Evaluating Hydrothermal System Evolution Using Geochronological Dating and Biological Diversity Analyses
To elucidate the evolution of hydrothermal activities, we conducted an interdisciplinary study including geochemistry and biology to develop a method of obtaining reliable age information. As geochemical dating techniques, two methods applicable for hydrothermal ore minerals were developed and improved: electron spin resonance method and uranium–thorium disequilibrium method. Cross checks between the two methods generally showed good agreement for the range of hundreds to thousands of years. As biological analysis, the biodiversity among faunal communities in the targeted areas was analyzed at the species and DNA levels. Species and genetic diversity of the local fauna were not always correlated to geochemical dating, either in the southern Mariana Trough region or in the Okinawa Trough region. Although the results are not simple, comparison of age information obtained from analyses of these two disciplines potentially provides important constraints for discussion of the history and evolution of hydrothermal activities.
KeywordsBiodiversity ESR Geochronology Life-span of hydrothermal activity Mitochondrial mismatch analysis U-Th disequilibrium
Deep-sea hydrothermal systems have strongly attracted broad interest from science since their discovery in the late 1970s (e.g. Hessler and Kaharl 1995). The time scale for hydrothermal activity is an important factor to estimate the extent of hydrothermal deposits and the evolutionary process of chemosynthesis-based communities in submarine hydrothermal systems. The chemical flux from hydrothermal systems constrains the supply of substances fostering chemosynthesis-based communities. Therefore, flux intensities affect the biogeochemical reactions and the biological diversities in vent areas. The geochemical diversity observed in vent fluids can be elucidated as a result from scavenging process of the infiltrated seawater, frequently forming impressive chimney structures on the seafloor (e.g. Tivey 1995). Such infiltrated seawater, passing through the subseafloor environment, contains complex biogeochemical ingredients extracted from host formations up to the wider regional scale (e.g. Tsuji et al. 2012). The circulation of hydrothermal systems between seawater and the subseafloor environment has played an important role in the geochemical evolution of the earth. It has provided volatiles and metals that have sustained unique communities and ecosystems in hydrothermal areas from the beginning of Earth’s ecosystems (e.g. Rasmussen 2000).
The dating of divergence events and the evolutionary timescale from phylogenetic analysis remains difficult because the molecular clock to estimate a rate of molecular evolution can vary markedly over time and among species. Although a common calibration method of molecular clock is an examination of the fossil record (e.g. Cruzan and Templeton 2000; Smith et al. 2006; Weir and Schluter 2008), fossil evidence of hydrothermal vent communities is rare because of the destruction of vent fauna by hydrothermal activity itself. Other methods of calibrating the molecular clock in gene sequences of hydrothermal vent fauna might be geochronological dating techniques of hydrothermal ore minerals and geological events.
A few studies have been conducted to constrain the lifetime of activity at hydrothermal sites (e.g. Jamieson et al. 2013, and references therein). Such a small amount of constraint is partly attributable to the limited availabilities of numerical dating methods applied to the hydrothermal deposits at inactive sites directly: a gap of 102–103 years (Jamieson et al. 2013). One early trial of geochronology is reported by Lalou et al. (1985). They applied 210Pb–Pb and 230Th–234U disequilibrium method to sulfide deposits collected on and near the ridge axis. The determined ages were as high as 2,100 years. Another trial is on TAG-site, which develops slightly off the MAR-axis in 26°45′N (Rona 1984). You and Bickle (1998) reported precipitated age variation within the hydrothermally formed “mound” at the TAG-site, which exceeds 10 ka for core samples obtained in ODP Leg158. Some efforts to generalize these results for ordinary hydrothermal systems have been undertaken for the following reasons: (1) unusually large size of mounds within present-day hydrothermal mounds and (2) the slow spreading environment in which TAG is formed. The former point implies that the duration of the hydrothermalism beneath TAG is unusually long-lived or vigorous or both. Consequently, coupling of methodological challenges to produce reliable age information was conducted for the TAIGA project (Urabe et al. Chap. 1).
