Geochemistry and minerals at DSDP Legs 68-70 Holes

The compositions of natural glasses and phenocrysts in basalts from Deep Sea Drilling Project Sites 501, 504, and 505, near the Costa Rica Rift, constitute evidence for the existence of a periodically replenished axial magma chamber that repeatedly erupted lavas of remarkably uniform composition. Magma compositions were affected by three general components: (1) injected magmas carrying (in decreasing order of abundance) Plagioclase, olivine, and chrome-spinel phenocrysts (spinel assemblage); (2) injected magmas carrying Plagioclase, clinopyroxene, and olivine phenocrysts, but no spinel (clinopyroxene assemblage); and (3) moderately evolved hybrids in the magma chamber itself. The compositions of the injected phenocrysts and minerals in glomerocrysts are as follows: Plagioclase - An85-94; olivine - Fo87-89; clinopyroxene - high Cr2O3 (0.7-1.1%), endiopside (Wo42En51Fs7), and aluminous chromian spinel (Cr/Cr + Al = 0.3). These minerals resemble those thought to occur in upper mantle sources (9 kbars and less) of ocean-ridge basalts and to crystallize in magmas near those sources. In the magma chamber, more sodic Plagioclase (An79-85), less magnesian olivine (Fo81-86) and low-Cr2O3 (0.1-0.4%) clinopyroxene formed rims on these crystals, grew as other phenocrysts, and formed cumulus segregations on the walls and floors of the magma chamber. In the spinel-assemblage magmas, magnesiochromite (Cr/Cr + Al = 0.4-0.5) also formed. Some cumulus segregations were later entrained in lavas as xenoliths. The glass compositions define 16 internally homogeneous eruptive units, 13 of which are in stratigraphic order in a single hole, Hole 504B, which was drilled 561.5 meters into the ocean crust. These units are defined as differing from each other by more than analytical uncertainty in one or more oxides. However, many of the glass groups in Hole 504B show virtually no differences in TiO2 contents, Mg/Mg + Fe2+, or normative An/An + Ab, all of which are sensitive indicators of crystallization differentiation. The differences are so small that they are only apparent in the glass compositions; they are almost completely obscured in whole-rock samples by the presence of phenocrysts and the effects of alteration. Moreover, several of the glass units at different depths in Hole 504B are compositionally identical, with all oxides falling within the range of analytical uncertainty, with only small variations in the rest of the suite. The repetition of identical chemical types requires (1) very regular injection of magmas into the magma chamber, (2) extreme similarity of injected magmas, and (3) displacement of very nearly the same proportion of the magmas in the chamber at each injection. Numerical modeling and thermal considerations have led some workers to propose the existence of such conditions at certain types of spreading centers, but the lava and glass compositions at Hole 504B represent the first direct evidence revealed by drilling of the existence of a compositionally nearly steady-state magma chamber, and this chapter examines the processes acting in it in some detail. The glass groups that are most similar are from clinopyroxene-assemblage lavas, which have a range of Mg/Mg + Fe2"1" of 0.59 to 0.65. Spinel-assemblage basalts are less evolved, with Mg/Mg + Fe2+ of 0.65 to 0.69, but both types have nearly identical normative An/An + Ab (0.65-0.66). However, the two lava types contain megacrysts (olivine, Plagioclase, clinopyroxene) that crystallized from melts with Mg/Mg + Fe2+ values of 0.70 to 0.72. Projection of glass compositions into ternary normative systems suggests that spinel-assemblage magmas originated deeper in the mantle than clinopyroxene-assemblage magmas, and mineral data indicate that the two types followed different fractionation paths before reaching the magma chamber. The two magma types therefore represent neither a low- nor a high-pressure fractionation sequence. Some of the spinel-assemblage magmas may have had picritic parents, but were coprecipitating all of the spinel-assemblage phenocrysts before reaching the magma chamber. Clinopyroxene-assemblage magmas did not have picritic parents, but the compositions of phenocrysts suggest that they originated at about 9 kbars, near the transition between plagioclase peridotite and spinel peridotite in the mantle. Two glass groups have higher contents of alkalis, TiO2, and P2O5 than the others, evidently as a result of the compositions of mantle sources. Eruption of these lavas implies that conduits and chambers containing magmas from dissimilar sources were not completely interconnected on the Costa Rica Rift. The data are used to draw comparisons with the East Pacific Rise and to consider the mechanisms that may have prevented the eruption of ferrobasalts at these sites.

