Jennifer B. Paduan

Distribution of chemosynthetic biological communities in Monterey Bay, California

We report the first quantitative evaluation of the distribution of seafloor chemosynthetic biological communities on a regional scale. The results are based on the analysis of video images and navigation from 792 benthic remotely operated vehicle dives conducted on the continental margin in Monterey Bay, California. These communities are common, occurring within 5% of the 25-m-square grid cells within which there have been bottom observations within 45 km of the bay9s head and within 9% of the visited cells that are below 550 m water depth. Although it has been previously assumed that these communities are associated with fluid seepage from faults, they are not more common within known fault zones. Surprisingly, the communities in Monterey Bay occur preferentially on steep slopes, which are commonly sites of recent erosion.

Eruptive and tectonic history of the Endeavour Segment, Juan de Fuca Ridge, based on AUV mapping data and lava flow ages

High-resolution bathymetric surveys from autonomous underwater vehicles ABE and D. Allan B. were merged to create a coregistered map of 71.7 km2 of the Endeavour Segment of the Juan de Fuca Ridge. Radiocarbon dating of foraminifera in cores from three dives of remotely operated vehicle Doc Ricketts provide minimum eruption ages for 40 lava flows that are combined with the bathymetric data to outline the eruptive and tectonic history. The ages range from Modern to 10,700 marine-calibrated years before present (yr BP). During a robust magmatic phase from >10,700 yr BP to ∼4300 yr BP, flows erupted from an axial high and many flowed >5 km down the flanks; some partly buried adjacent valleys. Axial magma chambers (AMCs) may have been wider than today to supply dike intrusions over a 2 km wide axial zone. Summit Seamount formed by ∼4770 yr BP and was subsequently dismembered during a period of extension with little volcanism starting ∼4300 yr BP. This tectonic phase with only rare volcanic eruptions lasted until ∼2300 yr BP and may have resulted in near-solidification of the AMCs. The axial graben formed by crustal extension during this period of low magmatic activity. Infrequent eruptions occurred on the flanks between 2620–1760 yr BP and within the axial graben since ∼1750 yr BP. This most recent phase of limited volcanic and intense hydrothermal activity that began ∼2300 yr BP defines a hydrothermal phase of ridge development that coincides with the present-day 1 km wide AMCs and overlying hydrothermal vent fields.

Volcanic morphology of West Mata Volcano, NE Lau Basin, based on high‐resolution bathymetry and depth changes

High-resolution (1.5 m) mapping from the autonomous underwater vehicle (AUV) D. Allan B. of West Mata Volcano in the northern Lau Basin is used to identify the processes that construct and modify the volcano. The surface consists largely of volcaniclastic debris that forms smooth slopes to the NW and SE, with smaller lava flows forming gently sloping plateaus concentrated along the ENE and WSW rift zones, and more elongate flows radiating from the summit. Two active volcanic vents, Prometheus and Hades, are located ∼50 and ∼150 m WSW of the 1159 m summit, respectively, and are slightly NW of the ridgeline so the most abundant clastic deposits are emplaced on the NW flank. This eruptive activity and the location of vents appears to have been persistent for more than a decade, based on comparison of ship-based bathymetric surveys in 1996 and 2008–2010, which show positive depth changes up to 96 m on the summit and north flank of the volcano. The widespread distribution of clastic deposits downslope from the rift zones, as well as from the current vents, suggests that pyroclastic activity occurs at least as deep as 2200 m. The similar morphology of additional nearby volcanoes suggests that they too have abundant pyroclastic deposits.