To conduct these studies, the following difficulties to be solved were recognized at the beginning of our project. The first is raised from the phylogeographic aspect. A dearth of precise information related to the molecular evolutionary rates (mutation rate) in genes might cause a discrepancy of interspecies or intraspecies phylogeography. Dispersal and transition events of vent fauna in hydrothermal system are affected strongly by many environmental constraints such as water depths, temperatures, deep-sea currents, and geochemical conditions. Not only calibration methods for such uncertainty but also investigations of life cycles for the vent endemic species must be conducted to produce better phylogeographic information. The latter must be pursued from a geochronological perspective. To obtain reliable numerical ages, a closed system is normally required. However, it is apparently easily disturbed for the samples of ore minerals in the vicinity of the hydrothermal vents. Near active vents, a non-steady and sometimes intermittent supply of vent fluid might occasionally replace the ore minerals once they have been formed. Further abrupt heating influenced by higher temperature fluids might break its closed system, causing a loss of accumulated daughter nuclei or erasure of radiation-related signals used in radiometric dating. These concerns are discussed in later sections.
5.2 Development of Dating Methods
5.2.1 Geochemical Approach for Ore Minerals
Here the ESR-dating is based on the accumulation of unpaired electron in the sample specimen induced by irradiation of high-energy particles and exposure to electromagnetic waves. Such irradiation mainly results from gamma-ray radiation resulting from the strong attenuation for charged particles in seawater. Consequently, the estimation of annual irradiation rate from ore deposits themselves is crucially important to calculate the irradiation age. Where the external radiation was effectively shielded by seawater, an approximation incorporating the internal radiation generated in and around ore precipitates is expected to be sufficient. In addition, 226Ra in barite of a black smoker is unsupported (decaying) because 230Th is negligible in black smokers as a result of its valence and charges, which considerably simplifies the determination of the ages of the target minerals. Here, Ba serves as a proximal stable isotope for Ra that has no stable isotopes of its own. In fact, 226Ra/Ba in chimney deposits will decrease as a result of radioactive decay of 226Ra after their formation. Within sulfate minerals, barite was chosen as the target mineral because the obtained signals are clear. To estimate the in-situ dose-rate in a deep-sea environment, we calibrated yields of the ESR signals in a known/measured dose-rate as reported by Toyoda et al. (2012). Such a dose rate includes α-radiation using an Al2O3-target. The target tube was deployed near an active vent site in the Mariana Trough and was left for 29 days. Subsequently, it was retrieved for calibration. Details were reported by Toyoda et al. (2012).
The obtained ESR ages from the TAIGA project have also been reported in this volume (Ishibashi et al. Chap. 23; Fujiwara et al. Chap. 34). The determined ages for the samples from the southern Mariana Trough were 1.5 × 102–5.3 × 103 years; those from the Okinawa Trough were 4.0–1.6 × 104 years.
The second one is the U–Th dating, which uses the deviation of radioactivity from the radioactive equilibrium. A well known characteristic of the radioactive decay series under secular equilibria is that the specific radioactivities of parent and daughter elements are equivalent. Before reaching this state, i.e. under transient equilibrium, the radioactivity of the daughter element is markedly less than that of the parent nuclei. During our project, a 234U–230Th system was adopted; respective half-lives 230Th and 234U are 7.6 × 104 years and 2.5 × 105 years. Here, thorium is less soluble in hydrothermal fluids and is not precipitated in hydrothermal minerals, although uranium is precipitated in these minerals. Therefore, 230Th in the mineral simply accumulates from 234U over time, which provides a criterion of the evaluation of data quality. In general, the direct determination is based on the radioactivity measurement, but the radioactivity of the targeted nuclei of the focused system is low because the half life of 238U, their ultimate parent, is much longer, >4.5 × 109 years. Consequently, direct measurements of the abundances of 234U and 230Th were applied in our TAIGA project, which required development of the precise quantification on trace amounts of both 230Th and of 234U. Because of the half-lives of our targeted parent–daughter pair, our method can be used to calculate the ages of minerals as young as 50 years when a sample contains sufficient uranium but little thorium. However, the upper limit of the range of this dating method is limited to 4.0 × 105 years, which is around eight times the half life of the daughter: 230Th.
Detailed protocols of U–Th dating were reported by Takamasa et al. (2013). We analyzed uranium and thorium isotope ratios in mineral specimens using a Multicollector-ICP-MS (IsoProbe; GV Instruments Ltd.). The powdered and sieved ore samples were separated electromagnetically using an isodynamic separator to concentrate pyrite and other sulfide minerals. Two mesh ranges were adopted for this study: 100–250 and 250–500 μm (see also Fujiwara et al. Chap. 34). Then, the chemically leached specimens were rinsed, dried, and decomposed using nitric acid. Before the ICP-MS measurements, uranium and thorium were separated from sample solutions under two steps of column chromatography. During this process, barium (Ba) and lead (Pb) were also separated carefully to suppress their interference of signals from targeted isotopes: 234U or 230Th.