High-pressure experiments on a natural olivine gabbro from ODP Hole 176-735B

We have conducted high-pressure experiments on a natural oceanic gabbro composition (Gb108). Our aim was to test recent proposals that Sr-enrichment in rare primitive melt inclusions from Mauna Loa, Hawaii, may have resulted from melting of garnet pyroxenite formed in the magma source regions by reaction of peridotite with siliceous, Sr-enriched partial melts of eclogite of gabbroic composition. Gb108 is a natural, Sr-enriched olivine gabbro, which has a strong positive Sr anomaly superimposed on an overall depleted incompatible trace element pattern, reflecting its origin as a plagioclase-rich cumulate. At high pressures it crystallises as a coesite eclogite assemblage, with the solidus between 1,300 and 1,350°C at 3.5 GPa and 1,450 and 1,500°C at 4.5 GPa. Clinopyroxenes contain 4-9% Ca-eskolaite component, which varies systematically with pressure and temperature. Garnets are almandine and grossular-rich. Low degree partial melts are highly siliceous in composition, resembling dacites. Coesite is eliminated between 50 and 100°C above the solidus. The whole-rock Sr-enrichment is primarily hosted by clinopyroxene. This phase dominates the mode (>75 wt%) at all investigated PT conditions, and is the major contributor to partial melts of this eclogite composition. Hence the partial melts have trace element patterns sub-parallel to those of clinopyroxene with ~10* greater overall abundances and with strong positive Sr anomalies. Recent studies of primitive Hawaiian volcanics have suggested the incorporation into their source regions of eclogite, formerly gabbroic material recycled through the mantle at subduction zones. The models suggest that formerly gabbroic material, present as eclogite in the Hawaiian plume, partially melted earlier than surrounding peridotite (i.e. at higher pressure) because of the lower solidus temperature of eclogite compared with peridotite. This produced highly siliceous melts which reacted with surrounding peridotite producing hybrid pyroxene + garnet lithologies. The Sr-enriched nature of the formerly plagioclase-rich gabbro was present in the siliceous partial melts, as demonstrated by these experiments, and was transferred to the reactive pyroxenite. These in turn partially melted, producing Sr-enriched picritic liquids which mixed with normal picritic partial melts of peridotite before eruption. On rare occasions these mixed, relatively Sr-rich melts were trapped as melt inclusions in primitive olivine phenocrysts.Yaxley-Sobolev

Chemistry of Pliocene-Pleistocene pumice from the Izu-Bonin Arc

Thick pumice deposits were found in the cored sequences of forearc, arc, and backarc sites of Leg 126 in the Izu-Bonin Arc. These deposits, composed of fragmental rhyolite pumice with the chemical composition of low-alkali tholeiites, are products of arc volcanism. Pumice deposits constitute more than half of the thickness of the sediment fill of the Sumisu Rift, a backarc rift of the Izu-Bonin Arc. They comprise five thick pumiceous beds separated by thin hemipelagic units; as such, they record four major episodes or pulses of explosive, rhyolitic volcanism during the last 0.15 Ma, separated by quiescent intervals that each lasted about 30-60 k.y. The thick pumiceous beds were deposited in the rift mainly by sediment gravity flows during and immediately after the eruption of arc volcanos, which were probably submarine. Initiation of rifting was also preceded in the Pliocene by submarine rhyolitic volcanism, as seen in samples from the top of the eastern rift flank. Thick pumice beds correlative with those in the backarc also occur in the forearc basin to the east.

Chemical composition of tephra from the Greenland-Iceland-Norwegian-Sea

Explosive ocean island volcanism in the Greenland-Iceland-Norwegian Sea (GIN Sea) is indicated by marine tephra layers at 10-300 ka. Peaks of explosive volcanism occurred in oxygen isotope stages 8, 7, 5 and 1. The depositional age of the tephra was estimated using the oxygen isotope stratigraphy and dating of marine records. Geochemical analyses of the tephra layers show that all originate from Iceland. Here we report the characteristics of tephra from these major Icelandic events in 30 deep-sea cores from the GIN Sea. Our findings provide constraints on the distribution of tephra from the eruption source. For the Vedde Ash (oxygen isotope stage 1) we estimate a minimum fallout area of 2*10**5 km**2, stretching from central Greenland in the west and southern Sweden in the east, to 71°N in the GIN Sea. The magnitude of the eruption and the regional wind conditions controlled the extent and concentrations of these ash fallout events. Oceanic circulation and differential settling may have affected the distribution and final deposition of ash particles such as bubble wall shards.

