Frank Sirocko

Climate change and the collapse of the Akkadian empire: Evidence from the deep sea

The Akkadian empire ruled Mesopotamia from the headwaters of the Tigris-Euphrates Rivers to the Persian Gulf during the late third millennium B.C. Archeological evidence has shown that this highly developed civilization collapsed abruptly near 4170 ± 150 calendar yr B.P., perhaps related to a shift to more arid conditions. Detailed paleoclimate records to test this assertion from Mesopotamia are rare, but changes in regional aridity are preserved in adjacent ocean basins. We document Holocene changes in regional aridity using mineralogic and geochemical analyses of a marine sediment core from the Gulf of Oman, which is directly downwind of Mesopotamian dust source areas and archeological sites. Our results document a very abrupt increase in eolian dust and Mesopotamian aridity, accelerator mass spectrometer radiocarbon dated to 4025 ± 125 calendar yr B.P., which persisted for ~300 yr. Radiogenic (Nd and Sr) isotope analyses confirm that the observed increase in mineral dust was derived from Mesopotamian source areas. Geochemical correlation of volcanic ash shards between the archeological site and marine sediment record establishes a direct temporal link between Mesopotamian aridification and social collapse, implicating a sudden shift to more arid conditions as a key factor contributing to the collapse of the Akkadian empire.

Teleconnections Between the Subtropical Monsoons and High-Latitude Climates During the Last Deglaciation

The major deglacial intensification of the southwest monsoon occurred at 11,450 ± 150 calendar years before present, synchronous with a major climate transition as recorded in Greenland ice. An earlier event of monsoon intensification at 16,000 ± 150 calendar years before present occurred at the end of Heinrich layer 1 in the Atlantic and parallels the initial rise in global atmospheric methane concentrations and the first abrupt climate changes in the Antarctic; thus, the evolution of the monsoonal and high-latitude climates show teleconnections but hemispheric asymmetries. Superimposed on abrupt events, the monsoonal climate shows high-frequency variability of 1785-, 1450-, and 1150-year oscillations, and abrupt climate change seems to occur when at least two of these oscillations are in phase.

El Niño variability off Peru during the last 20,000 years

Here we present a high-resolution marine sediment record from the El Nino region off the coast of Peru spanning the last 20,000 years. Sea surface temperature, photosynthetic pigments, and a lithic proxy for El Nino flood events on the continent are used as paleo–El Nino–Southern Oscillation proxy data. The onset of stronger El Nino activity in Peru started around 17,000 calibrated years before the present, which is later than modeling experiments show but contemporaneous with the Heinrich event 1. Maximum El Nino activity occurred during the early and late Holocene, especially during the second and third millennium B.P. The recurrence period of very strong El Nino events is 60–80 years. El Nino events were weak before and during the beginning of the Younger Dryas, during the middle of the Holocene, and during medieval times. The strength of El Nino flood events during the last millennium has positive and negative relationships to global and Northern Hemisphere temperature reconstructions.

Ancient DNA Reveals Key Stages in the Formation of Central European Mitochondrial Genetic Diversity

The Origins of Europeans To investigate the genetic origins of modern Europeans, Brandt et al. (p. 257) examined ancient mitochondrial DNA (mtDNA) and were able to identify genetic differences in 364 Central Europeans spanning the early Neolithic to the Early Bronze Age. Observed changes in mitochondrial haplotypes corresponded with hypothesized human migration across Eurasia and revealed the complexity of the demographic changes and evidence of a Late Neolithic origin for the European mtDNA gene pool. This transect through time reveals four key population events associated with well-known archaeological cultures, which involved genetic influx into Central Europe from various directions at various times. Mitochondrial DNA profiles of 364 prehistoric people reveal human demography and migration patterns in Neolithic Germany. The processes that shaped modern European mitochondrial DNA (mtDNA) variation remain unclear. The initial peopling by Palaeolithic hunter-gatherers ~42,000 years ago and the immigration of Neolithic farmers into Europe ~8000 years ago appear to have played important roles but do not explain present-day mtDNA diversity. We generated mtDNA profiles of 364 individuals from prehistoric cultures in Central Europe to perform a chronological study, spanning the Early Neolithic to the Early Bronze Age (5500 to 1550 calibrated years before the common era). We used this transect through time to identify four marked shifts in genetic composition during the Neolithic period, revealing a key role for Late Neolithic cultures in shaping modern Central European genetic diversity.

