Oceanic Melting Research Articles

Geochemical evidence for melting of carbonated peridotite on Santa Maria Island, Azores

The islands of the Azores archipelago emerge from an oceanic plateau built on lithosphere increasing in age with distance from the Mid-Atlantic Ridge from 10 to 45 Ma. Here, we present the first comprehensive major and trace element and Sr–Nd–Pb isotope data from Santa Maria, the easternmost island of the archipelago, along with published data from the other Azores islands situated much closer to the Mid-Atlantic Ridge axis. We can show that the distinctively more variable and more enriched trace element ratios at Santa Maria combined with a relatively small range in Sr–Nd–Pb isotope ratios are the result of low degrees of partial melting of a common Azores mantle plume source underneath thicker lithosphere. This implies that melt extraction processes and melting dynamics may be able to better preserve the trace element mantle source variability underneath thicker lithosphere. These conclusions may apply widely for oceanic melts erupted on relatively thick lithosphere. In addition, lower Ti/Sm and K/La ratios and SiO2 contents of Santa Maria lavas imply melting of a carbonated peridotite source. Mixing of variable portions of deep small-degree carbonated peridotite melts and shallow volatile-free garnet peridotite could explain the geochemical variability underneath Santa Maria in agreement with the volatile-rich nature of the Azores mantle source. However, Santa Maria is the Azores island where the CO2-rich nature of the mantle source is more evident, reflecting a combination of a smaller extent of partial melting and the positioning at the edge of the tilted Azores mantle plume.

Two types of alkaline volcanics in the southwestern Iberian margin: The causes of their diversity

The diverse geodynamic conditions of the parental magma’s melting are responsible for the compositional diversity of the alkaline volcanics near the southwestern margin of Iberia. The petrological-geochemical data show that the volcanics of the Gorringe Bank originated within the continental plate. The parental melilitite melts depleted in silica and anomalously enriched with trace elements could have been generated only in deep settings with a low degree of metasomatically enriched mantle matter melting. The volcanic melilitite-nephelinite-phonolite series is widespread in alkaline provinces of the Eurasian, African, and other continental plates. The Ampere, Josephine, and other seamounts and islands of the region are largely composed of volcanic rocks belonging to the picrobasalt-hawaiite-mugearite association. Their parental magmas were generated within the oceanic plate at shallower depths under a higher degree of moderately enriched oceanic lithospheric mantle melting. Both series of volcanics were formed under the influence of mantle plumes.

Reprint of: Modeling Antarctic ice sheet and climate variations during Marine Isotope Stage 31

Marine Isotope Stage 31 (MIS-31) is one of the major interglacials of the early Pleistocene ~1.08 to 1.06Ma. Data from proximal sediment cores around several sectors of Antarctica indicate strong sea surface warming and ice shelf and sea ice retreat. Benthic deep-sea-core δ18O values at this time are some of the lowest of the Pleistocene, indicating both deep sea warming and reduced global ice volume. A coeval alignment of orbital parameters produces one of the strongest high-latitude summer insolation anomalies of the last several million years. Here we use a 3-D ice sheet-shelf model to simulate the evolution of Antarctic ice sheets through the event, and a global climate model to simulate temperatures and sea ice during peak Antarctic warmth. The ice model predicts nearly complete collapse and subsequent recovery of marine ice in West Antarctica, and the ice and climate model results agree well with proximal sediment core data in the Ross Embayment recovered by the ANDRILL and Cape Roberts drilling projects. The dominant forcing is found to be variations in sub-ice-shelf oceanic melting, with insignificant surface melting of terrestrial ice flanks even during peak warmth. Implications are noted in light of other observations and theories of Pliocene–Pleistocene Antarctic ice sheet variability that do involve surface melt.

