Gary M. McMurtry

Giant landslides, mega-tsunamis, and paleo-sea level in the Hawaiian Islands

Abstract Landslide tsunami simulations have advanced to the point where the tsunamigenic potential of giant submarine landslides (GSL) can be affirmed, while the subsidence history of different Hawaiian Islands is still subject to debate. We show that mega-tsunamis are a sufficient explanation for the observed pattern of debris height of calcareous marine deposits on some of the Hawaiian Islands. Further, our tsunami simulations, using the Alika GSL as example, can be used to reduce the considerable uncertainty in subsidence history of the different Hawaiian Islands, a current obstacle to interpreting the deposits from large waves. We also show that the onset of interglacials provides a probable explanation for the timing of these giant landslides over at least the last five million years. The climate change mechanism both explains the confusion with eustatic sea-level rise and provides a reasonable triggering mechanism for giant landslides from oceanic island volcanoes.

Rare-earth element geochemistry of ferromanganese crusts from the Hawaiian Archipelago, central Pacific

Abstract The rare-earth element (REE) composition of ferromanganese deposits from the Hawaiian Archipelago was determined as part of a geochemical investigation into the origin and environment of deposition of crusts. Most samples exhibit shale-normalized REE patterns which are typical of hydrogenous ferromanganese deposits. Patterns of two todorokite- and manjiroite-rich crusts are significantly different from those of the other samples. The range of concentrations in typical Hawaiian crusts (sum of REE 0.15–0.25 wt.%) is in the uppermost portion of that reported for deep-sea Fe-Mn nodules. Shallow-water ( 2000 m) as well as slightly greater REE fractionation. Large positive Ce anomalies attributed to the oxidative scavenging at the MnO 2 surface are observed on all samples except one. The slight light REE (LREE) depletion in shallow-water crusts relative to deeper samples is discussed in terms of variations in the REE concentrations in the water column and the relative complexing behavior of the REE. No significant trend in Ce concentration with depth is observed. Partition coefficient calculations suggest that concentrations of trivalent REE in crusts are affected by their availability in seawater to a greater extent than by the composition of the scavenging ferromanganese oxide flocs.

Chemistry of hydrothermal solutions from Pele's Vents, Loihi Seamount, Hawaii

Abstract Hydrothermal fluids were sampled from Peles Vents on the summit of Loihi Seamount, an intraplate, hotspot volcano, on four occasions from February 1987 to September 1990. The warm (≤ 31°C) vent solutions are enriched in dissolved Si, CO 2 , H 2 S, alkalinity, K + , Li + , Rb + , Ca 2+ , Sr 2+ , Ba 2+ , Fe 2+ , Mn 2+ , NH 4 + , and possibly Ni 2+ , and depleted in SO 4 2− , O 2 , Mg 2+ , 87 Sr 86 Sr , NO 3 − , and sometimes Cl − and Na + (calculated), relative to ambient seawater. Dissolved Si correlates linearly with sample temperature, suggesting that the solutions sampled from numerous vents in the ~ 20 m diameter field have a common source and that Si can be used as a conservative tracer for mixing of the vent fluids with ambient seawater. There are general similarities of the vent waters with ridge-axis warm springs on the Galapagos Rift and Axial Seamount, but also striking differences: very high total dissolved CO 2 (> 200 mmol/kg), high alkalinity (> 8 meq/kg) and dissolved Fe 2+ (almost 1 mmol/kg), and relatively low pH (~ 4.2–4.4 estimated, in situ, sws) and dissolved H 2 S (several μmol/kg). The Mg 2+ and SO 4 2− data are inconsistent with the “Galapagos model” proposed for the warm springs at 86°W, Galapagos Rift, whereby the warm fluids result from sub-seafloor mixing of a high-temperature (~ 350°C) hydrothermal endmember with essentially unaltered seawater. The variable Cl − depletions in the vent fluids, however, suggest that the warm vent fluids do contain a high-temperature (> 200°C) component. The fluid history can be qualitatively described by a modified “Galapagos model” which includes the overprint of reactions resulting from the addition of juvenile CO 2 and SO 2 to the circulating fluids; the CO 2 attacks the basalt releasing metal cations, HCO 3 − and possibly Si into solution, and the SO 2 is hydrolyzed to SO 4 2− . These juvenile inputs likely reflect the shallow, hotspot setting of this hydrothermal system. A simple quantitative fluidhistory model is considered and shown to be consistent with mass-balance constraints and saturationstate calculations, which suggest that the Si concentration of the fluids may be controlled by amorphous silica saturation at ~ 31°C. Observed temporal variations in fluid composition between expeditions—specifically, in Cl − , A T , C T , Na + (calculated), Mg 2+ , Ca 2+ , Sr 2+ , 87 Sr 86 Sr , Fe 2+ , Mn 2+ and perhaps NH 4 + , relative to Si—are, excepting Mg 2+ , 87 Sr 86 Sr , and Mn 2+ , consistent with the effects of variable phase segregation at the proposed high-temperature endmember.

