Geochemistry, Geophysics, Geosystems

Woodruff, Jonathan D.; Donnelly, Jeffrey P.; Emanuel, Kerry; Lane, Philip

doi: 10.1029/2008GC002043pmid: N/A

Patterns of overwash deposition observed within back‐barrier sediment archives can indicate past changes in tropical cyclone activity; however, it is necessary to evaluate the significance of observed trends in the context of the full range of variability under modern climate conditions. Here we present a method for assessing the statistical significance of patterns observed within a sedimentary hurricane‐overwash reconstruction. To alleviate restrictions associated with the limited number of historical hurricanes affecting a specific site, we apply a recently published technique for generating a large number of synthetic storms using a coupled ocean‐atmosphere hurricane model set to simulate modern climatology. Thousands of overwash records are generated for a site using a random draw of these synthetic hurricanes, a prescribed threshold for overwash, and a specified temporal resolution based on sedimentation rates observed at a particular site. As a test case we apply this Monte Carlo technique to a hurricane‐induced overwash reconstruction developed from Laguna Playa Grande (LPG), a coastal lagoon located on the island of Vieques, Puerto Rico in the northeastern Caribbean. Apparent overwash rates in the LPG overwash record are observed to be four times lower between 2500 and 1000 years B.P. when compared to apparent overwash rates during the last 300 years. However, probability distributions based on Monte Carlo simulations indicate that as much as 65% of this drop can be explained by a reduction in the temporal resolution for older sediments due to a decrease in sedimentation rates. Periods of no apparent overwash activity at LPG between 2500 and 3600 years B.P. and 500–1000 years B.P. are exceptionally long and are unlikely to occur (above 99% confidence) under the current climate conditions. In addition, breaks in activity are difficult to produce even when the hurricane model is forced to a constant El Niño state. Results from this study continue to support the interpretation that the western North Atlantic has exhibited significant changes in hurricane climatology over the last 5500 years.

Dixon, Jacqueline; Clague, David A.; Cousens, Brian; Monsalve, Maria Luisa; Uhl, Jessika

doi: 10.1029/2008GC002076pmid: N/A

We present new volatile, trace element, and radiogenic isotopic compositions for rejuvenated‐stage lavas erupted on Niihau and its submarine northwest flank. Niihau rejuvenated‐stage Kiekie Basalt lavas are mildly alkalic and are isotopically similar to, though shifted to higher 87Sr/86Sr and lower 206Pb/204Pb than, rejuvenated‐stage lavas erupted on other islands and marginal seafloor settings. Kiekie lavas display trace element heterogeneity greater than that of other rejuvenated‐stage lavas, with enrichments in Ba, Sr, and light‐rare earth elements resulting in high and highly variable Ba/Th and Sr/Ce. The high Ba/Th lavas are among the least silica‐undersaturated of the rejuvenated‐stage suite, implying that the greatest enrichments are associated with the largest extents of melting. Kiekie lavas also have high and variable H2O/Ce and Cl/La, up to 620 and 39, respectively. We model the trace element concentrations of most rejuvenated‐stage lavas by small degrees (∼1% to 9%) of melting of depleted peridotite recently metasomatized by a few percent of an enriched incipient melt (0.5% melting) of the Hawaiian plume. Kiekie lavas are best explained by 4% to 13% partial melting of a peridotite source metasomatized by up to 0.2% carbonatite, similar in composition to oceanic carbonatites from the Canary and Cape Verde Islands, with lower proportion of incipient melt than that for other rejuvenated‐stage lavas. Primary H2O and Cl of the carbonatite component must be high, but variability in the volatile data may be caused by heterogeneity in the carbonatite composition and/or interaction with seawater. Our model is consistent with predictions based on carbonated eclogite and peridotite melting experiments in which (1) carbonated eclogite and peridotite within the Hawaiian plume are the first to melt during plume ascent; (2) carbonatite melt metasomatizes plume and surrounding depleted peridotite; (3) as the plume rises, silica‐undersaturated silicate melts are also produced and contribute to the metasomatic signature. The metasomatic component is best preserved at the margins of the plume, where low extents of melting of the metasomatized depleted mantle surrounding the plume are sampled during flexural uplift. Formation of carbonatite melts may provide a mechanism to transfer plume He to the margins of the plume.