Modeling Antarctic ice sheet and climate variations during Marine Isotope Stage 31

Marine Isotope Stage 31 (MIS-31) is one of the major interglacials of the early Pleistocene ~1.08 to 1.06Ma. Data from proximal sediment cores around several sectors of Antarctica indicate strong sea surface warming and ice shelf and sea ice retreat. Benthic deep-sea-core δ18O values at this time are some of the lowest of the Pleistocene, indicating both deep sea warming and reduced global ice volume. A coeval alignment of orbital parameters produces one of the strongest high-latitude summer insolation anomalies of the last several million years. Here we use a 3-D ice sheet-shelf model to simulate the evolution of Antarctic ice sheets through the event, and a global climate model to simulate temperatures and sea ice during peak Antarctic warmth. The ice model predicts nearly complete collapse and subsequent recovery of marine ice in West Antarctica, and the ice and climate model results agree well with proximal sediment core data in the Ross Embayment recovered by the ANDRILL and Cape Roberts drilling projects. The dominant forcing is found to be variations in sub-ice-shelf oceanic melting, with insignificant surface melting of terrestrial ice flanks even during peak warmth. Implications are noted in light of other observations and theories of Pliocene–Pleistocene Antarctic ice sheet variability that do involve surface melt.

Ice-shelf basal channels in a coupled ice/ocean model

AbstractA numerical model for an interacting ice shelf and ocean is presented in which the ice- shelf base exhibits a channelized morphology similar to that observed beneath Petermann Gletscher’s (Greenland) floating ice shelf. Channels are initiated by irregularities in the ice along the grounding line and then enlarged by ocean melting. To a first approximation, spatially variable basal melting seaward of the grounding line acts as a steel-rule die or a stencil, imparting a channelized form to the ice base as it passes by. Ocean circulation in the region of high melt is inertial in the along-channel direction and geostrophically balanced in the transverse direction. Melt rates depend on the wavelength of imposed variations in ice thickness where it enters the shelf, with shorter wavelengths reducing overall melting. Petermann Gletscher’s narrow basal channels may therefore act to preserve the ice shelf against excessive melting. Overall melting in the model increases for a warming of the subsurface water. The same sensitivity holds for very slight cooling, but for cooling of a few tenths of a degree a reorganization of the spatial pattern of melting leads, surprisingly, to catastrophic thinning of the ice shelf 12 km from the grounding line. Subglacial discharge of fresh water along the grounding line increases overall melting. The eventual steady state depends on when discharge is initiated in the transient history of the ice, showing that multiple steady states of the coupled system exist in general.

The response of Petermann Glacier, Greenland, to large calving events, and its future stability in the context of atmospheric and oceanic warming

AbstractThis study assesses the impact of a large 2010 calving event on the current and future stability of Petermann Glacier, Greenland, and ascertains the glacier’s interaction with different components of the climate and ocean system. We use a numerical ice-flow model that captures the major aspects of the glacier’s mass budget, the resistive forces controlling glacier flow, and includes dynamic calving. Satellite observations and model results show that the recent break-off of 25% of the floating tongue did not result in a significant glacier speed-up due to the low lateral resistance of this relatively wide and thin ice tongue. We demonstrate that seasonal speed-up at Petermann Glacier is mainly driven by meltwater lubrication rather than freeze-up conditions in the fjord. Results also show that sub-shelf ocean melt may have a profound effect on the future stability of Petermann Glacier, emphasizing the urgent need for more observations, and a better understanding of fjord temperature variability and circulation.

Impact of ice-shelf basal melting on inland ice-sheet thickness: a model study

AbstractIce flow from the ice sheets to the ocean contains the maximum potential contributing to future eustatic sea-level rise. In Antarctica most mass fluxes occur via the extended ice-shelf regions covering more than half the Antarctic coastline. The most extended ice shelves are the Filchner–Ronne and Ross Ice Shelves, which contribute ~30% to the total mass loss caused by basal melting. Basal melt rates here show small to moderate average amplitudes of <0.5ma–1. By comparison, the smaller but most vulnerable ice shelves in the Amundsen and Bellinghausen Seas show much higher melt rates (up to 30 ma–1), but overall basal mass loss is comparably small due to the small size of the ice shelves. The pivotal question for both characteristic ice-shelf regions, however, is the impact of ocean melting, and, coevally, change in ice-shelf thickness, on the flow dynamics of the hinterland ice masses. In theory, ice-shelf back-pressure acts to stabilize the ice sheet, and thus the ice volume stored above sea level. We use the three-dimensional (3-D) thermomechanical ice-flow model RIMBAY to investigate the ice flow in a regularly shaped model domain, including ice-sheet, ice-shelf and open-ocean regions. By using melting scenarios for perturbation studies, we find a hysteresis-like behaviour. The experiments show that the system regains its initial state when perturbations are switched off. Average basal melt rates of up to 2 ma–1 as well as spatially variable melting calculated by our 3-D ocean model ROMBAX act as basal boundary conditions in time-dependent model studies. Changes in ice volume and grounding-line position are monitored after 1000 years of modelling and reveal mass losses of up to 40 Gt a–1.

