Ulrich von Rad

Correlation between Arabian Sea and Greenland climate oscillations of the past 110,000 years

Palaeoclimate studies have revealed the general high-frequency instability of Late Pleistocene climate—for example, the so-called Dansgaard–Oeschger and Heinrich events—on timescales of a few millennia, centuries or even decades. Here we present evidence for a general relationship between low-latitude monsoonal climate variability and the rapid temperature fluctuations of high northern latitudes that are recorded in the Greenland ice records. Sediment cores from the northeastern Arabian Sea show laminated, organic-carbon-rich bands, reflecting strong monsoon-induced biological productivity, that correlate with the mild interstadial climate events in the northern North Atlantic region. In contrast, periods of lowered southwest monsoonal intensity, indicated by bioturbated, organic-carbon-poor bands, are associated with intervals of high-latitude atmospheric cooling and the injection of melt water into the North Atlantic basin. Our records suggest that Dansgaard–Oeschger and Heinrich events are strongly expressed in low-latitude (monsoonal) climate variability, suggesting the importance of common forcing agents such as atmospheric moisture and other greenhouse gases.Palaeoclimate studies have revealed the general high-frequency instability of Late Pleistocene climate—for example, the so-called Dansgaard–Oeschger and Heinrich events—on timescales of a few millennia, centuries or even decades1,11. Here we present evidence for a general relationship between low-latitude monsoonal climate variability and the rapid temperature fluctuations of high northern latitudes that are recorded in the Greenland ice records. Sediment cores from the northeastern Arabian Sea show laminated, organic-carbon-rich bands, reflecting strong monsoon-induced biological productivity, that correlate with the mild interstadial climate events in the northern North Atlantic region. In contrast, periods of lowered southwest monsoonal intensity, indicated by bioturbated, organic-carbon-poor bands, are associated with intervals of high-latitude atmospheric cooling and the injection of melt water into the North Atlantic basin. Our records suggest that Dansgaard–Oeschger and Heinrich events are strongly expressed in low-latitude (monsoonal) climate variability, suggesting the importance of common forcing agents such as atmospheric moisture and other greenhouse gases.

Multiple monsoon-controlled breakdown of oxygen-minimum conditions during the past 30,000 years documented in laminated sediments off Pakistan

Abstract Late Holocene laminated sediments from a core transect centred in the oxygen minimum zone (OMZ) impinging at the continental slope off Pakistan indicate stable oxygen minimum conditions for the past 7000 calendar years. High SW-monsoon-controlled biological productivity and enhanced organic matter preservation during this period is reflected in high contents of total organic carbon (TOC) and redox-sensitive elements (Ni, V), as well as by a low-diversity, high-abundance benthic foraminiferal Buliminacea association and high abundance of the planktonic species Globigerina bulloides indicative of upwelling conditions. Surface-water productivity was strongest during SW monsoon maxima. Stable OMZ conditions (reflected by laminated sediments) were found also during warm interstadial events (Preboreal, Bolling–Allerod, and Dansgaard–Oeschger events), as well as during peak glacial times (17–22.5 ka, all ages in calendar years). Sediment mass accumulation rates were at a maximum during the Preboreal and Younger Dryas periods due to strong riverine input and mobilisation of fine-grained sediment coinciding with rapid deglacial sea-level rise, whereas eolian input generally decreased from glacial to interglacial times. In contrast, the occurrence of bioturbated intervals from 7 to 10.5 ka (early Holocene), in the Younger Dryas (11.7–13 ka), from 15 to 17 ka (Heinrich event 1) and from 22.5 to 25 ka (Heinrich event 2) suggests completely different conditions of oxygen-rich bottom waters, extremely low mass and organic carbon accumulation rates, a high-diversity benthic fauna, all indicating lowered surface-water productivity. During these intervals the OMZ was very poorly developed or absent and a sharp fall of the aragonite compensation depth favoured the preservation of pteropods. The abundance of lithogenic proxies suggests aridity and wind transport by northwesterly or northeasterly winds during these periods coinciding with the North Atlantic Heinrich events and dust peaks in the Tibetan Loess records. The correlation of the monsoon-driven OMZ variability in the Arabian Sea with the rapid climatic fluctuations in the high northern latitudes suggests a close coupling between the climates of the high and low latitudes at a global scale.