Atmospheric summer circulation and coastal upwelling in the Arabian Sea during the Holocene and the last glaciation

Accumulation rates of biogenic and lithogenic components were studied in 39 turbidite-free, well-dated sediment cores from the northern Indian Ocean to define the proportions of fluvial and eolian input and to reconstruct Quaternary patterns of coastal upwelling. The majority of dust deposited in the western Arabian Sea during the Holocene (about 100 × 106 t yr−1) is advected from Arabia by northwesterly winds, which overlie the low-level southwest monsoon. The glacial increase in dust flux to 160 × 106 t yr−1 culminated in the northern Arabian Sea, most probably due to (i) entrainment of dust, rich in chlorite, dolomite, and lithogenic carbonate in the then-dry Persian Gulf, and (ii) a southward shift of the mean position of the southwest monsoon during glacial summer. This shift is recorded in reduced accumulation rates of biogenic opal and increased rates of marine carbonate off Somalia and Oman. Both the terrigenous and biogenic sediment records show that the northwesterly winds and the southwest monsoon persisted over the last 27,000 yr, as well as the Asian continental summer heat low. However, the glacial seasonal time span of the southwest monsoon season was much reduced, most likely because of a delay in the seasonal onset of the southwest monsoon.

The low-latitude monsoon climate during Dansgaard–Oeschger cycles and Heinrich Events

During the last 100,000 years Dansgaard–Oeschger cycles (D/O cycles) and Heinrich Events have been the dominant signal of past climate variability over Greenland and the North Atlantic. The succession of stadials (cold) and interstadials (warm) associated with these cycles has been documented in records from the entire northern hemisphere, South America, New Zealand, Antarctica, the South Atlantic and the Southern Ocean. Evidently, climate forcing in the D/O band affects both hemispheres. The origin and cause of these teleconnected patterns is still unknown, even if a large proportion of the cooling in Europe and northern Asia during Heinrich Events is a meteorological response to cold surface water in the North Atlantic resulting from the surge of the Laurentian and Scandinavian ice sheets. But, this does not imply that the D/O cycles are a priori caused by the collapse of these large icesheets. A severe challenge to a primary origin from the northern ice sheet and North Atlantic comes from the observation that warming in Antarctica and the circumpolar current leads the North Atlantic changes by some 1500 years (Charles et al., 1996; Blunier et al., 1998). In the second part of the paper we investigate new deep sea records from the low-latitude monsoon system, mainly corroborating the result of Schulz et al. (1998) that the Indian monsoon showed D/O variability. The abundance of eolian dust (indicating continental humidity/aridity) in core 70KL from the Arabian Sea shows humid intervals which seem to correlate with temperature maxima in the Antarctic Vostok ice core. Apparently, the low-frequency, sub-Milankovitch variability of the monsoon is associated with the southern hemisphere. The D/O-scale component in the monsoonal climate, on the other hand, shows a succession of short humid intervals. The sequence is most closely comparable to the Greenland temperature record and to the stadial/interstadial succession in the Pacific Santa Barbara Basin ODP 893. In order to understand the global forcing of climate change in the D/O band, it appears necessary to quantify the phase relationships not only between D/O cycles in the polar ice cores (Bender et al., 1994; Blunier et al., 1998; Steig et al., 1998), but also between the low latitude Pacific, Indian Ocean and the high-latitude records.

Processes controlling trace element geochemistry of Arabian Sea sediments during the last 25,000 years