Ito, Garrett; Behn, Mark D.

doi: 10.1029/2008GC001970pmid: N/A

We investigate the origin of mid‐ocean ridge morphology with numerical models that successfully predict axial topographic highs, axial valleys, and the transition between the two. The models are time‐dependent, simulating alternating tectonic and magmatic periods where far‐field extension is accommodated by faulting and by magmatism, respectively. During tectonic phases, models predict faults to grow on either side of the ridge axis and axial height to decrease. During magmatic phases, models simulate magmatic extension by allowing the axial lithosphere to open freely in response to extension. Results show that fault size and spacing decreases with increasing time fraction spent in the magmatic phase FM. Magmatic phases also simulate the growth of topography in response to local buoyancy forces. The fundamental variable that controls the transition between axial highs and valleys is the “rise‐sink ratio,” (FM/FT)(τT/τM), where FM/FT is the ratio of the time spent in the magmatic and tectonic periods and τT/τM is the ratio of the characteristic rates for growing topography during magmatic phases (1/τM) and for reducing topography during tectonic phases (1/τT). Models predict the tallest axial highs when (FM/FT)(τT/τM) ≫ 1, faulted topography without a high or valley when (FM/FT)(τT/τM) ∼ 1, and the deepest median valleys when (FM/FT)(τM/τT) < 1. New scaling laws explain a global negative correlation between axial topography and lithosphere thickness as measured by the depths of axial magma lenses and microearthquakes. Exceptions to this trend reveal the importance of other behaviors such as a predicted inverse relation between axial topography and spreading rate as evident along the Lau Spreading Center. Still other factors related to the frequency and spatial pervasiveness of magmatic intrusions and eruptions, as evident at the Mid‐Atlantic and Juan de Fuca ridges, influence the rise‐sink‐ratio (FM/FT)(τT/τM) and thus axial morphology.

Sriver, Ryan L.; Huber, Matthew; Nusbaumer, Jesse

doi: 10.1029/2007GC001842pmid: N/A

Sea surface temperature (SST) and near‐surface winds from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and the Quick Scatterometer (QuikScat) are used to calculate globally integrated tropical cyclone‐induced SST anomalies and power dissipation (PD). We estimate tropical cyclone‐induced upper ocean cooling to be ∼35% higher than our previous estimates based on reanalyzed ERA40 and NCEP surface data. Annually averaged, global PD estimates from TMI are ∼5 × 1019 J for the years 1998 to 2006 (roughly 30% greater than ERA40 PD for overlapping years). QuikScat PD is estimated to be ∼1.7 × 1020 J for the years 2000 to 2006. On the basis of these results, we conclude that the cyclone‐induced cooling signal appears to be underrepresented in ERA40 and NCEP reanalysis, as postulated in recent observational and modeling studies. Furthermore, we observe a strong positive relationship between PD and ocean surface cooling, providing further evidence for the likelihood of cyclone‐induced climatic feedbacks. These results support the hypothesis that tropical cyclones play an active role in the tropical surface ocean heat budget by cooling the tropical upper oceans through enhanced vertical mixing, which likely represents a net warming beneath the oceanic mixed layer. Thus, to the degree that vertical mixing is important for regulating the ocean's meridional overturning circulation and poleward heat transport, tropical cyclones may be an important contributor to Earth's climate system. This further confirms the results of Emanuel (2001, 2002) and Sriver and Huber (2007b) that possible future changes in integrated cyclone intensity associated with warmer SST may provide possible climatic feedbacks through enhanced vertical mixing and increased ocean heat transport, thus buffering the tropics to increased temperatures while amplifying the warming at higher latitudes.

White, Scott M.; Meyer, Jason D.; Haymon, Rachel M.; Macdonald, Ken C.; Baker, Edward T.; Resing, Joseph A.