Geochemistry of tephra from Lake Ohrid

Here we present a tephrostratigraphic record (core Co1202) recovered from the northeastern part of Lake Ohrid (Republics of Macedonia and Albania) reaching back to Marine Isotope Stage (MIS) 6. Overall ten horizons (OT0702-1 to OT0702-10) containing volcanic tephra have been recognised throughout the 14.94 m long sediment succession. Four tephra layers were visible at macroscopic inspection (OT0702-4, OT0702-6, OT0702-8 and OT0702-9), while the remaining six are cryptotephras (OT0702-1, OT0702-2, OT0702-3, OT0702-5, OT0702-7 and OT0702-10) identified from peaks in K, Zr and Sr intensities, magnetic susceptibility measurements, and washing and sieving of the sediments. Glass shards of tephra layers and cryptotephras were analysed with respect to their major element composition, and correlated to explosive eruptions of Italian volcanoes. The stratigraphy and the major element composition of tephra layers and cryptotephras allowed the correlation of OT0702-1 to AD 472 or AD 512 eruptions of Somma-Vesuvius, OT0702-2 to the FL eruption of Mount Etna, OT0702-3 to the Mercato from Somma-Vesuvius, OT0702-4 to SMP1-e/Y-3 eruption from the Campi Flegrei caldera, OT0702-5 to the Codola eruption (Somma-Vesuvius or Campi Flegrei), OT0702-6 to the Campanian Ignimbrite/Y-5 from the Campi Flegrei caldera, OT0702-7 to the Green Tuff/Y-6 eruption from Pantelleria Island, OT0702-8 to the X-5 eruption probably originating from the Campi Flegrei caldera, OT0702-9 to the X-6 eruption of generic Campanian origin, and OT0702-10 to the P-11 eruption from Pantelleria Island. The fairly well-known ages of these tephra layers and parent eruptions provide new data on the dispersal and deposition of these tephras and, furthermore, allow the establishment of a chronological framework for core Co1202 for a first interpretation of major sedimentological changes.

Table 1: Potassium-argon dates on whole rock lavas from Mt. Giluwe

A series of K-Ar dates from Mt Giluwe volcano is reported and its relevance to the Quaternary history of the volcano is discussed. The period between about 380 000 and 220 000 years BP seems to have been one of major volcanic activity. During the volcanic activity there were periods of ice cover probably of short duration. The oldest evidence of glacial action predates a lava flow dated at between 340 000 and 380 000 years. At about 290 000 years an ice cap of a thickness of at least 100 m covered the summit area and one or a series of subglacial eruption(s) led to the formation of palagonitic breccia. This event was probably associated with a complete melting of the ice since it was followed almost immediately by the eruption of a thick sequence of normal lava flows which range in age from about 289 000 years to about 220 000 years. Subsequent volcanic activity was less significant and no dates are available on this.

(Table 1) Radiocarbon results from this study

The Indonesian Arc represents the subduction of the Indian-Australian plate beneath Asia. It has been the scene of catastrophic tectonic activity, including the recent 2004 M=9.1 Aceh earthquake and resulting Indian Ocean tsunami. We have dated planktonic forams associated with historic tephras (Tambora, 1815 and Krakatau, 1883) in marine sediment cores to determine radiocarbon reservoir ages for 2 locations along the arc. Our best estimates for 19th century regional reservoir corrections (DeltaR) are +90 ± 40 yr for surface-dwelling species and +220 ± 40 yr for mixed planktic assemblages containing some upper thermocline species, but scatter in the data suggests that past surface reservoir ages may have varied by about ±100 yr. We used the results of this study to investigate a proposed very large AD 535 eruption at or near Krakatau. We find no evidence for ash from such an eruption, and although this is negative evidence, we consider it sufficiently strong to rule out any possibility that one took place.