Authigenic carbonates derived from oxidized methane vented from the Makran accretionary prism off Pakistan

We report the first discovery and sampling of a methane-hydrogen sulfide rich “cold seep” from the Makran accretionary prism off Pakistan (Arabian Sea). A variety of cm- to m-scale pockmarks and gas seepage structures were identified from underwater TV-photo sled profiles crossing the oxygen minimum zone (OMZ), related to highresolution seismic (PARASOUND) records. From the seeps isotopically light, bacterially formed methane is emanating that is partly oxidized to HCO3− in the bacterial sulfate reduction zone. This results in the precipitation of irregular dark-gray to black crusts of indurated authigenic carbonates, mainly cryptocrystalline magnesian calcite and Ca-rich dolomite, near the sediment/seawater interface within the OMZ. Downcore, the crusts grade into hemipelagic carbonate-poor silty clays with transitional lithologies in between. Similar to the pockmark carbonates from the Oregon margin and in the North Sea, the authigenic carbonates are extremely depleted in 13C (δ13Ccarbonate < − 40%.). This suggests that they were derived from bacterial methane (δ13Cmethane: − 77.8%.) that was oxidized under anaerobic conditions. The authigenic carbonates are associated with white fluffy mats of chemoautotrophic H2S-oxidizing bacteria. Small pockmarks appear to be concentrated at small-scale escarpments, suggesting focussed fault-controlled pore fluid expulsion due to the tectonic dewatering and degassing of the accretionary prism, whereas diffuse discharge of pore fluids is inferred from the widespread occurrence of tiny gas bubble tubes.

Organic content and preservational controls in surficial shelf and slope sediments from the Arabian Sea (Pakistan margin)

Abstract In the Arabian Sea, organic-rich deposits occur on the continental slope at depths roughly coincident with the impingement of an intense water-column oxygen minimum zone (OMZ). This relationship has often been taken as causal, and such deposits are often proposed as modern analogues of organic-rich facies from the palaeorecord. However, there are numerous other potential preservational controls on continental margins, and their relative importance remains unclear and much debated. We report the results of organic, inorganic and stable C isotope analyses on surficial sediments from three transects of the Pakistan margin, ranging in depth from well above to well below the OMZ. Organic carbon (C org ) and total nitrogen (N total ) contents show expected mid-slope maxima, although these do not mirror bottom water dissolved O 2 profiles. Maximal values are observed near the lower boundary of the OMZ, or even in homogenous sediments below it. Together, lignin yields and δ 13 C org values indicate a general paucity of terrigenous organic matter (OM). This information, combined with C org :N total relationships, suggests that the OM contents of all sediments are dictated by differing accumulation of a predominant marine fraction. While there are no obvious redox-related differences in C org :N total or OM hydrogen richness across the margin, there are clear minima in δ 13 C org within the OMZ, in apparent contradiction with maximal accumulation of marine OM. The latter phenomenon is ascribed either to local source effects, such as variable algal isotopic signatures, or to differences in OM preservation state, possibly linked to bottom water redox conditions. This remains to be clarified, but, along with the rough correspondence between mid-slope sedimentary C org maxima and bottom water O 2 minima, may indicate that O 2 availability is an important factor in determining sedimentary OM content on this margin. However, evidence from redox-sensitive indicators (I/C org , Mn/Al) for persistent accumulation of OM-rich deposits under oxidizing conditions in homogeneous sediments below the OMZ, clearly indicates that O 2 availability is not the sole or over-riding control on OM distributions.

High-resolution temperature and evaporation changes during the Late Holocene in the northeastern Arabian Sea

In order to reconstruct the monsoonal variability during the late Holocene we investigated a complete, annually laminated sediment record from the oxygen minimum zone (OMZ) off Pakistan for oxygen isotopes of planktic foraminifera and alkenone-derived sea surface temperatures (SST). Significant SST changes of up to 3°C which cannot be explained by changes in the alkenone-producing coccolithophorid species (inferred from the Gephyrocapsa oceanica / Emiliania huxleyi ratio) suggest that SST changes are driven by changes in the monsoon strength. Our high-(decadal)-resolution data indicate that the late Holocene in the northeastern Arabian Sea was not characterized by a stable uniform climate, as inferred from the Greenland ice cores, but by variations in the dominance of the SW monsoon conditions with significant effects on temperatures. Highest SST fluctuations of up to 3.0°C and 2.5°C were observed for the time interval from 4600 to 3300 years B.P. and during the past 500 years. The significant, short-term SST changes during the past 500 years might be related to climatic instabilities known from the northern latitudes (“Little Ice Age”) and confirm global effects. Surface salinity values, reconstructed from δ18O records after correction for temperature-related oxygen isotope fractionation, suggest that in general, the past 5000 years were characterized by higher-than-recent evaporation and more intense SW monsoon conditions. However, between 4600 and 3700 years B.P., evaporation dropped, SW monsoon weakened, and NE monsoon conditions were comparatively enhanced. For the past 1500 years we infer strongly fluctuating monsoon conditions. Comparisons of reconstructed salinity records with ice accumulation data from published Tibetan ice core and Tibetan tree ring width data reveal that during the past 2000 years, enhanced evaporation in the northeastern Arabian Sea correlates with periods of increased ice accumulation in Tibet, and vice versa. This suggests a strong climatic relationship between both monsoon-controlled areas.