Large variations in vent fluid CO2/3He ratios signal rapid changes in magma chemistry at Loihi seamount, Hawaii

Loihi seamount, an active submarine volcano situated about 30 km south of the island of Hawaii, is the youngest manifestation of the hotspot responsible for the Emperor–Hawaiian seamount chain and Hawaiian islands. This seamount has been the focus of numerous studies characterizing the geophysical, geochemical and biological features of an active intraplate volcano. In July–August 1996, Loihi seamount experienced the most intense period of seismic activity yet recorded for any Hawaiian volcano. Within two months of the ‘seismic crisis’, summit and flank hydrothermal vent fluids were collected using a manned submersible. Here we report data from these samples that indicate large and systematic changes in the CO2/3He ratios of the vent fluids compared to pre-seismic-crisis values,. These changes are consistent with an abrupt transition from alkalic to tholeiitic basaltic magma having supplied volatiles to the vents. This rapid change in magma chemistry has been discernible only through CO2/3He monitoring, and suggests that the anticipated evolution of the Hawaiian plume to a phase of shield-building tholeiitic magmatism is highly episodic at Loihi and not yet complete.

Chemical and isotopic investigations into the origin of clay minerals from the Galapagos hydrothermal mounds field

Abstract A dark green authigenic nontronite is the major component of the Galapagos hydrothermal mounds field sediments. Oxygen isotopic compositions of the chemically purified, in situ mounds temperatures of up to 15°C. Assuming an authigenic origin, the Fe-rich montmorillonite that dominates in the noncarbonate clay fraction of the surrounding pelagic ooze has isotopic formation temperatures of 27° to 39°C, compared with measured in situ temperatures of ca. 3.5° to 6.5°C. The higher isotopic formation temperatures calculated for the hydrothermal nontronite suggest either complex patterns of fluid circulation and nontronite precipitation presently within the mounds or a higher thermal history associated with rapid and episodic periods of deposition during the Holocene-Pleistocene. The apparent high isotopic temperature of the Fe-rich montmorillonite may reflect: (1) formation under hydrothermal conditions at spreading centers with subsequent dispersal by bottom currents, (2) a detrital origin of the mineral, or (3) a mixture of authigenic Fe-montmorillonite and detrital Al-montmorillonite in this region.

Hydrothermal clay mineral formation of East Pacific rise and Bauer Basin sediments

Samples of surface metalliferous sediment recovered from the crest of the East Pacific Rise at 6°S and 10°S latitudes and from the adjacent Bauer Basin are characterized by an authigenically formed, Fe-rich montmorillonite that dominates the non-carbonate mineralogy of the clay fraction (< 2 μm). Oxygen-isotopic formation temperatures indicate that the Fe-montmorillonites are created by low-temperature, hydrothermal processes (30°–50°C) in the 10°S region of the East Pacific Rise and Bauer Basin, possibly as a result of cooling and oxidation of unstable, high-temperature (380° ± 30°C) sulfide assemblages or as a result of the percolation of hydrothermally altered seawater solutions through underlying basalt and sediments. The widespread sedimentation of the clay mineral is suggested to be caused by colloidal transport, possibly as a result of erosion of hydrothermal mounds by bottom currents. Hydrothermal Fe-montmorillonite-nontronite formation may act as a direct and significant oceanic sink for Si and Fe released by the high-temperature, hydrothermal alteration of basalt at ocean spreading centers.

Continuous sampling of hydrothermal fluids from Loihi Seamount after the 1996 event

For at least 9 years prior to July 1996, hydrothermal fluids flowed from Peles Vents on Loihi Seamount, Hawaii. In July–August 1996 a tectonic-volcanic event occurred that destroyed Peles Vents, creating a pit crater (Peles Pit) and several sites with hydrothermal venting. In October 1996 we deployed two new continuous water samplers (OsmoSamplers) at two of these hydrothermal sites and collected fluids using traditional sampling techniques to monitor the evolution of crustal and hydrothermal conditions after the event. The samplers were recovered in September 1997, and additional discrete vent fluid samples were collected. The OsmoSampler located along the south rift at Naha Vents captured a change in composition from a low-chlorinity, high-K fluid (relative to bottom seawater) to a high-chlorinity, low-K fluid. These changes are consistent with the fluid cooling during ascent and being derived from several different sources, which include high- (>330°C) and low- ( 330°C) into which magmatic volatiles were added. During the deployment, thermal and fluid fluxes decreased. At Naha the transport of heat and chemicals was decoupled. The chemical and thermal evolution of hydrothermal fluids after the event on Loihi is consistent with previous models based on events that have occurred along mid-ocean ridges. The event at Loihi clearly had an effect on the local hydrography; however, the integrated effect of chemical fluxes to global budgets from similar events is uncertain. Chemical fluxes from similar events may have a global impact, if ratios of chemical (e.g., CO2, Fe/Mn, Mg, sulfate, and K) to thermal anomalies greatly exceed, or are in the opposite direction to, fluxes from mid-ocean ridge hydrothermal systems.