Summary of 40Ar/39Ar results from sediment core CRP-2A (Table 1)

40Ar/39Ar analyses of tephra and clasts of volcanic rock provide age constraints for upper parts of the CRP-2A core. Single-crystal laser-fusion analyses of anorthoclase phenocrysts from three tephra-bearing layers yielded the most precise age constraints for CRP-2A. The dated tephra layers are: 1) a 2.7-m-thick interval of pumice and ash layers between 111.5 and 114.2 meters below sea floor (mbsf) (weighted mean age = 21.44 ± 0.05 Ma, +2.2); 2) a concentration of pumice near 193.4 mbsf (23.98 ± 0.13 Ma): and 3) a concentration of pumice near 280 mbsf (24,22 ± 0.03 Ma) (all ages are calibrated relative to Fish Canyon Tuff sanidine at 27.84 Ma). The 111 to 114 mbsf tephra is almost entirely non-reworked, and the 193 mbsf and 280 mbsf tephra concentrations are interpreted as being reworked and redeposited soon after eruption. All three of the tephra ages are therefore considered to be equivalent to depositional ages. The variation in precision of these three age determinations is largely a function of phenocryst size and abundance. The accuracy of these ages is equal to the accuracy of the current calibration of the 40Ar/39Ar methode (about ± 1 %). 40Ar/39Ar results from volcanic clasts provide three additional maximum age constraints for the CRP-2A core. Single-crystal laser-fusion of sanidine phenocrysts from a rhyolitic clast from 294 mbsf yielded a precise maximum depositional age of 24.98 ± 0.08 Ma, and plateau ages of groundmass concentrates from basaltic clasts near 36.02 mbsf and 125.92 mbsf yielded maximum depositional ages of 19.18 ± 0.12 Ma, and 22.56 ± 0.14 Ma, respectively. The 40Ar/39Ar data, in association with biostratigraphic, paleomagnetic, and isotopic age constraints for CRP-2A, confirm interpretation for rapid sedimentation rates in the 36 to 280 mbsf interval, particularly in the 193 to 280 mbsf interval where they support interpretations for sedimentation cycles spanning 100 k.y. intervals. In addition to the 19 to 25 Ma ages measured from thephra layers and clasts, provenance-related ages ranging from 150 to 450 Ma were determined from clasts and individual detrital or xenocrystic crystals from CRP-2A.

Magnetic properties of maar lake sediments from Otago, New Zealand

Foulden Maar is a highly resolved maar lake deposit from the South Island of New Zealand comprising laminated diatomite punctuated by numerous diatomaceous turbidites. Basaltic clasts found in debris flow deposits at the base of the cored sedimentary sequence yielded two new 40Ar/39Ar dates of 24.51±0.24 Ma and 23.38±0.24 Ma (2sigma). The younger date agrees within error with a previously published 40Ar/39Ar date of 23.17±0.19 Ma from a basaltic dyke adjacent to the maar crater. The diatomite is inferred to have been deposited over several tens of thousands of years in the latest Oligocene/earliest Miocene, and may have overlapped with the period of rapid glaciation and subsequent deglaciation of Antarctica known as the Mi-1 event. Sediment magnetic properties and SEM measurements indicate that the magnetic signal is dominated by pseudo-single domain pyrrhotite. The most likely source of detrital pyrrhotite is schist country rock fragments from the inferred tephra ring created by the phreatomagmatic eruption that formed the maar. Variations in magnetic concentration and lamina thickness indicate a decrease in erosional input and increase in diatom productivity throughout the depositional period, suggesting a long-term (tens of thousands of years) climatic change in New Zealand in the latest Oligocene/earliest Miocene.

Temperature measurements, gas chemistry, and gas hydrates of the Dvurechenskii mud volcano

The sediment temperature distribution at mud volcanoes provides insights into their activity and into the occurrence of gas hydrates. If ambient pressure and temperature conditions are close to the limits of the gas hydrate stability field, the sediment temperature distribution not only limits the occurrence of gas hydrates, but is itself influenced by heat production and consumption related to the formation and dissociation of gas hydrates. Located in the Sorokin Trough in the northern Black Sea, the Dvurechenskii mud volcano (DMV) was in the focus of detailed investigations during the M72/2 and M73/3a cruises of the German R/V Meteor and the ROV Quest 4000 m in February and March 2007. A large number of in-situ sediment temperature measurements were conducted from the ROV and with a sensor-equipped gravity corer. Gas hydrates were sampled in pressurized cores using a dynamic autoclave piston corer (DAPC). The thermal structure of the DMV suggests a regime of fluid flow at rates decreasing from the summit towards the edges of the mud volcano, accompanied by intermittent mud expulsion at the summit. Modeled gas hydrate dissociation temperatures reveal that the gas hydrates at the DMV are very close to the stability limits. Changes in heat flow due to variable seepage rates probably do not result in changes in sediment temperature but are compensated by gas hydrate dissociation and formation.