Five million years of compositionally diverse, episodic volcanism: Construction of Davidson Seamount atop an abandoned spreading center

Davidson Seamount, a volcano located about 80 km off the central California coast, has a volume of ∼320 km3 and consists of a series of parallel ridges serrated with steep cones. Davidson was sampled and its morphology observed during 27 ROV Tiburon dives. During those dives, 286 samples of lava, volcaniclastite, and erratics from the continental margin were collected, with additional samples from one ROV-collected push core and four gravity cores. We report glass compositions for 99 samples and 40Ar-39Ar incremental heating age data for 20 of the samples. The glass analyses are of hawaiite (62%), mugearite (13%), alkalic basalt (9%), and tephrite (8%), with minor transitional basalt (2%), benmoreite (2%), and trachyandesite (2%). The lithologies are irregularly distributed in space and time. The volcano erupted onto crust inferred to be 20 Ma from seafloor magnetic anomalies. Ages of the lavas range from 9.8 to 14.8 Ma. The oldest rocks are from the central ridge, and the youngest are from the flanks and southern end of the edifice. The compositions of the 18 reliably dated volcanic cones vary with age such that the oldest lavas are the most fractionated. The melts lost 65% to nearly 95% of their initial S because of bubble loss during vesiculation, and the shallowest samples have S contents similar to lava erupted subaerially in Hawaii. Despite this similarity in S contents, there is scant other evidence to suggest that Davidson was ever an island. The numerous small cones of disparate chemistry and the long eruptive period suggest episodic growth of the volcano over at least 5 Myr and perhaps as long as 10 Myr if it began to grow when the spreading ridge was abandoned.

Origin of volcanic seamounts at the continental margin of California related to changes in plate margins

Volcanic samples collected with the Monterey Bay Aquarium Research Institutes ROV Tiburon from eight seamounts at the continental margin offshore central to southern California comprise a diverse suite of mainly alkalic basalt to trachyte but also include rare tholeiitic basalt and basanite. All samples experienced complex crystal fractionation probably near the crust/mantle boundary, based on the presence in some of mantle xenoliths. Incompatible trace elements, poorly correlated with isotopic compositions, suggest variable degrees of partial melting of compositionally heterogeneous mantle sources, ranging from MORB-like to relatively enriched OIB. High-precision 40Ar/39Ar ages indicate episodes of volcanic activity mainly from 16 to 7 Ma but document one eruption as recent as 2.8 Ma at San Juan Seamount. Synchronous episodes of volcanism occurred at geographically widely separated locations offshore and within the continental borderland. Collectively, the samples from these seamounts have age ranges and chemical compositions similar to those from Davidson Seamount, identified as being located atop an abandoned spreading center. These seamounts appear to have a common origin ultimately related to abandonment and partial subduction of spreading center segments when the plate boundary changed from subduction-dominated to a transform margin. They differ in composition, age, and origin from other more widespread near-ridge seamounts, which commonly have circular plans with nested calderas, and from age progressive volcanoes in linear arrays, such as the Fieberling-Guadalupe chain, that occur in the same region. Each volcanic episode represents decompression melting of discrete enriched material in the suboceanic mantle with melts rising along zones of weakness in the oceanic crust fabric. The process may be aided by transtensional tectonics related to continued faulting along the continental margin.

Unraveling the tilting history of the submerged reefs surrounding Oahu and the Maui‐Nui Complex, Hawaii

There is considerable uncertainty concerning the history of lithospheric flexure of Oahu and the Maui-Nui Complex associated with the Hawaiian hot spot. Previous reconstructions based on models and sparse observational data place the zone of maximum uplift (forebulge) either under Oahu or between Oahu and Molokai. To address this issue we have compiled and analyzed all existing and new high‐resolution multibeam bathymetry, dive observations, and chronologic data from submerged terraces in the region. We have identified 89 separate submarine terraces and mapped their distribution across the volcanic flanks of Oahu and the Maui‐Nui Complex. These data are used to systematically measure the direction and amplitude of terrace tilting within the Maui‐Nui Complex and to provide new constraints on lithospheric flexure associated with volcanic loading at the hot spot. The terraces are divided into six separate geographic regions (Oahu, Molokai, Penguin Bank, Lanai, Maui, and East Maui) defined by significant discontinuities in either terrace morphology or depth. Following the sequential development of new substrate as the new volcanoes formed throughout the development of the MNC, 18 new and 51 published 87Sr/86Sr isotopic ages show that the terraces in these regions were also initiated at different times, and the tilting data show that they also responded to lithospheric flexure independently. The general pattern of terrace tilting records the plate’s response to lithospheric flexure with each region continually tilting toward the zone of maximum subsidence since their formation. Our individual tilt calculations are based on the most complete and highest‐ resolution bathymetric data available, and they show that plate response to volcanic loading is a gradual process. Finally, we demonstrate that the shallowest terraces we identified in the Oahu and Molokai regions tilt toward the north, away from the current zone of maximum subsidence, indicating that these regions have recently passed the zone of maximum uplift. We compare our results with two separate published numerical models and differentiate between these, resolving a long‐standing uncertainty in the position of the forebulge.