doi: 10.1029/2008GC002036pmid: N/A

The Galápagos Spreading Center (GSC) at 89°–95°W exhibits large gradients in magma supply at a relatively constant intermediate spreading rate, making this area an ideal natural laboratory to study the effects of magma supply on volcanism at seafloor spreading ridges. Prior work shows that the GSC develops from axial valley to shallow axial rise and a shallow magma sill, much like a typical fast spreading ridge, as the contribution of the hot spot increases. The volcanic morphology varies with magma supply in a predictable manner that we divide into three terrains based on the characteristic style of volcanic emplacement and edifice construction within each terrain. The volcanic cone terrain comprises most of the GSC and is characterized by prominent volcanic cones within a >1 km wide and >100 m deep axial graben. Approaching the area of maximum mantle plume influence at 91°W, the GSC axis lies along an elevated axial rise split by a <1 km wide and <100 m deep axial graben, and the style of volcanism shifts to axial volcanic ridge terrain characterized by axis‐elongate, low‐relief ridges of pillow lava. The lava channel terrain comprises only one segment on either side of the maximum magma supply at 91°W, where sheet lava flows and lava channels are relatively widespread. A general lengthening of seafloor fissures with increasing magma supply suggests a greater tendency toward linear source eruptions, in agreement with the volcanic observations. These results suggest that magma supply rather than magma chamber depth or rate of tectonic extension is the primary influence on lava morphology, hence eruptive processes, at seafloor spreading ridges in general. In both the axial volcanic ridge and lava channel terrains, a single prominent volcanic cone exists within each volcanic segment, suggesting a segment‐centered magma focusing.

Dunlop, David J.

doi: 10.1029/2008GC002054pmid: N/A

A vibrating thermomagnetometer which measures magnetization M continuously at high temperature is used to test a new method of paleointensity determination involving single rather than double heatings. Loss of natural remanent magnetization (NRM) is recorded by the average of 20–25 measurements at the peak temperature T achieved in a zero‐field heating step. Partial thermoremanent magnetization (pTRM) is calculated from the average M after in‐field cooling to room temperature T0. For comparison with data taken at T0, values of M measured at T are multiplied by Ms(T0)/Ms(T), with the aid of the thermomagnetic or Ms(T) curve measured on a chip of the sample. NRM versus pTRM results from 11 heating‐cooling steps on a diabase containing both single‐domain magnetite inclusions in plagioclase and coarser multidomain magnetite grains reproduce the features of Thellier double‐heating paleointensity results for samples from the same site. The NRM – pTRM plot is nonlinear, with convex‐down curvature. For a stringent validation of the single‐heating method, a truly single‐domain sample needs to be tested.

Ridente, Domenico; Trincardi, Fabio; Piva, Andrea; Asioli, Alessandra; Cattaneo, Antonio

doi: 10.1029/2007GC001783pmid: N/A

Borehole PRAD1–2 was drilled in ∼186 m water depth on the upper slope of the central Adriatic, in the frame of Profiles across Mediterranean Sedimentary Systems (PROMESS1) European Union‐funded project. The borehole penetrated 71.2 m through a stratigraphic interval characterized by subparallel seismic reflections and uniform seismic units. According to an age‐depth model based on several independent proxies (including foraminifera and nannoplankton stratigraphy, ∂18O curves, and magnetostratigraphy) the cored interval records Marine Isotope Stages and Substages (MIS) from MIS1 to the top of MIS11, thus encompassing the past ∼370 ka. PRAD1–2 therefore represents an unprecedented continuous record through the last four glacial‐interglacial cycles from a proximal continental margin setting where depositional sequences are typically composed of progradational units. These progradational units record dominantly interglacial intervals (MIS5, MIS7, and MIS9) and appear composed of thicker highstand deposits (HST) formed during interstadials and thinner forced‐regression units (FSST) deposited during stadials above distinctive downward shift surfaces. The development of thicker highstand deposits with a distinctively thicker bottomset reflects enhanced shore‐parallel advection any time sea level rise leads to the drowning of the Adriatic shelf, triggering the formation of dense water and vigorous cyclonic circulation. This advection mechanism persisted in each cycle throughout the early phases of the sea level fall but progressively decreased as sea level fall proceeded approaching the maximum lowstand position, when most of the shelf became exposed. Relative sea level falls punctuating interglacials within each 100‐ka cycle were thus accompanied by a dearth in sediment flux on the outer shelf. The alternation of HST and FSST progradational wedges with markedly different thickness and downlap geometry of their bottomsets is the most evident stratigraphic signature, within each 100‐ka depositional cycle, of the impact on the shelf of higher‐frequency (∼20 ka) sea level cycles and concomitant supply fluctuations.