In the Okinawa Trough, it is difficult to find such a simple relation with a geographical position of the vent area, e.g. with distance from the spreading axis. The Okinawa Trough is still in the early rifting stage before the normal seafloor/backarc spreading (Ishibashi et al. Chap. 29 and references therein). Therefore, no clear spreading center is apparent yet. The respective ranges of the determined ages of vent deposits are up to 11, 18, 144, 97, and 12 ka for Daiyon-Yonaguni Knoll, Hatoma Knoll, Hakurei-site of Izena-Hole, Iheya North Knoll, and Yoron-Hole (Fujiwara et al. Chap. 34). All ranges include one or two extremely old values, as discussed later. The longest duration of hydrothermal activity was estimated for Hakurei-site of Izena Hole, 16,000 years, even excluding one extreme value. The Izena Hole is a tectonically formed seafloor depression. It might be influenced by rather long activity of the Hakurei-site in the same order of magnitude as that of the TAG mound reported by You and Bickle (1998).
5.2.2 Comparison between ESR and U–Th ages
The selected age pairs determined using ESR and U-Th methods for samples from active hydrothermal fields of Southern Mariana Trough are presented in Fig. 5.3. The U-Th ages were obtained from sulfide minerals that had been sieved and magnetically separated (mostly pyrite and sphalerite). The ESR ages were from barite that had been concentrated from sulfide-rich breccia. Some sulfide samples, from the Archean site, are also analyzed using the ESR method for cross-checking. The ages obtained using the two methods mostly show a correlation in the diagram. Both ages are well matched down to the order of hundreds of years (~102 years). This time range is well within the range of the vent complex consisting of several feed paths in the particular field. Although the reason causing the discrepancy remains unclear, the ESR ages tend to be lower than U–Th ages in the case of Southern Mariana sulfides (Fig. 5.3). This phenomenon might be the result of the robustness of a closed-system of each method. Here ESR signals might be reset by thermal disturbances, whereas U–Th ages might be disturbed by the dissolution or the adsorption of targeted isotopes.
5.2.3 Ecological Analyses of Vent Fauna
The estimation for evolutionary processes of hydrothermal activities has been done using three biological features: (1) species diversity of vent fauna, (2) genetic diversity of a population, and (3) speciation events (Fig. 5.1). The species diversity fluctuates in the change of habitat conditions. In the case of hydrothermal systems, drastic ecological succession starts from a newly created vent site or a sterilized site after volcanic eruption (Govenar 2010; and references therein). The first majority of inhabitants at this primary stage includes thermophilic prokaryotes (Lutz et al. 1994; Shank et al. 1998). Planktonic larvae or adults of vent-specific faunal species migrate from other vent areas, settle on a suitable niche, and gradually make a large and complex-structured faunal aggregation with high diversity of species. Therefore, high species diversity was only observable in continuous vent fields. Species diversity of vents conveys the short-scale fluctuation of vent activity.
Once an organism settles in a suitable environment of the vent field, the demographic expansion of the fauna begins. The genetic diversity of a population should be correlated positively with its effective population size (number of individuals involved in reproduction), and it is regarded as tracer of past demographic expansion of a population (Rogers and Harpending 1992). Therefore, the genetic diversity of a population can be an estimator of the past population expansion after settlement of vent species, or formation of a vent environment that is suitable for the species. The temporal change of genetic variation can be examined using a molecular clock hypothesis, which has been used to estimate divergence times. The molecular clock hypothesis states that DNA and protein sequences evolve at a mutation rate, which is constant over time and among populations. For example, the partial sequence of mitochondrial cytochrome oxidase c subunit I (COI) gene, which is commonly used as a barcode of species, has been investigated. Reportedly 1.4 % sequence divergence per 106 years occurred on the gene sequence of snapping shrimp (Knowlton and Weigt 1998). However, the rates of accumulation of genetic variation are not the same among species (e.g. large animals have lower genetic diversity than small ones; Beebee and Rowe 2008), and the dating of vent activity with genetic diversity can only provide relative ages for many species.
The expanded population might be divided into two or more subpopulations either sympatrically or allopatrically. Without gene exchange or connectivity among the subpopulations, genetic differentiation accumulates and eventually causes speciation, which is an irreversible event. With calibration of the molecular clock, speciation ages of some vent animals have been estimated in million year order with geologic events (Shank et al. 1999; Johnson et al. 2006, 2010). Estimation of the speciation date of vent species provides the longest scale.