Understanding and Modelling Rapid Dynamic Changes of Tidewater Outlet Glaciers: Issues and Implications

Recent dramatic acceleration, thinning and retreat of tidewater outlet glaciers in Greenland raises concern regarding their contribution to future sea-level rise. These dynamic changes seem to be parallel to oceanic and climatic warming but the linking mechanisms and forcings are poorly understood and, furthermore, large-scale ice sheet models are currently unable to realistically simulate such changes which provides a major limitation in our ability to predict dynamic mass losses. In this paper we apply a specifically designed numerical flowband model to Jakobshavn Isbrae (JIB), a major marine outlet glacier of the Greenland ice sheet, and we explore and discuss the basic concepts and emerging issues in our understanding and modelling ability of the dynamics of tidewater outlet glaciers. The modelling demonstrates that enhanced ocean melt is able to trigger the observed dynamic changes of JIB but it heavily relies on the feedback between calving and terminus retreat and therefore the loss of buttressing. Through the same feedback, other forcings such as reduced winter sea-ice duration can produce similar rapid retreat. This highlights the need for a robust representation of the calving process and for improvements in the understanding and implementation of forcings at the marine boundary in predictive ice sheet models. Furthermore, the modelling uncovers high sensitivity and rapid adjustment of marine outlet glaciers to perturbations at their marine boundary implying that care should be taken in interpreting or extrapolating such rapid dynamic changes as recently observed in Greenland.

Sensitivity of 21st century sea level to ocean‐induced thinning of Pine Island Glacier, Antarctica

Pine Island Glacier (PIG), Antarctica, is rapidly losing mass, supporting arguments that it may play a major role in 21st century sea‐level rise. Yet this glacier's quantitative contribution to sea level based on theoretical and computational models is poorly known. We have developed a basin‐scale glaciological model to examine the sensitivity of PIG to a range of environmental forcings. While oceanic melt likely played the leading role in recent thinning and retreat, we find that the particular grounding‐line geometry with an extended ice plain in the 1990s made it susceptible to such forcing. Our model further indicates that while the rate of grounding‐line retreat should diminish soon, the glacier's mass loss may continue at rates similar to, or moderately elevated from, the present. While substantial, our model‐derived maximum rate of 2.7 cm/century is considerably smaller than previous heuristically‐derived bounds on the sea‐level contribution.

Tides, the PIG, and "warm" water

The present rapid melting of the Pine Island Glacier (PIG) has been attributed to basal melting driven by the ocean. Specifically, this ocean melting is attributed to currents and tides pumping "warm" Circumpolar Deep Water (CDW) into the ice shelf cavity. To identify tidal activity in the region, an observational time series of yo-yo CTD (Conductivity, Temperature, and Depth) data collected in the PIG outflow region was analyzed. The water column in front of the PIG consisted of two primary layers, a meltwater layer exiting the ice shelf cavity over a layer of CDW. Semidiurnal tides were present in both layers, with both the strength and direction of the tides differing between the two layers. The upper layer tides were stronger and directed in and out of the cavity, while the lower layer tides were primarily directed along the front of the cavity. Energy was found to be transferred from the semidiurnal tide to other frequencies and to be reflected by the ice shelf front. These mechanisms were most prominent at the interfaces between layers and indicate potential mixing between the layers. In conclusion, tides were found to contribute to the circulation into the ice shelf cavity and also to mixing of the exiting water, which influences pumping of the CDW into the ice shelf cavity and melting of the PIG.

