Dragoslav Ninkovich

Paleomagnetic stratigraphy, rates of deposition and tephrachronology in North Pacific deep-sea sediments

The paleomagnetic stratigraphy of 12 North Pacific deep-sea sediment cores has been investigated and has been used to date volcanic eruptions and to determine rates of deposition of pelagic sediments. Only four of the cores penetrated sediments deposited before the last reversal of the earths magnetic field (0.7 m.y.). Of these, one penetrated to the Gauss series, two to sediments deposited during the Olduvai event and one penetrated to the middle of the Matuyama series. Eight other cores, 10–16 meters long, taken within 1000 km of the Japan-Kuril-Kamtchatka arc failed to reach the Matuyama series. The rate of deposition in North Pacific pelagic sediments vary from2cm/1000y in the area east of the Asiatic continent to< 0.8cm/1000y in the mid Pacific. Assuming continuous deposition, the length of the Jaramillo event can be established as 50 000 y and the Olduvai event as 14 000 y. The apparent length of time during which the dipole field of the earth was reduced during reversals of the earths magnetic field is approximately 20 000 y. In one of the cores the top of the Olduvai event is split. This may represent the Gilsa event. The brown volcanic ash present in three of the cores apparently originated in an eruption 1.2 m.y. ago in the Aleutian Arc near the Andreanof Islands.

Distribution, stratigraphic position and age of ash layer “L”, in the Panama Basin region

Abstract Deep-sea sediments in the Panama Basin-Carnegie Ridge area contain biogenic material, detrital carbonate, mineral clay and volcanic ash layers. Ash layer “L” of Bowles et al. (1973) is correlated mineralogically and by the physical property of glass shards through sixteen cores. Isopachs and grain-size analyses of the ash layer indicate that it originated in Colombia or Ecuador, and was carried by easterly winds. The distribution of the ash and of mica percentage in the ash form a W-shaped pattern opening towards the west. This suggests that two branches of the Cromwell current, one moving along the equator and one along 3°S, had a significant influence on the distribution of the ash. CaCO 3 content has been measured down thirteen cores and oxygen isotope content of benthonic Foraminifera obtained in two. Two cores penetrate the Stylatractus universus extinction datum. Ash “L” fell 230,000 years ago during the cold substage 7b of isotope stage 7. Deposition rates vary between 2.5 and 6 cm/1000 yr and show no relationship with bottom topography or proximity to land.

Mediterranean island arcs and origin of high potash volcanoes

Abstract Active volcanoes of the Mediterranean Sea are distributed along two arc structures: the Hellenic arc in the Aegean Sea and the Calabrian arc in the Tyrrhenian Sea. The active volcanoes in both arcs lie above earthquakes with focal depth greater than 100 km. The depth of these earthquakes increases generally northward reaching a maximum depth of about 200 km in the Aegean Sea and more than 300 km in the Tyrrhenian Sea. The K 2 O versus SiO 2 value in volcanic rocks of the active volcanoes of the Hellenic and Calabrin arcs increases with increasing depth of underlying earthquakes which is similar to the pattern found in Pacific and Indonesian volcanic arcs. The high potash rocks of the Mediterranean suite are the culmination of this trend toward increasing potash as earthquake depth increases. The ratio of the trace element rubidium to silica also increases with increasing depth of earthquakes. Both the intermediate earthquakes and volcanic activity in the Mediterranean, as in Indo-Pacific volcanic arcs, are considered to have originated in dehydration of oceanic crust underthrusting the Aegean and Tyrrhenian Seas as a consequence of a counterclockwise rotation of Africa relative to Eurasia. The released water, we believe, works its way toward the surface in a hydrous melt scavenging potash and other alkali on the way. The ultimate ratio of K 2 O versus SiO 2 and Rb versus SiO 2 will be determined by the temperature of the hydrous melt and the distance traveled through the asthenosphere.

Pleistocene volcanic eruptions in New Zealand recorded in deep-sea sediments

Abstract Five layers of rhyolitic ash have been identified in deep-sea cores, taken within about 1000 km east of New Zealand. Paleomagnetic stratigraphy has been established in the cores and used for dating of the ash layers. The earliest sediment penetrated is about 3 m.y. old, and the ages of the ash layers are as follows: 0.86; 0.73; 0.67; 0.31, and 0.27 m.y. B.P. The ash layers have the same age range as the New Zealand ignimbrites, suggesting a significant ash fall phase, until now little known, associated with eruptions of ignimbrites.

The paleomagnetism of two Aegean deep-sea cores

Abstract Two cores from the Aegean Sea were continuously sampled for paleomagnetic measurements. Sedimentation rates of 75.5 cm/1000 yr (V10-50) and 18.2 cm/1000 yr (V10-58) were derived from 14 C dates of sapropelic mud and ash layers, one of which, the Santorini ash, was present in both cores and was used for correlation. Most of the sediment in each core consisted of calcareous lutite of probable detrital origin. The magnetism of the cores was due to fine-grained magnetite, also of probable detrital origin and uniformly distributed in the lutite. For the total time represented by these cores (27 000 yr) all the samples were normally magnetized. The average inclination corresponded to that of an axial dipole. A cyclic fluctuation in inclination with an approximate period of 6000 yr is thought to be associated with anticlockwise precession of the dipole axis around the rotation axis. Another smaller fluctuation with period around 1000 yr is probably due to westward drift of the non-dipole field. The intensity data were in general agreement with trends established by archeomagnetic investigators.