Nedimović, Mladen R.; Carbotte, Suzanne M.; Diebold, John B.; Harding, Alistair J.; Canales, J. Pablo; Kent, Graham M.

doi: 10.1029/2008GC002085pmid: N/A

Recent P wave velocity compilations of the oceanic crust indicate that the velocity of the uppermost layer 2A doubles or reaches ∼4.3 km/s found in mature crust in <10 Ma after crustal formation. This velocity change is commonly attributed to precipitation of low‐temperature alteration minerals within the extrusive rocks associated with ridge‐flank hydrothermal circulation. Sediment blanketing, acting as a thermal insulator, has been proposed to further accelerate layer 2A evolution by enhancing mineral precipitation. We carried out 1‐D traveltime modeling on common midpoint supergathers from our 2002 Juan de Fuca ridge multichannel seismic data to determine upper crustal structure at ∼3 km intervals along 300 km long transects crossing the Endeavor, Northern Symmetric, and Cleft ridge segments. Our results show a regional correlation between upper crustal velocity and crustal age. The measured velocity increase with crustal age is not uniform across the investigated ridge flanks. For the ridge flanks blanketed with a sealing sedimentary cover, the velocity increase is double that observed on the sparsely and discontinuously sedimented flanks (∼60% increase versus ∼28%) over the same crustal age range of 5–9 Ma. Extrapolation of velocity‐age gradients indicates that layer 2A velocity reaches 4.3 km/s by ∼8 Ma on the sediment blanketed flanks compared to ∼16 Ma on the flanks with thin and discontinuous sediment cover. The computed thickness gradients show that layer 2A does not thin and disappear in the Juan de Fuca region with increasing crustal age or sediment blanketing but persists as a relatively low seismic velocity layer capping the deeper oceanic crust. However, layer 2A on the fully sedimented ridge‐flank sections is on average thinner than on the sparsely and discontinuously sedimented flanks (330 ± 80 versus 430 ± 80 m). The change in thickness occurs over a 10–20 km distance coincident with the onset of sediment burial. Our results also suggest that propagator wakes can have atypical layer 2A thickness and velocity. Impact of propagator wakes is evident in the chemical signature of the fluids sampled by ODP drill holes along the east Endeavor transect, providing further indication that these crustal discontinuities may be sites of localized fluid flow and alteration.

Herzberg, C.; Asimow, P. D.

doi: 10.1029/2008GC002057pmid: N/A

PRIMELT2.XLS software is introduced for calculating primary magma composition and mantle potential temperature (TP) from an observed lava composition. It is an upgrade over a previous version in that it includes garnet peridotite melting and it detects complexities that can lead to overestimates in TP by >100°C. These are variations in source lithology, source volatile content, source oxidation state, and clinopyroxene fractionation. Nevertheless, application of PRIMELT2.XLS to lavas from a wide range of oceanic islands reveals no evidence that volatile‐enrichment and source fertility are sufficient to produce them. All are associated with thermal anomalies, and this appears to be a prerequisite for their formation. For the ocean islands considered in this work, TP maxima are typically ∼1450–1500°C in the Atlantic and 1500–1600°C in the Pacific, substantially greater than ∼1350°C for ambient mantle. Lavas from the Galápagos Islands and Hawaii record in their geochemistry high TP maxima and large ranges in both TP and melt fraction over short horizontal distances, a result that is predicted by the mantle plume model.

Mazzarini, F.; Fornaciai, A.; Bistacchi, A.; Pasquarè, F. A.

doi: 10.1029/2008GC002037pmid: N/A

Shield volcanoes, caldera‐bearing stratovolcanoes, and monogenetic cones compose the large fissural Payen Volcanic Complex, located in the Andes foreland between latitude 35°S and 38°S. The late Pliocene‐Pleistocene and recent volcanic activity along E‐W trending eruptive fissures produced basaltic lavas showing a within‐plate geochemical signature. The spatial distribution of fractures and monogenetic vents is characterized by self‐similar clustering with well defined power law distributions. Vents have average spacing of 1.27 km and fractal exponent D = 1.33 defined in the range 0.7–49.3 km. The fractal exponent of fractures is 1.62 in the range 1.5–48.1 km. The upper cutoffs of fractures and vent fractal distributions (about 48–49 km) scale to the crustal thickness in the area, as derived from geophysical data. This analysis determines fractured media (crust) thickness associated with basaltic retroarc eruptions. We propose that the Payen Volcanic Complex was and is still active under an E‐W crustal shortening regime.