Potassium and Argon data for dating the core, and radiochemical data of a series of measurements made on layers inside various manganese nodules from the Pacific Ocean

Although various models have been proposed to explain the origin of manganese nodules (see Goldberg and Arrhenius), two major hypotheses have received extensive attention. One concept suggests that manganese nodules form as the result of interaction between submarine volcanic products and sea water. The common association of manganese nodules with volcanic materials constitutes the main evidence for this theory. The second theory involves a direct inorganic precipitation of manganese from sea water. Goldberg and Arrhenius view this process as the oxidation of divalent manganese to tetravalent manganese by oxygen under the catalytic action of particulate iron hydroxides. Manganese accumulation by the Goldberg and Arrhenius theory would be a relatively slow and comparatively steady process, whereas Bonatti and Nayudu believe manganese nodule formation takes place subsequent to the eruption of submarine volcanoes by the acidic leaching of lava.

Global impacts of the 1980s regime shift

Despite evidence from a number of Earth systems that abrupt temporal changes known as regime shifts are important, their nature, scale and mechanisms remain poorly documented and understood. Applying principal component analysis, change-point analysis and a sequential t-test analysis of regime shifts to 72 time series, we confirm that the 1980s regime shift represented a major change in the Earth's biophysical systems from the upper atmosphere to the depths of the ocean and from the Arctic to the Antarctic, and occurred at slightly different times around the world. Using historical climate model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and statistical modelling of historical temperatures, we then demonstrate that this event was triggered by rapid global warming from anthropogenic plus natural forcing, the latter associated with the recovery from the El Chichón volcanic eruption. The shift in temperature that occurred at this time is hypothesized as the main forcing for a cascade of abrupt environmental changes. Within the context of the last century or more, the 1980s event was unique in terms of its global scope and scale; our observed consequences imply that if unavoidable natural events such as major volcanic eruptions interact with anthropogenic warming unforeseen multiplier effects may occur.

Global impacts of the 1980s regime shift

Despite evidence from a number of Earth systems that abrupt temporal changes known as regime shifts are important, their nature, scale and mechanisms remain poorly documented and understood. Applying principal component analysis, change-point analysis and a sequential t-test analysis of regime shifts to 72 time series, we confirm that the 1980s regime shift represented a major change in the Earth's biophysical systems from the upper atmosphere to the depths of the ocean and from the Arctic to the Antarctic, and occurred at slightly different times around the world. Using historical climate model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and statistical modelling of historical temperatures, we then demonstrate that this event was triggered by rapid global warming from anthropogenic plus natural forcing, the latter associated with the recovery from the El Chichón volcanic eruption. The shift in temperature that occurred at this time is hypothesized as the main forcing for a cascade of abrupt environmental changes. Within the context of the last century or more, the 1980s event was unique in terms of its global scope and scale; our observed consequences imply that if unavoidable natural events such as major volcanic eruptions interact with anthropogenic warming unforeseen multiplier effects may occur.

Argaric Sociology: Sex and Death

The detailed, rich and diverse Argaric funerary record offers an opportunity to explore social dimensions that usually remain elusive for prehistoric research, such us social rules on kinship rights and obligations, sexual tolerance and the role of funerary practices in preserving the economic and political organization. This paper addresses these topics through an analysis of the social meaning of Argaric double tombs by looking at body treatment and composition of grave goods assemblages according to gender and class affiliation. The Argaric seems to have been a conservative society, scarcely tolerant regarding homosexuality, and willing to celebrate ancestry associated to certain places as a means of asserting residence and property rights.

Late Bronze Age in Eastern Iberian Peninsula: Archaeological Bases and Periodization

Research surrounding the transition from II to I millennium cal BC in Eastern Iberian Peninsula has a large and extensive tradition of investigation. However, the chances to do research on this historical process have been limited by the lack of a well contextualized and dated stratigraphic sequence. For this reason, recent studies in this topic have followed the periodic proposals which were developed in closer regions and areas, especially in the South East and North East of the Peninsula. The investigation perspective about the Late Bronze in Eastern Iberian has however now improved, with the development of several archaeological investigations, the increase in the number of sites being dated and more recent studies into the region helping to bring about this change. As such, it is now in the correct state to be able to propose a new periodization and delve into the changes which occurred in the transition between 1500 and 725 cal BC.