Hydrothermal vent fields discovered in the southern Gulf of California clarify role of habitat in augmenting regional diversity

Hydrothermal vent communities are distributed along mid-ocean spreading ridges as isolated patches. While distance is a key factor influencing connectivity among sites, habitat characteristics are also critical. The Pescadero Basin (PB) and Alarcón Rise (AR) vent fields, recently discovered in the southern Gulf of California, are bounded by previously known vent localities (e.g. Guaymas Basin and 21° N East Pacific Rise); yet, the newly discovered vents differ markedly in substrata and vent fluid attributes. Out of 116 macrofaunal species observed or collected, only three species are shared among all four vent fields, while 73 occur at only one locality. Foundation species at basalt-hosted sulfide chimneys on the AR differ from the functional equivalents inhabiting sediment-hosted carbonate chimneys in the PB, only 75 km away. The dominant species of symbiont-hosting tubeworms and clams, and peripheral suspension-feeding taxa, differ between the sites. Notably, the PB vents host a limited and specialized fauna in which 17 of 26 species are unknown at other regional vents and many are new species. Rare sightings and captured larvae of the ‘missing’ species revealed that dispersal limitation is not responsible for differences in community composition at the neighbouring vent localities. Instead, larval recruitment-limiting habitat suitability probably favours species differentially. As scenarios develop to design conservation strategies around mining of seafloor sulfide deposits, these results illustrate that models encompassing habitat characteristics are needed to predict metacommunity structure.

Geology of the Alarcon Rise, Southern Gulf of California

Author(s): Clague, DA; Caress, DW; Dreyer, BM; Lundsten, L; Paduan, JB; Portner, RA; Spelz-Madero, R; Bowles, JA; Castillo, PR; Guardado-France, R; Le Saout, M; Martin, JF; Santa Rosa-del Rio, MA; Zierenberg, RA | Abstract:

High-resolution AUV mapping and ROV sampling of mid-ocean ridges

Since the mid-1980s, eight eruptions have occurred along the Juan de Fuca and Gorda Ridges in the NE Pacific. MBARI has examined seven of them, including the April 2015 eruption at Axial Seamount, using autonomous underwater vehicles (AUVs) to map at 1-m lateral resolution and remotely operated vehicles (ROVs) to observe and sample the lava flows and hydrothermal deposits. We have done similar work on the Alarcon Rise at the mouth of the Gulf of California, where all eruptions pre-date our studies of the ridge system, but are recent enough to host impressive active hydrothermal systems.AUV sonar data were processed with MB-System software and the resulting maps were brought into a geographical information system (GIS) for display and analysis. The highresolution AUV maps were used at sea as basemaps in a real-time GIS to guide our ROV dives with greater efficiency than possible before. From the maps and observations, we defined lava flow boundaries and channel systems, assessed fault activity, calculated flow sizes, evaluated age relationships between flows, and then sampled lavas for chemistry. Those data were coupled with age dates from sediment samples to place changes in eruption styles, lava chemistry, and hydrothermal systems on the ridges into the previously elusive framework of time. Using these tools, we have constructed geologic field maps and volcanic histories of the spreading ridges, much like volcanologists do on land, but all unprecedented for submarine volcanoes. The synergistic high-resolution mapping and targeted ROV sampling of the ridges has permitted better understanding of when, how, how much, and how often spreading ridges erupt, and how their magmatic and hydrothermal systems change over time.