough to make it geologically or hydrologically significant, whether or not it is hotter than the rock containing it", and decided to avoid the strict implication of this definition. The Panel also considered using either SELTHF ("salinity-enriched low temperature hydrothermal fluids") or SAM ("slightly-warmer and more-saline water"), and finally settled on SHEF as a good and understandable compromise. Some concern has been expressed in the limnology section of the Panel's report on the validity of the original assumption that the lake has actually been losing transparency. A close examination of the available Secchi data suggests that there may not have been a significant loss. Evidence from sedimentary profiles suggests that phytoplankton productivity is correlated with hydrothermal fluid inputs on a 100-1000 year time scale. But in shorter time scales, anthropogenic influences may be much greater than hydrothermal ones. Refinement of the nitrogen budget, as noted by the previous Panel, remains one of the most important research needs for understanding Crater Lake's water quality. Geochemical information presented by Dymond and Collier has shown that an input of SHEF occurs into the deep waters of the lake, particularly in the South Basin. These fluids are primarily sodium/magnesium/calcium bicarbonate waters with a moderate amount of sulfate and chloride, with total dissolved solids (TDS) of about 5 to 7 times higher concentration than the 90 mg/liter TDS of the lake water, which is a very dilute, near-neutral NaCl-S04, water. These SHEF waters are also characterized by enhanced levels of 3He, 222Rn, and reduced iron, but their TDS contents are orders of magnitude lower than those typical for geothermal fluids from a wide variety of environments. Because of their dilute nature and unusual chemistry, caution is necessary in using geochemical geothermometers on these waters, although some evidence suggests that the original temperatures may have been above 19 °C, the highest measured SHEF water temperature. We have no knowledge of the subterranean fluid pathway and, therefore, can only speculate on the depth of origin of the i fluids and the temperature of the interaction. Of importance, however, is the observation that SHEF fluids are a major contributor to the salt balance of this lake, with the heat balance being affected only locally. The geochemical evidence agrees with the postulate that the chemistry of the SHEF fluids is affected by water-rock interaction at some elevated, although unspecified, temperatures* The freon data reported provide the most sensitive indicators currently available of the time scale of deep water ventilation and establish the deep water renewal time to be about two years, assuming a steady-state vertical mixing process. This finding is a critical new result which helps constrain the magnitude of chemical fluxes from SHEF fluids into the deep waters of the lake, averaged over the mean vertical mixing time. The 222Rn act ivities observed in samples of deep water clearly establish the depth and general location at which the SHEF fluids are delivered to the deep waters of the lake. The distribution of this tracer in the lake water provides unequivocal evidence of influx of high 2"Rn fluids to the deep waters of the lake at the time of sampling during August 1989. With regard to the microbiological component, the Panel feels that the discovery of the microbial mat communities in Crater Lake was a major finding and may, if followed up, lead to an appreciation of several aspects of biogeochemical cycling in Crater Lake, as well as making possible its comparison with other lacustrine and oceanic environments. Considerable laboratory and fieldwork will be required before the most positive aspects of the microbiology can be realized* Future work should be done in close collaboration with microbiologists and biogeochemists familiar with the biology and biochemistry of iron cycling. In retrospect, one problem with the report was that its experimental design was limited to consideration of only part of the important relevant issues. Answering the overriding question, "Are there hydrothermal inputs to the lake which are significant in influencing its transparency or its ecology?", breaks down into answering the following more specific questions: (a) "What is the nature and size of detectable or probable inputs of hotter water into the lake?"; (b) "Is the chemical budget from such hotter sources significant in affecting the lake's clarity or ecology?"; and (c) "Is-the thermal budget from such sources significant in affecting the clarity and ecology?". At the outset, it seems that the investigators made the implicit ii assumption that finding hotter water inputs of any level of intensity would be enough to show that they were significant, i.e., they are defined as hydrothermal. Thus, attention was focused almost entirely on question (a), above, so that questions (b) and (c) to some extent still remain unanswered. The research to date has laid much of the groundwork for a better understanding of Crater Lake, and the Panel has been convinced of the input of SHEF to the lake bottom and its significance for lake chemistry. It remains for future studies to determine just how important these inputs are to the dynamics of biological and other processes related to water clarity in Crater Lake. A great effort was made by the principal investigators despite the limited funding available for a research effort which included expensive submersible time. iii
