Holocene Sediment Records From the Continental Shelf of Mac. Robertson Land, East Antarctica

Geochemical records are presented for five sediment cores from basins on the continental shelf of Mac. Robertson Land, East Antarctica. The cores contain 2-4 m thick sequences of hemipelagic, siliceous mud and ooze (SMO) deposited under seasonally open marine conditions. The inner and middle shelf SMO sequences are massive dark olive green material, whereas the outer shelf SMO sequences are dark olive material interspersed with light olive green layers similar to1-10 cm thick. The biogenic material is dominated by marine diatoms including Fragilariopsis curta, Fragilariopsis cylindrus, and Chaetoceros spp. in the dark-colored SMO and Corethron criophilum in the light-colored layers. Radiocarbon dates suggest that the cores provide continuous accumulation records extending from \u3c 1 kyr before present (B.P.) back as far as 4-15 kyr B.P., with estimated accumulation rates of 0.07-5 mm yr(-1). The three core records from the middle and outer shelf suggest six episodes of increased accumulation of biogenic material at 5.5 kyr B.P. tall three cores), 1, 2, and 6.2 kyr B.P. (two of the three cores), and 3.8 and 10.8 kyr B.P. tone core), most of which coincide with Corethron layers. We interpret these features as the result of enhanced diatom production over the outer shelf, possibly related to climatic warm periods. The absence of such features in the inner shelf core records is thought to reflect a relatively constant level of seasonal diatom production in adjacent waters maintained by a coastal polynya

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 Pele\u27s Vents on Loihi Seamount, Hawaii. In July–August 1996 a tectonic-volcanic event occurred that destroyed Pele\u27s Vents, creating a pit crater (Pele\u27s 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- (\u3e330°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

Carbon release from submarine seeps at the Costa Rica fore arc: implications for the volatile cycle at the Central America convergent margin

We report total dissolved inorganic carbon (DIC) abundances and isotope ratios, as well as helium isotope ratios (3He/4He), of cold seep fluids sampled at the Costa Rica fore arc in order to evaluate the extent of carbon loss from the submarine segment of the Central America convergent margin. Seep fluids were collected over a 12 month period at Mound 11, Mound 12, and Jaco Scar using copper tubing attached to submarine flux meters operating in continuous pumping mode. The fluids show minimum 3He/4He ratios of 1.3 RA (where RA is air 3He/4He), consistent with a small but discernable contribution of mantle-derived helium. At Mound 11, δ13C∑CO2 values between −23.9‰ and −11.6‰ indicate that DIC is predominantly derived from deep methanogenesis and is carried to the surface by fluids derived from sediments of the subducting slab. In contrast, at Mound 12, most of the ascending dissolved methane is oxidized due to lower flow rates, giving extremely low δ13C∑CO2 values ranging from −68.2‰ to −60.3‰. We estimate that the carbon flux (CO2 plus methane) through submarine fluid venting at the outer fore arc is 8.0 × 105 g C km−1 yr−1, which is virtually negligible compared to the total sedimentary carbon input to the margin and the output at the volcanic front. Unless there is a significant but hitherto unidentified carbon flux at the inner fore arc, the implication is that most of the carbon being subducted in Costa Rica must be transferred to the (deeper) mantle, i.e., beyond the depth of arc magma generation

Scientific Studies and History of the Ala Wai Canal, an Artificial Tropical Estuary in Honolulu

Fifteen studies of the Ala Wai Canal, O'ahu, Hawai'i, initially were spawned by two federally funded summer research programs designed to introduce high-school students from around the state of Hawai'i to the challenges, practicalities, and excitement of work in the natural sciences and engineering. This special issue reports on the end products of 10 of those studies. The canal is an artificial estuary created in the 1920s to drain coastal wetlands and borders the present tourist mecca of Waikiki. Today, it is polluted and hypereutrophic, and it receives high levels of nutrients that sustain levels of primary production that rival all but a few of the world's water bodies. Acting as a sediment trap for the combined drainage of the Manoa and Palolo Streams, the midportion of the canal contains two large sedimentary sills that restrict seawater exchange. This restricted flow and the high rain rate of organic matter result in severe oxygen depletion behind the sill. The canal's small reservoir size, variably oxygenated water column and sediments, single oceanic outlet, and receipt of natural freshwater drainage-within the confines of a rapidly developed major metropolitan area-combine to make it an excellent aquatic laboratory for the study of present and historical water exchange characteristics; phytoplankton, zooplankton, and benthic foraminifer behavior; biogeochemical responses of shallow, tropical water masses to hypereutrophication; and historical records of heavy metals, radionuclides, and other pollutants over the past 60 yr. We believe this special issue will attract the attention of a variety of scientists and academicians, as well as administrators and others interested in the environmental quality of Hawai'i

Mercury Accumulation in Sediments of the Ala Wai Canal and in Soils and Stream Sediments of the Central Honolulu Watershed