LATE QUATERNARY SEDIMENTATION ON THE OUTER INDUS SHELF AND SLOPE (PAKISTAN) : EVIDENCE FROM HIGH-RESOLUTION SEISMIC DATA AND CORING

High-resolution seismic (Parasound) profiles, multibeam bathymetry (Hydrosweep), under-water photography and sediment cores are used to map the morphology and echo-facies distribution, and to describe the late Quaternary sedimentation on the outer shelf and slope off the Indus delta (Pakistan). The morphology and origin of the Indus Canyon were studied in detail. The upper and middle canyon (from 20- to 1350-m axial depth) is an erosional (degradational) canyon or delta-front trough with steep erosional walls and a meandering axial channel without levees. The lower Indus Canyon (1350- to 1500-m axial depth) is a transitional type between a degradational canyon and the aggradational channel-levee system of the upper Indus Fan. The late Quaternary sedimentation is influenced by fluctuations of fluvial input, delta progradation, canyon erosion and fan aggradation. It is especially controlled by sea-level changes. Four episodes can be distinguished: 1. (1) During interglacial (?Eemian) times and a relatively high sea-level position, a layered sequence of V-shaped, channel-like erosional features were formed below the shelfbreak (135 m) down to an upper slope scarp by slumping or erosion as delta-front or prodelta gullies. 2. (2) During the Last Glacial Maximum, the sea-level lowstand caused the Indus delta to advance across the present shelfbreak and shed fluvial silty clays directly onto the upper slope. This is indicated by layered, gently seaward dipping subparallel reflectors which are interpreted as being prograded prodelta mud units. At this time the Indus Canyon experienced maximum erosion and funnelled turbidity currents to the aggradational channel-levee system on the Indus Fan. Major slumps and debris flows were triggered at the continental slope. 3. (3) During deglaciation and the beginning of the Holocene transgression, several sea-level stillstands are indicated by the formation of biogenic sediments on the outer shelf and the build-up of shallow-water algal bioherms around 9–12 ka BP (presently at a water depth of ~90–100 m). 4. (4) During the late Holocene sea-level highstand, the shelf was flooded and the delta-front sediments of the Indus River were deposited on the innermost shelf. The outer shelf is characterized by a lack of deposition and erosion. The Indus Canyon experienced ongoing (but much decreased) activity of low-density turbidity currents with overbank spilling. The middle and lower continental slope down to ~ 2000 m is covered by hemipelagic sediments showing a layered sequence of distinct subbottom reflectors explained as hemipelagic drape interbedded with turbid layer sediments and/or thin mud turbidites from spillover along natural levees bordering the Indus Canyon. For the past 50 ka, the continental slope (outside the canyon and channels) experienced no turbidite sedimentation.

Stratigraphy, Facies and Tectonic Development of the On- and Offshore Aaiun-Tarfaya Basin — A Review

Based mainly on published data, we attempt a synthesis of the stratigraphy, facies and tectonic evolution of the onshore Aaiun-Tarfaya Basin and its offshore extension, the West Saharan Marginal Basin. Basement rocks are Precambrian, and folded Paleozoic sediments (Mauritanides belt): they dip gently westward and are overlain by a seaward thickening wedge of Mesozoic to Cenozoic continental to shallow-marine sediments. Jurassic to Cretaceous sediments extend from the onshore basin to the present shelf and upper slope, where they are more than 12 km thick.

Upper Triassic Tethyan Carbonates off Northwest Australia (Wombat Plateau, ODP Leg 122)