Megatsunami deposits on Kohala volcano, Hawaii, from flank collapse of Mauna Loa

The origin of coastal and high-elevation marine gravels on the Hawaiian islands of Lanai and Molokai is controversial, because the vertical tectonics of these islands is poorly constrained. The gravels are either from eustatic highstands or were left by massive tsunamis from offshore giant landslides. In contrast, at Kohala on the island of Hawaii, where continuous subsidence is well established, lithofacies analysis and dating of a fossiliferous marine conglomerate 1.5–61 m above present sea level support a tsunami origin and indicate a runup of >400 m >6 km inland. The conglomerate age, 110 ± 10 ka, suggests a tsunami caused by the ca. 120 ka giant Alika 2 landslide from nearby Mauna Loa volcano.

Stratigraphic constraints on the timing and emplacement of the Alika 2 giant Hawaiian submarine landslide

Abstract Previous work has found evidence for giant tsunami waves that impacted the coasts of Lanai, Molokai and other southern Hawaiian Islands, tentatively dated at 100+ and 200+ ka by U-series methods on uplifted coral clasts. Seafloor imaging and related work off Hawaii Island has suggested the Alika phase 2 debris avalanche as the source of the ∼100 ka “giant wave deposits”, although its precise age has been elusive. More recently, a basaltic sand bed in ODP site 842 (∼300 km west of Hawaii) estimated at 100±20 ka has been suggested to correlate with this or another large Hawaiian landslide. Our approach to the timing and linkage of giant submarine landslides and paleo-tsunami deposits is a detailed stratigraphic survey of pelagic deposits proximal to the landslide feature, beginning with a suite of seven piston, gravity and box cores collected in the vicinity of the Alika 2 slide. We used U-series dating techniques, including excess 230 Th and 210 Pb profiling, high-resolution paleomagnetic stratigraphy, including continuous, U-channel analysis, δ 18 O stratigraphy, visual and X-ray sediment lithology, and the petrology and geochemistry of the included turbidites and ash layers. Minimum ages for the Alika phase 2a slide from detailed investigation of two of the cores are 112±15 ka and 125±24 ka (2σ) based on excess 230 Th dating. A less precise age for the Alika phase 1 and/or South Kona slide is 242±80 ka (2σ), consistent with previous geological estimates. Oxygen isotope analyses of entrained planktonic foraminifera better constrain the Alika phase 2a maximum age at 127±5 ka, which corresponds to the beginning of the stage 5e interglacial period. It is proposed that triggering of these giant landslides may be related to climate change when wetter periods increase the possibility of groundwater intrusion and consequent phreatomagmatic eruptions of shallow magma chambers. Our study indicates the contemporaneity of the Alika giant submarine landslides and distal deposits from enormous turbidity currents as well as coral clasts reported to be tsunami deposits on Lanai and Molokai through direct dating and compositional analysis of the landslide deposits.

Geochemistry of hydrothermal deposits from Loihi submarine volcano, Hawaii

Abstract Dredging across the northeast rim of the summit crater of Loihi Seamount recovered several morphologically similar but chemically and mineralogically distinct hydrothermal deposits encrusting the surface of fresh pillow lava talus. The multicolored deposits suggest a precipitation sequence that may be controlled by an oxidation-reduction gradient in which smectite ranging in composition from Fe-montmorillonite to nontronite has precipitated along with iron oxide under slightly reducing conditions. This deposition was apparently followed by amorphous iron oxide and silica precipitation, possibly under more oxic conditions. Oxygen isotope geothermometry indicates formation temperatures in the range of 31–57°C for the Loihi smectites. Trace element enrichments appear to be positively correlated with the isotopic formation temperature of the smectite, suggesting either increased trace element solubility within the higher-temperature vent fluids or increased smectite and iron oxide scavenging with increased precipitation rates. The trace element abundances further suggest the presence of polymetallic sulfides that either are directly associated with the smectites as amorphous phases or occur beneath these deposits in the volcanic pile of the seamount.