In this study we determined the historical trend of both natural and anthropogenic sources of mercury deposition as preserved in anoxic estuarine sediments of the Ala Wai Canal, an estuary situated within a heavily urbanized area of Honolulu. Analysis of sediments from the Ala Wai Canal revealed that the total mercury content is highest at the Ala Wai Yacht Harbor (0.054-2.810 ug/g) and decreases exponentially toward the most distal portion of the canal (0.009-0.237 ug/g). In contrast, the mercury content of soil and stream samples taken from the central Honolulu watershed ranges from only 0.001 to 0.058 ug/g. This pattern suggests tidal transport of mercury into the canal from the Ala Wai Yacht Harbor. A chronological analysis of core samples shows a peak in mercury concentrations in the late 1950s, which corresponds to the use of antifouling paints on boats in the harbor and is the probable source of the majority of the mercury found in the Ala Wai Canal. High mercury accumulation ends in the early 1970s in two of the cores investigated, suggesting that antifouling paint-based accumulation ceased rapidly after the U.S. Environmental Protection Agency (EPA) ban. An exception is noted in a comparatively smaller peak coincident with 1986, the last year of a 3-yr intense fire-fountaining period of the ongoing Pu'u '0'o eruption of nearby Kilauea Volcano

Modeling Sediment Accumulation and Soil Erosion with 137Cs and 210Pb in the Ala Wai Canal and Central Honolulu Watershed, Hawai'i

Radiochemical studies of sediments from the Ala Wai Canal, an urban estuary in Honolulu, and of soils and stream sediments from the central Honolulu watershed were undertaken to investigate the sediment accumulation history and estimate the sediment yield and denudation rate of the watershed. Modern high-purity Ge gamma spectrometry techniques were used to assess the activities of U-series and 137Cs radioisotopes in stratigraphic subsamples of three 1- to 2-m-long sediment cores, 14 watershed soil horizons, and grab samples of seven tributary stream sediments. Geochronology based on excess 210Pb, using either steady-state constant flux or constant activity models, yields ages that exceed the known age of the Ala Wai Canal. Geochronology based on a nonsteady-state, two-box, erosion/redeposition model of fallout 137CS yields sedimentation rates for the canal of between ca. 2 and 22 cm yr-1. These rates generally exceed those based upon excess 210Pb by more than a factor of two and agree with the known age of the canal and with sedimentation rate estimates based upon bathymetry changes. Based on the 137Cs-model chronology from 1957 to 1991, the Ala Wai Canal collects bulk sediment at a mean rate of ca. 3100 tons annually. About 80% of the sediment is detrital clays from erosion of the central Honolulu watershed, whereas about 20% of the sediment is composed of marine authigenic and biogenous phases. The sediment yield for the central Honolulu watershed of ca. 60 metric tons km-2 yr-1 equates to a physical denudation rate of ca. 6 mg cm-2 yr-1 --at the low end of the range of physical denudation rate estimates for the island of O'ahu. Based on the mean 137Cs sedimentation rates and an average canal water depth of 2 m, the average time to fill the canal is about 60 yr, assuming that little sediment escapes. The mean fill time is only about 40 yr for the middle canal segment, which receives most sill development from the Manoa-Palolo Stream drainage canal, whereas for the outer and inner canal sediments, mean fill times are about 70 yr. Fallout 137Cs-derived sedimentation rates for each 4-cm interval range from <0.1 to >1.0 g cm-2 month-1 and reveal three episodes of relatively high sediment accumulation in the canal over the ca. 35-yr period before 1991: 1957-1967, 1979-1982, and 1986-1991. The two earlier episodes appear to coincide with periods of high rainfall, but are generally preceded by dry periods where accumulation of marine authigenic phases are high. The most recent high sediment accumulation episode does not appear to correlate with high rainfall, although the annual rainfall trend has increased toward 1990 from a low in 1983. For the Ala Wai Canal, the flux of excess 210Pb generally follows the sedimentation rate and is not constant with time. Two possible causes of higher excess 210Pb fluxes than those expected from a linear relationship are nonsteady-state atmospheric input to the Hawaiian Islands from 222Rn_rich air masses that originate in Asia, and 222Rn from local volcanic eruptions. The variable excess 210Pb flux into the canal sediments may, however, be related to a complex mechanism of soil erosion