Late cenozoic clockwise rotation of Sumatra

Abstract A clockwise rotation of Sumatra of about 20° about an axis located in or near the Sunda Strait has been inferred on the basis of the following data: (1) The portion of the Indonesian volcanic arc between the Sunda Strait and the island of Timor lies along a small circle whose center is located about 32°N, 119°E. The volcanic chain of Sumatra makes an angle of 20° with this portion of the arc. (2) The Benioff zone of Indonesia has a maximum depth of 600 km to the east of the Sunda Strait, but the maximum depth decreases to 200 km northwestward along the island of Sumatra. (3) The age of the present phase of volcanic activity in Indonesia is proportional to the maximum depth of the Benioff zone; rhyolitic tuffs of the Sunda Strait range in age from Late Miocene to Pleistocene, while ignimbrites of north Sumatra are about 70,000 years old. It is suggested that the increase in sea-floor spreading rate since 10 m.y. B.P. pushed north Sumatra and Malaya northeastward for about 500 km along the system of presently inactive faults, causing a clockwise rotation of both Sumatra and Malaya about an axis located in or near the Sunda Strait. Only when this rotation ceased did the underthrusting of north Sumatra begin, producing a shallow and short Benioff zone, and delayed volcanic activity.

Use of K2O, Rb, Zr, and Y versus SiO2 in volcanic ash layers of the eastern Mediterranean to trace their source

The two main provinces of Pleistocene tephra eruption in the Mediterranean Sea are the eastern half of the Hellenic arc, represented by calc-alkalic and weakly alkalic material, and the Neapolitan area, represented by high-K 2 O material. Of twenty distinctive ash layers that have been identified in Pleistocene deep-sea cores of the eastern Mediterranean, one is calc-alkalic ash that originated in the Minoan eruption of Santorini at about 3500 B.P., and another is trachytic ash that originated in the Citara-Serrara eruption on Ischia Island at about 25,000 B.P. Rb, Zr, and Y content has been analyzed in the upper and the lower ash from both the land and deep-sea cores and in three tephra deposits from the east Hellenic volcanoes of Nisyros, Yali, and Kos. In addition to the K 2 O/SiO 2 ratio, values of the trace elements Rb, Zr, and Y versus SiO 2 in the ash layers can be used in correlating the ash layers and in tracing the source of ash. All relative K 2 O, Rb, Zr, and Y contents can be used to trace the source of ash to one of the two main provinces of tephra eruption. Zr and Y are most useful for identifying individual volcanoes as sources. Good agreement of chemical analyses of correlatable samples of the two ash layers from land and deep-sea cores argues against major alteration of the glass shards or a measurable cation exchange between glass shards and sea water on the time scale of 25,000 yr.

South Sandwich tephra in deep-sea sediments☆

Abstract Eleven cores containing volcanic detritus, taken from the bottom of the South Atlantic between the South Sandwich Islands and the Mid-Atlantic Ridge, have been examined. The mineralogical composition, texture, type of transportation and the source of the volcanic detritus are discussed. Black volcanic sand layers and olivine-rich basaltic sand occur interbedded and mixed with diatomaceous ooze which is the predominant sediment in the cores. Thick black sand layers composed of basaltic ash and deposited by turbidity currents occur in the cores in the South Sandwich Islands area. Several volcanic ash layers occur in the cores east of the South Sandwich Islands and can be traced a distance of 400 miles towards the Mid-Atlantic Ridge. Three ash layers can be correlated on the basis of refractive index. The thickness and the grain size of the tephra decrease eastward from the South Sandwich Trench for 400 miles. Beyond this area tephra is dispersed in other sediment and does not form discrete layers. The tephra which consists of a mixture of brown volcanic glass, minerals, and rock fragments is most probably derived from andesite-basaltic and basaltic eruptions in the South Sandwich Islands. A study of the diatom floras indicates that the last tephra fall occurred subsequent to the last cold period, that the middle fall occurred during the penultimate warm period and that the lower tephra fell during the penultimate cold period.

Volcanogenic effects on the rates of deposition of sediments in the Northwest Pacific Ocean

Abstract Seven piston cores, 7–16 m long, taken between the Kuril Islands and Emperor Seamounts, have been dated using radiolarian and diatom extinction levels and correlated using volcanic ash layers. The average rate of deposition in the cores decreases from 6 cm/1000 years near the Kuril Trench to about 3.5 cm/1000 years near the seamounts. Dispersed volcanic ash is the main constituent of the cores and it comprises up to 80% of the sediments. The percentage of the ash in the sediments decreases eastward from the Kuril Islands as the rates of deposition decrease. The total thickness of the sediments in the same latitudinal belt also decreases eastward. The thickness of the sediment inferred from seismic data near the Kuril trench is about 600 m and rates of deposition are approximately 6 cm/1000 years in the Pleistocene cores. Sediment thickness near the seamounts is about 300 m, and rates of deposition are approximately 3 cm/1000 years in the Pleistocene cores. Extrapolated rates of deposition in these cores suggest that the age of the base of the sediment to the east of the Kurils is only about 10 m.y. The anomalously young age for the base of the sediments obtained by extrapolation of an assumed constant rate of deposition can be explained by Deep Sea Drilling Project data from the northwest Pacific. The sediment thickness at DSDP site 192 east of Kamchatka includes sediments from all the Cenozoic epochs except the Paleocene. Rates of deposition of sediment younger than Middle Miocene are an order of magnitude higher than those prior to this time. At DSDP sites east of Japan, either Late Miocene sediments lie directly on the basement, or sediments older than Late Miocene are very thin. Post-Middle Miocene sediments are composed primarily of glass shards. Thus, about 90% of the total thickness of sediments in the northwest Pacific is composed of sediments younger than Middle Miocene with volcanic ash as the main constituent. The volcanic ash results from the present phase of explosive volcanic activity which began in the Late Miocene in the northwest Pacific volcanic arcs.