Impact of drifting icebergs on surface phytoplankton biomass in the Southern Ocean: Ocean colour remote sensing and in situ iceberg tracking

Icebergs that calve from the Antarctic ice shelves and drift in the Southern Ocean melt to deliver fresh water, dust and minerogenic particles to the surface ocean along the iceberg's path. Each of these components may have an effect on growth conditions for phytoplankton, as might the mechanical effects of the iceberg keel disturbing the water. Although anecdotal and small-scale surveys suggest that drifting icebergs increase local primary production, no large-scale studies have been reported. An analysis of satellite and automated iceberg tracking data from the Weddell Sea, covering the months October to March, from 1999 to 2004, showed that the probability of increased surface phytoplankton biomass was up to one-third higher in the wake of a tracked iceberg compared to background biomass fluctuations. Only during the month of February were the effects of icebergs on surface biomass likely to be negative, whereas background biomass fluctuations were likely to be negative during March. These results confirm icebergs as a factor affecting phytoplankton in the Southern Ocean and highlight the need for detailed process studies so that responses to future changes in the Antarctic ice sheets may be predicted.

Geochemistry, petrogenesis, and tectonic setting of lower Paleozoic alkalic and potassic volcanic rocks, Northern Canadian Cordilleran Miogeocline

The Northern Canadian Cordilleran Miogeocline developed intermittently during the early Paleozoic and hosts alkalic and ultrapotassic volcanic rocks that are spatially restricted in thin beds and lenses and isolated volcanic piles. On the basis of geochemistry and geographic location, these volcanic rocks are subdivided into five main groups. Group I rocks (Porter Puddle and Macmillan rocks) are potassic basanites characterized by high Nb, Ce, and NbIY and low ZrINb. They are chemically similar to the Mountain Diatreme, indicating a genetic link. Group 11 rocks (Porter Puddle, Niddery, and Macmillan rocks) are also potassic but have lower abundances of Nb and Ce, higher ZrINb, and lower NbIY. Group I11 rocks (Vulcan and Itsi Lakes) are also potassic but are chemically variable, have lower contents of high field strength elements (HFSE) than the groups I and I1 rocks, and contain elevated Ba contents. Groups 1-111 are characterized by mica (biotite and phlogopite) phenocrysts, sanidine, augite, and Ba-feldspar, a mineral assemblage typical of ultrapotassic lavas. Group IV (Whale Mountain) alkali basalts are the least enriched in the large ion lithophile elements and have relatively low contents of HFSE compared with Groups I and I1 basalts. Groups I-111 are consistent with partial melting of a previously metasomatized lithospheric mantle that was variably enriched in Ba, Nb, and Ce, whereas the group IV rocks are more typical of asthenospherically derived oceanic island basalt partial melts. The geochemistry of the volcanic rocks is consistent with paleotectonic models of the Selwyn Basin. The Selwyn Basin is a passive continental rift that underwent episodic extension and associated subsidence throughout the lower Paleozoic. Alkalic volcanism, and spatially and temporally associated Ba and base metal mineralization, is concentrated along rift-parallel normal faults, particularly where these faults are offset by transform faults.

Mantle plumes, continental magmatism and asymmetry in the South Atlantic

Melt production rates are estimated along the ocean tracks (Rio Grande Rise, Walvis Ridge) and the onshore Paraná/Etendeka Flood Basalt Province for the mantle plume currently associated with Tristan da Cunha in the South Atlantic. Assuming that the volcanic edifice is locally compensated with a thickened crust, the maximum oceanic rate (∼ 0.3 km 3 a - 1) is similar to the value for the continental flood basalt, calculated using the surface volume of lava and a duration of 5 m.y. However, this latter estimate is conservative, and the appropriate rate is probably at least a factor of 5 higher. The maximum mantle potential temperature for the ocean plume tracks is estimated to have been 1450–1540°C, assuming a dry peridotite source. Using a similar model, temperatures in excess of 1600°C are required to explain the observed magma volumes onshore. This is because there is no significant extension in the region where the oldest lavas occur. It is argued that the observed volumes of melt can be generated with small amounts of extension and with plume temperatures of < 1500°C if melting in the lithosphere is strongly influenced by the relative enrichment of volatiles. Moreover, the model predicts the inferred change to an asthenosphere-dominated source when significant extension occurs. Recent geochonological data have shown that the continental volcanics are ∼ 138 Ma in the northwest of the Paraná province and become progressively younger to the southeast. Oceanic melt production rates decrease by a factor of 3–5 towards the mid-ocean ridge/Tristan da Cunha. Intriguingly, melt production rates on the Rio Grande Rise vary to more than twice that of the Walvis Ridge for oceanic crust of similar age. This asymmetry is also seen in the continental regions, most obviously in the preserved volume of lava in the Paraná and Etendeka, but also in the topography of the underplated continental margin and the orientation of dykes and oblique extension structures. These observations reflect the combined roles of the plume temperature, lithospheric thickness, crustal tectonics and magma source region composition during the evolution of the South Atlantic over the last 150 Ma.