Heavy Metal Anomalies in Coastal Sediments of O'ahu, Hawai'i

Interelement ratios to Cr in surface samples of coastal sediments and watershed soils of Oah'u, Hawai'i, show widespread, anomalous concentrations of Pb, Cd, and Hg when compared with basalt, the ubiquitous rock type. Enrichments of these heavy metals are especially pronounced in the carbonate sands of Kahana, Maunalua, and Mamala Bays, where enrichment factors for Pb, Cd, and Hg range from 84 to 240, 67 to 180, and 43 to 72, respectively, based on samples collected in the early 1970s. Lesser enrichments of Cu, Zn, and Ni generally parallel those of Pb, Cd, and Hg in highly contaminated areas at Pearl and Honolulu Harbors, and in cultivated watershed soils. Estimated deposition rates for Pb, Cd, and Hg from three major local source categories-motor vehicle, agriculture, and volcanic-indicate that motor vehicles are by far the largest source of Pb enrichments in O'ahu soils and sediments. Widespread mercury deposition is apparently dominated by local volcanic sources, whereas Cd deposition is more evenly dispersed among the three major sources. The estimated Pb and Cd deposition rates are in reasonable agreement with their observed sediment and soil burdens in the early 1970s. The estimated Hg deposition rates are higher than necessary to explain the observed burdens for this metal, possibly as a result of soil leaching, postdepositional volatility, or Hg uptake and re-emission by biota

Isotopic Clues to Sources of Natural and Anthropogenic Lead in Sediments and Soils from O'ahu, Hawai'i

Stable Pb isotopes, Pb elemental concentrations, and, for some samples, Nd and Sr isotopes and concentrations have been analyzed on soils and on stream and estuarine sediments to evaluate the provenance of major inputs of Pb to the O'ahu, Hawai'i, environment. Core samples from the Ala Wai Canal, a major estuary draining urban Honolulu, preserve a historical record of anthropogenic lead input that peaked during the 1970s, the period of heaviest leaded-gas usage in Hawai'i. The timing of the Pb concentration peak and the simultaneous rise in Zn and Cd concentrations, two elements used in tire vulcanization, strongly suggest that the source of this Pb was tetraethyl Pb used in leaded gasoline. The changing Pb isotopic composition in these sediments reflects changing sources of ore from which tetraethyllead was produced. These isotopic signatures can be used to fingerprint anthropogenic Pb elsewhere on O'ahu. Although leaded gasoline has been phased out of production in the United States and in many other countries, elevated amounts of lead continue to deposit from the Ala Wai Canal's watershed. Sediment samples from Manoa Stream, a principal tributary, suggest that relatively uncontaminated sediments are eroded from its headwaters while a source (or sources) of lead continues to discharge into the stream as it nears the south end of Manoa Valley. The isotopic composition of this lead is similar to that measured in recently deposited sediments cored from the Ala Wai Canal. An atmospheric dust-enriched soil collected on the island of Hawai'i contains elevated Pb concentrations (55 ppm) and a Pb isotopic composition similar to North Pacific pelagic sediment. In addition, this sample contains unradiogenic Nd (E = -6) and radiogenic Sr (87Sr/86Sr = 0.722527) confirming an old, continentally derived provenance. Soils collected in Ha'ikii Valley, a windward O'ahu valley subject to high rainfall, contain variable Pb concentrations and Sr, Nd, and Pb isotopes trending toward the isotopic composition of the dust-enriched sample. This confirms that the Ha'ikii Valley soils contain an aerosol component. Soils enriched in this component could have natural lead concentrations higher than soils made up solely of weathered Hawaiian rocks. Hawai'i's soils and sediments have naturally derived variations in Pb concentration that are caused by differences in provenance and degree of weathering. Superimposed on this natural concentration variation is a variable anthropogenic signal. These variations should be factored into environmental monitoring programs

In-situ monitoring of 3He/4He in summit gases of Kilauea Volcano (Hawaii) prior to the 2020 eruption

Abstract We present He isotope (3He/4He) data from a fumarole and near-ground gases measured in-situ at the Sulfur Banks solfatara field at the summit of Kilauea Volcano, Hawaii. We used a field-deployable mass-spectrometer-based system: the Helium Isotope Monitor (HIM) previously described in McMurtry et al. (2019a, b). The in-situ instrument was deployed using solar power for the first time and results were ground-truthed against data determined using conventional gas analytical and noble gas mass spectrometry techniques. The HIM instrumentation, associated Vent Gas Purification System (VGPS), and related sampling equipment and strategy are described. Cloudy and rainy weather conditions hampered the deployment, which was reorganized to reduce power loads and resulted in less sampling than planned. Nevertheless, we obtained daily sampling of the volcanic vent gas. Results from the Old Well fumarole indicate a ~ 2 RA increase in 3He/4He on the day of the December 20th, 2020 eruption of nearby Halema‘uma‘u Crater, reaching 17.0 RA using the in-situ instrument and 16.0 ± 0.67 RA using conventional techniques. This finding suggests that a new 3He-enriched magma source is driving the current, ongoing eruption phase of Kilauea and, if so, confirms that the deep summit caldera fault system that hosts the Sulfur Banks field is connected to the Halema‘uma‘u Crater magmatic system. Overall, these findings illustrate how time-series helium isotope data, which are well established by ongoing discrete monitoring at low temporal resolution, can help forecast forthcoming eruptive events that may not be foreseen by other volcanic monitoring methods