VII. SUMMARY A. Important results from the 1989 and 1990 field seasons The Panel is impressed by the effort made by Collier, Dymond and their coworkers to determine if hydrothermal inputs are present in Crater Lake. The data has been primarily descriptive and geochemical in nature, reflecting the expertise of the researchers and limitations of submersible and ROV research. Their work supports the following conclusions: 1. The Panel agrees that a slightly warmer and more saline water (SHEF) is entering the lake bottom. 2. The SHEF strongly influence the major element geochemistry and environmental isotope compositions of the lake waters. 3. The SHEF fluid "venting" is associated with interesting features. The bacterial mats are particularly unusual and fascinating. Descriptive evidence indicates that these mats are dynamic features. The presence of pools of relatively high density enhanced salinity water on the lake bottom is also an interesting observation. Descriptive evidence indicates that these pools form from the sinking and transport of fluids from vents or seep zones, some of which find their immediate source under rocks. 4. The work described in the 1990 report helps establish that SHEF includes a "magmatic" component, and that the He isotope composition is dominated by a mantle or magmatic source leaking into the lake. 5. The Panel notes the discovery of water with temperature in excess of 15 °C, which is appreciably higher than regional meteoric water, that is entering the lake near the bottom. It should also be noted that a reasonable geotherm within a Holocene volcanic edifice could reach these temperatures at a depth of 0.5 km. 6. The discovery of the Palisades Point features is important because it demonstrates the SHEF are not restricted to the Chaski Slide portion of the Detailed Study Area. However, water column measurements indicate that the greatest influx of fluids occurs in the South Basin, which tends to support the decision to utilize most of the submersible vehicle observation time within that basin. 7. The discovery of the siliceous spires at Skell Head indicates that influx of higher temperature buoyant fluids has occurred on the lake bottom sometime 35 in the past. The time when this influx occurred cannot be established from observations available up to the present. 8. There is evidence that the SHEF fluids are mixed in the bottom layer and that advection into the East Basin occurs at the depth of the sill between the South and East Basins. 9. The freon data reported provide the most sensitive indicators currently available of the time scale of deep water ventilation and establish the deep water renewal time to be about two years, assuming a steady-state vertical mixing process. This finding is a critical new result which helps constrain the magnitude of chemical fluxes from SHEF fluids into the deep waters of the lake, averaged over the mean vertical mixing time. 10. The 2"Rn activities observed in samples of deep water clearly establish the depth and general location at which the SHEF fluids are delivered to the deep waters of the lake. The distribution of this tracer in the lake water provides unequivocal evidence of influx of high "^Rn fluids to the deep waters of the lake at the time of sampling during August 1989. B. Other comments While the above results and conclusions are both interesting and important there remain some uncertainties about the origin and characteristics of the SHEF inputs to the deep waters of Crater Lake. These uncertainties include: 1. The role of the SHEF fluids in the mixing of Crater Lake cannot be defined at the present time. Mixing dynamics are not well established from the available data, and improved sampling is recommended as part of any future monitoring program. Nitrate levels suggest that the lake is not completely mixed to its deepest level. 2. The nature of the system that supplies SHEF fluids to the lake bottom is \/ery poorly defined. The size of the reservoir and the maximum temperatures in the reservoir are not constrained by the present data. The possibility that Crater Lake is underlain by a large high-temperature system still remains to be proved or refuted. Oxygen isotope work would be very helpful, as well as more reliable heat flow data, in resolving this issue. One Panel member believes that the work reported has established that the SHEF fluids form from reactions of lake or spring water with hydrothermally-altered volcanic rocks. This alteration may have occurred during earlier volcanic episodes. He argues 36 that the SHEF water chemistry is not consistent with being derived from a moderate or high-temperature hydrothermal system. 3. Arguments that the siliceous spires at Skell Head (which strongly indicate high temperature fluid input) are "recent" features are not conclusively supported by existing evidence. The submersible did not make a thorough study of the features, and the video tape shows the features to appear broken up. Without better knowledge of water movement, the lack of sediments is not conclusive evidence of recent activity. 4. The nature of the bacterial mats is still unknown despite recommendations from the previous Panel. The Panel recognized the investigators' efforts to secure volunteer help in this area; still, very little quantitative information about mat growth or metabolic rates has been obtained. 5. A geological model of the hydrothermal system cannot be made. The hypothesis that SHEF fluids enter the lake along the ring fractures that bound the caldera remains largely untested. We appreciate the limitations of submersible or ROV observations in collecting structural information. Even obtaining the strike of features is very difficult. However, until such data is obtained, the geological context of the SHEF fluids remains unknown. 