Mantle Convection and Volcanic Periodicity in the Pacific; Evidence from Hawaii

The thermal-feedback theory of mantle melting proposed by Shaw in 1969 is found to be quantitatively consistent with data pertaining to the evolution of the Hawaiian Ridge. Applicable rate factors are estimated from relations between lava volumes and position along the ridge given in this paper and the radio-metric age distributions given by Jackson and others in 1972. Rate curves derived from these data provide a new method of age extrapolation or interpolation; results indicate that previous methods used to estimate the age of the Hawaiian-Emperor Bend are in error. No definite age is established, but calculations suggest an age greater than 50 m.y. Much more extensive radiometric data are required to define kinematic relations between the Hawaiian Ridge and Emperor Seamount chain. It appears to be firmly established from the work of Jackson and others and from the present study that the evolution of the Hawaiian Ridge has been episodic, with episodes of several different time scales. Average growth rates of the entire ridge system are divided into two regimes with a discontinuity at a position roughly 1,000 km northwest of Kilauea; the estimated age of this discontinuity is about 10 m.y. Other episodes relate to the durations of eruptive sequences along individual or contiguous lines of volcanoes within the en echelon set of locus lines defined by Jackson and others. The latest of these episodes, beginning about 6 m.y. ago, is marked by accelerating volume rates of eruption and accelerating rates of ridge propagation; this episode appears to be approaching a culminating stage represented by the present activity of Kilauea Volcano. The calculated rate of eruption of Kilauea (0.11 km 3 per yr) is virtually identical with a rate independently estimated by Swanson in 1972 using different data. Calculated durations for older locus lines are generally greater than 6 m.y., but major time overlaps occur that are not adequately understood. Episodic behavior of shorter durations also exists relative to growth of individual shields or to synchronous activity on neighboring shields (for example, Mauna Loa and Kilauea). Some of these shorter term effects are partly explained in terms of isostatic factors acting on the lithosphere and asthenosphere. The longer episodes are explained in terms of variations of melting rates in the asthenosphere, governed by viscous heating produced by the interaction of lithosphere translation and both vertical and horizontal shear flows in the subjacent mantle. Accelerations of eruption and propagation rates are explained by melting instabilities in the upper zones of the asthenosphere as a result of thermal feedback. During the latest melting episode, shear stresses in the asthenosphere derived from the rate data as interpreted by the thermal feedback model are in the range 100 to 200 bars; apparent viscosities range from 2 × 10 21 to 4 × 10 20 poise, decreasing with increasing melting rate. In general, a thermomechanical model is shown to be consistent with the idea that oceanic melting spots can be fixed relative to the deep mantle, although this invariance is not completely established. The thermal plume model of Morgan is not definitely ruled out but does not seem to be required for internally consistent interpretations of oceanic chains of volcanism. It is concluded that motion vectors of the Pacific plate cannot be inferred directly from rates of propagation of volcanic chains, because these rates reflect local, not average, relative velocities of lithosphere versus mantle flow. During growth of the Hawaiian Ridge, propagation speeds calculated on the basis of rate data for the southeastern Hawaiian Islands ranged from less than 1 cm per yr near the Hawaiian-Emperor Bend to nearly 30 cm per yr at the present ridge front.