SummaryLeg 122 of the Ocean Drilling Program (ODP) recovered Upper Triassic (Carnian to Rhaetian) sediments at the sediment-starved passive continental margin off Northwestern Australia.The early-rift series at the Wombat Plateau, a northern sub-plateau of the Exmouth Plateau, consists of Upper Triassic fluviodeltaic siliciclastics and shallow-marine carbonates including reefal facies. Twenty-five microfacies types could be distinguished.These sequences are capped by an erosional ‘post-rift unconformity’ with a 70 m.y. hiatus during the Jurassic. The Wombat Plateau bears only a thin post-rift sedimentary cover of Cretaceous to Cenozoic age.The Carnian and Norian sequences are dominated by fluviodeltaic sediments that contain many carbonate intercalations. Their frequency and the kind and amount of allochems allow the reconstruction of a storm-influenced deltaic to prodeltaic environment with restricted estuarine (intradeltaic) lagoons and high-energy carbonate sand shoals in front of the delta lobes.The presence of the foraminifersTriasina oberhauseri andTriasina hantkeni in Sites 762 and 764 indicate a Norian to Rhaetian age. The reefal platform can be differentiated in a lagoon to patch reef environment with abundant Aulotortidae, and a patch reef to shelf zone with smaller foraminifers.The ‘Rhaetian’ starts with a global sequence boundary. Several shallowing-upward cycles from bioturbated wackestones to dolomitic algal bindstones suggest a shallow-subtidal to intertidal environment at Site 761. Typical reef development was observed at the more “distal’ Site 764. The limestone-marl alternations of the open marine shelf grade into local bioclastic and oolitic grainstones, which are the base for the incipient carbonate buildup. Calcisponge patch reefs developed into coral reefs. Several cycles characterize a ‘catch-up’ system grading into a ‘keep-up’ carbonate system. The reef growth ended abruptly with the second sequence boundary (211 Ma afterHaq et al., 1987), coinciding with the worldwide latest Rhaetian sea level fall, followed by renewed transgression.By comparison with Upper Triassic carbonates of the western Tethys (e.g., the Northern Calcareous Alps), several microfacies types could be combined to characteristic facies units: biolithite facies, different reef talus types, grapestoneoncoid facies, and calcareous algae-foraminifera detritus facies showing the reef-backreef/fore-reef-lagoon transitions.Detailed investigations of microfacies, wireline logs and high-resolution seismics allow the determination of depositional sequences (sequence stratigraphy). We distinguish influences of regional or global tectonics and/or eustatic sea-level changes. The results show that regional tectonic movements are of minor importance in the Rhaetian and that theHaq et al. (1987) cycle chart could also be used at the passive margin of Northwest Australia.

Cretaceous-Cenozoic History of the West Saharan Continental Margin (NW Africa): Development, Destruction and Gravitational Sedimentation

The most importantconstructional phases of the mature passive West Saharan continental margin involved the buildup of a thick Jurassic carbonate platform, overwhelmed by up to 4 km of Early Cretaceous Wealden-type deltaic sediments. Rollover structures between antithetic growth faults produced the “lower slope anticline” and a convex Cretaceous paleoslope. We recognized no salt diapirs, but local piercement structures probably caused by mobilized Early Cretaceous prodeltaic shale. During late Cretaceous to Tertiary times the continental slope was hardly prograded. During Tertiary (especially Neogene) times the depocenter shifted to the upper rise.

Planktic foraminifera, particle flux and oceanic productivity off Pakistan, NE Arabian Sea: modern analogues and application to the palaeoclimatic record

Abstract We use the flux of bulk sediment (CaCO3, biogenic opal, organic carbon, lithogenic material), and of planktic foraminifera (PF) and other shell-bearing plankton from sediment trap EPT-2 off Pakistan to (1) constrain the seasonal pattern of regional productivity and (2) search for indications of the NE monsoon winter situation that may serve as a modern analogue to better reveal the seasonal climatic signals preserved in the sedimentary record of the Arabian Sea. Our trap data show a clear seasonality of fluxes that can also be traced in the composition of non-bioturbated (varved) summer and winter sediment laminae preserved within the oxygen minimum zone. In EPT-2, the flux of PF is low during summer, but during winter and late spring it is higher, as at trap station WAST, in the upwelling area of the western Arabian Sea. Globigerina bulloides, a PF species linked to summer upwelling and high productivity, is of minor importance off Pakistan. In contrast, Globigerina falconensis dominates in flux and relative abundance, and is indicative of winter mixing, when NE monsoonal winds cool the highly saline surface waters and break up stratification. An enhanced horizontal flux of suspended sediments stirred up on the shelf and upper slope is clearly shown by the peak in occurrence of small benthic foraminifera during winter. Altogether, our data suggest that the particle flux in the northeastern Arabian Sea is determined by local sediment resuspension and winter productivity rather than by summer monsoonal upwelling, representing a ‘non-upwelling’ environment, in contrast to the ‘summer upwelling’ regime off Oman, Somalia and southern India. We used this evidence to reconstruct the seasonal intensity of both monsoons for the past 25 ka: the SW and NE monsoon both were weak during the last glacial period. The NE monsoon peaked during the cool phases of the glacial to interglacial climatic transition (i.e. during the Younger Dryas (YD) and Heinrich Event H1). The SW monsoon was reinforced after the YD. Both monsoons were enhanced during early Holocene time, when summer insolation and hence atmospheric forcing was at a maximum.