37 EXECUTIVE SUMMARY The Panel has discussed in great detail exactly what warm, slightly-saline inflows to Crater Lake should be called, and settled on "Salinity- and Heat-Enriched Fluids", or SHEF. We considered using "hydrothermal water", the American Geological Institute's definition of which is "subsurface water whose temperature is high enough to make it geologically or hydrologically significant, whether or not it is hotter than the rock containing it", and decided to avoid the strict implication of this definition. The Panel also considered using either SELTHF ("salinity-enriched low temperature hydrothermal fluids") or SAM ("slightly-warmer and more-saline water"), and finally settled on SHEF as a good and understandable compromise. Some concern has been expressed in the limnology section of the Panel's report on the validity of the original assumption that the lake has actually been losing transparency. A close examination of the available Secchi data suggests that there may not have been a significant loss. Evidence from sedimentary profiles suggests that phytoplankton productivity is correlated with hydrothermal fluid inputs on a 100-1000 year time scale. But in shorter time scales, anthropogenic influences may be much greater than hydrothermal ones. Refinement of the nitrogen budget, as noted by the previous Panel, remains one of the most important research needs for understanding Crater Lake's water quality. Geochemical information presented by Dymond and Collier has shown that an input of SHEF occurs into the deep waters of the lake, particularly in the South Basin. These fluids are primarily sodium/magnesium/calcium bicarbonate waters with a moderate amount of sulfate and chloride, with total dissolved solids (TDS) of about 5 to 7 times higher concentration than the 90 mg/liter TDS of the lake water, which is a very dilute, near-neutral NaCl-S04, water. These SHEF waters are also characterized by enhanced levels of 3He, 222Rn, and reduced iron, but their TDS contents are orders of magnitude lower than those typical for geothermal fluids from a wide variety of environments. Because of their dilute nature and unusual chemistry, caution is necessary in using geochemical geothermometers on these waters, although some evidence suggests that the original temperatures may have been above 19 °C, the highest measured SHEF water temperature. We have no knowledge of the subterranean fluid pathway and, therefore, can only speculate on the depth of origin of the i fluids and the temperature of the interaction. Of importance, however, is the observation that SHEF fluids are a major contributor to the salt balance of this lake, with the heat balance being affected only locally. The geochemical evidence agrees with the postulate that the chemistry of the SHEF fluids is affected by water-rock interaction at some elevated, although unspecified, temperatures* The freon data reported provide the most sensitive indicators currently available of the time scale of deep water ventilation and establish the deep water renewal time to be about two years, assuming a steady-state vertical mixing process. This finding is a critical new result which helps constrain the magnitude of chemical fluxes from SHEF fluids into the deep waters of the lake, averaged over the mean vertical mixing time. The 222Rn act ivities observed in samples of deep water clearly establish the depth and general location at which the SHEF fluids are delivered to the deep waters of the lake. The distribution of this tracer in the lake water provides unequivocal evidence of influx of high 2"Rn fluids to the deep waters of the lake at the time of sampling during August 1989. With regard to the microbiological component, the Panel feels that the discovery of the microbial mat communities in Crater Lake was a major finding and may, if followed up, lead to an appreciation of several aspects of biogeochemical cycling in Crater Lake, as well as making possible its comparison with other lacustrine and oceanic environments. Considerable laboratory and fieldwork will be required before the most positive aspects of the microbiology can be realized* Future work should be done in close collaboration with microbiologists and biogeochemists familiar with the biology and biochemistry of iron cycling. In retrospect, one problem with the report was that its experimental design was limited to consideration of only part of the important relevant issues. Answering the overriding question, "Are there hydrothermal inputs to the lake which are significant in influencing its transparency or its ecology?", breaks down into answering the following more specific questions: (a) "What is the nature and size of detectable or probable inputs of hotter water into the lake?"; (b) "Is the chemical budget from such hotter sources significant in affecting the lake's clarity or ecology?"; and (c) "Is-the thermal budget from such sources significant in affecting the clarity and ecology?". At the outset, it seems that the investigators made the implicit ii assumption that finding hotter water inputs of any level of intensity would be enough to show that they were significant, i.e., they are defined as hydrothermal. Thus, attention was focused almost entirely on question (a), above, so that questions (b) and (c) to some extent still remain unanswered. The research to date has laid much of the groundwork for a better understanding of Crater Lake, and the Panel has been convinced of the input of SHEF to the lake bottom and its significance for lake chemistry. It remains for future studies to determine just how important these inputs are to the dynamics of biological and other processes related to water clarity in Crater Lake. A great effort was made by the principal investigators despite the limited funding available for a research effort which included expensive submersible time. iii