To investigate the relation between biodiversity described above and hydrothermal activity, we examine the dataset of species diversity as well as genetic diversity based on partial COI sequences of vent species in the southern Mariana Trough and the Okinawa Trough.
5.3 Comparisons Between Ecological and Geochemical Age Information
As a challenge to estimate the evolution history of hydrothermal system in the aspect of the relations between geochemical environment and ecological responses, a comparative analysis of the chronological information obtained both from geochemical dating and from ecological analyses was conducted.
Results obtained from two different approaches were not simply comparable even under a simple geographical setting in the southern Mariana Trough. From the geochemical data, the ages of the hydrothermal sites composed of multiple vents were fairly correlated with the distance from the present day's spreading center where the seafloor is newly formed, which implies that the durations of the faunal communities could be longer in the distant vent field from the spreading axis. However, that inference is not fully supported by the ecological approach. The vent endemic communities might be severely damaged or exterminated if the older vent field experienced later eruption or seafloor renewal by lava flows. Such is not the case of an observed reversal in the numbers of species between Archean and Urashima/Pika sites because such seafloor renewal is not implied at the Urashima/Pika site, but it is at the Archean site, as indicated by high magnetization (Fujii et al. 2013). Another complexity is apparent from the genetic diversity at the Snail site: it is as large as those at the other three sites (Fig. 5.7b, c). This point is partly supported by the rather old age, up to 7.2 ka, at the Snail site, although only one exceptional datum shows such an old age. In contrast, a phylogeographical approach suggests a 0.2–1.0 Ma history for populations in the area (Hidaka et al. Chap. 27). This discrepancy has yet to be investigated.
Furthermore, a complicated situation was observed in the Okinawa Trough. Bathyacmaea secunda as a vent endemic species showed greater genetic diversity at the Hakurei site in the Izena Hole, which was indicated as the number of mismatches of the COI sequences (Fig. 5.9b). It is apparently consistent with the longer life of the vents at the Hakurei site estimated as exceeding 16 ka from geochemical data. However, genetic diversities of the Shinkaicaris at Daiyon-Yonaguni Knoll were as great as those at Yoron Hole, although their ranges of geochronological ages were different. They were also considerably younger than results show for the Hakurei-site (Figs. 5.5 and 5.9a). This point might be explained by the high-larval dispersal potentials of the species, resulting in a higher rate of gene flow among local populations. In terms of the inter-species comparison, the trend in genetic diversities of local populations of two limpets Bathyacmaea secunda and Lepetodrilus nux differed. Higher genetic diversity was observed in the southern populations in Bathyacmaea secunda, but lower genetic diversity was observed in the same ranges of populations in Lepetodrilus nux (Fig. 5.9b, c). Ecological features of vent faunas are necessary for more precise estimation of biological and geological histories of vent fields.
Ecological information will help to interpret these results of phylogeographic analyses and will be useful to estimate the ages of hydrothermal vent sites in the Okinawa Trough. Even for the same vent sites, inhabitants show various levels of genetic diversity, which are expected to correspond to their ecological characteristics. Consequently, ecological information of respective species on their life-history strategies, including reproduction and larval dispersal ability, are necessary to interpret genetic diversity of faunal populations and for application to estimate the history of hydrothermal activity.
The duration of present-day hydrothermal activities estimated from geochemical dating of ore minerals has approximately up to a few tens of thousands of years as reliable maxima in the Okinawa Trough and in the Southern Mariana Trough as back-arc tectonic settings. This duration is apparently in the same order of magnitude of duration as the TAG mound activity under the slow spreading condition of Mid-Atlantic Ridges. The results of ecological analyses presented herein showed various genetic structures among species, although they inhabit the same vent fields. Whereas populations of some species simply increase their genetic diversity along with the duration of hydrothermal activity, those of others show no such relation. Consequently, we can expect to select adequate species for dating the activities based on the genetic population structure of vent-endemic species. Correspondence to results obtained using geochemical dating will offer a useful evaluation criterion for the selection of the adequate species.
All maps were tabulated using GMT software, a free mapping tool (Wessel and Smith 1998). This study was supported by the “TAIGA project,” which was funded by a Grant-in-Aid for Scientific Research on Innovative Areas (KAKENHI, No. 20109004) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The final stage of this study was also supported by Submarine Resources Research Project, JAMSTEC. Constructive comments by three reviewers, Prof. H. Hyodo of Okayama Univ. Sci. and Drs. K. Sato and H. Yamamoto of JAMSTEC, greatly improved this article.
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