Thirty seven deep-sea sediment cores from the Arabian Sea were studied geochemically (49 major and trace elements) for four time slices during the Holocene and the last glacial, and in one high sedimentation rate core (century scale resolution) to detect tracers of past variations in the intensity of the atmospheric monsoon circulation and its hydrographic expression in the ocean surface. This geochemical multi-tracer approach, coupled with additional information on the grain size composition of the clastic fraction, the bulk carbonate and biogenic opal contents makes it possible to characterize the sedimentological regime in detail. Sediments characterized by a specific elemental composition (enrichment) originated from the following sources: river suspensions from the Tapti and Narbada, draining the Indian Deccan traps (Ti, Sr); Indus sediments and dust from Rajasthan and Pakistan (Rb, Cs); dust from Iran and the Persian Gulf (Al, Cr); dust from central Arabia (Mg); dust from East Africa and the Red Sea (Zr/Hf, Ti/Al). Corg, Cd, Zn, Ba, Pb, U, and the HREE are associated with the intensity of upwelling in the western Arabian Sea, but only those patterns that are consistently reproduced by all of these elements can be directly linked with the intensity of the southwest monsoon. Relying on information from a single element can be misleading, as each element is affected by various other processes than upwelling intensity and nutrient content of surface water alone. The application of the geochemical multi-tracer approach indicates that the intensity of the southwest monsoon was low during the LGM, declined to a minimum from 15,000–13,000 14C year BP, intensified slightly at the end of this interval, was almost stable during the Bolling, Allerod and the Younger Dryas, but then intensified in two abrupt successions at the end of the Younger Dryas (9900 14C year BP) and especially in a second event during the early Holocene (8800 14C year BP). Dust discharge by northwesterly winds from Arabia exhibited a similar evolution, but followed an opposite course: high during the LGM with two primary sources—the central Arabian desert and the dry Persian Gulf region. Dust discharge from both regions reached a pronounced maximum at 15,000–13,000 14C year. At the end of this interval, however, the dust plumes from the Persian Gulf area ceased dramatically, whereas dust discharge from central Arabia decreased only slightly. Dust discharge from East Africa and the Red Sea increased synchronously with the two major events of southwest monsoon intensification as recorded in the nutrient content of surface waters. In addition to the tracers of past dust flux and surface water nutrient content, the geochemical multi-tracer approach provides information on the history of deep sea ventilation (Mo, S), which was much lower during the last glacial maximum than during the Holocene. The multi-tracer approach—i.e. a few sedimentological parameters plus a set of geochemical tracers widely available from various multi-element analysis techniques—is a highly applicable technique for studying the complex sedimentation patterns of an ocean basin, and, specifically in the case of the Arabian Sea, can even reveal the seasonal structure of climate change.

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.

A late Eemian aridity pulse in central Europe during the last glacial inception.

Investigating the processes that led to the end of the last interglacial period is relevant for understanding how our ongoing interglacial will end, which has been a matter of much debate (see, for example, refs 1, 2). A recent ice core from Greenland demonstrates climate cooling from 122,000 years ago driven by orbitally controlled insolation, with glacial inception at 118,000 years ago. Here we present an annually resolved, layer-counted record of varve thickness, quartz grain size and pollen assemblages from a maar lake in the Eifel (Germany), which documents a late Eemian aridity pulse lasting 468 years with dust storms, aridity, bushfire and a decline of thermophilous trees at the time of glacial inception. We interpret the decrease in both precipitation and temperature as an indication of a close link of this extreme climate event to a sudden southward shift of the position of the North Atlantic drift, the ocean current that brings warm surface waters to the northern European region. The late Eemian aridity pulse occurred at a 65° N July insolation of 416 W m-2, close to todays value of 428 W m-2 (ref. 9), and may therefore be relevant for the interpretation of present-day climate variability.

Orbital insolation forcing of the Indian Monsoon – a motor for global climate changes?

Abstract Both modern and ancient Indian summer monsoons are driven by transequatorial pressure differences, directly coupled with the insolation difference between the Northern and Southern subtropical Hemispheres. A high-resolution record of upwelling and dust flux from the western Arabian Sea resembles an insolation-based Indian Summer Monsoon Index. This index and the observed monsoonal climate variations share major elements on the orbital obliquity and precessional band with the Specmap marine oxygen isotope record, representing global ice volume. The long-term evolution of the index mirrors almost exactly the insolation changes at 65°N, showing that the forcing of low latitude climate variability has a structure similar to that of the insolation forcing in the high northern latitudes. Moreover, insolation forcing in the low latitudes directly controls atmospheric processes in the African, Indian and Asian Monsoon, being responsible for a huge amount of transequatorial water vapour and therefore latent heat transport. Millennial-scale variability of the monsoonal climate is concentrated at periodicities near 1100, 1450, 1750 and 2300 years. These cycles are not strictly periodic, but occur in bands, with specific activity phases and amplitude increases during warm stages and interstadials for the 1100- and 1750-yr cycles, whereas the 1450-yr cycle dominates the cold intervals.