Major Climate Transitions Research Articles

A 200 year sub-annual record of sulfate in West Antarctica, from 16 ice cores

AbstractSixteen high-resolution ice-core records from West Antarctica and South Pole are used to examine the spatial and temporal distribution of sulfate for the last 200 years. The preservation of seasonal layers throughout the length of each record results in a dating accuracy of better than 1 year based on known global-scale volcanic events. A dual transport source for West Antarctic sea-salt (ss) SO42– and excess (xs) SO42– is observed: lower-tropospheric for areas below 1000m elevation and mid-/upper-tropospheric/stratospheric for areas located above 1000 m. Our xsSO42– records with volcanic peaks removed do not display any evidence of an anthropogenic impact on West Antarctic SO42– concentrations but do reveal that a major climate transition takes place over West Antarctica at ∼1940. Global-scale volcanic eruptions appear as significant peaks in the robust-spline residual xsSO42– records from sites located above 1000m elevation but do not appear in the residual records from sites located below 1000 m.

Terminal Pleistocene through Mid-Holocene archaeological sites as paleoclimatic archives for the Peruvian coast

Along the Peruvian coast, standard paleoclimatic archives such as pollen cores or corals are absent or undeveloped. However, anthropogenic deposits offer paleoclimatic information for the last 13 000 years at temporal scales ranging from centennial for long-term records to seasonal for short-term records. This paper summarizes archaeologically based paleoclimatic data for the Peruvian coast from the Terminal Pleistocene through the Middle Holocene. South of 12°S, coastal waters have been cool–temperate and terrestrial conditions have been hyperarid for the entire period. Between 8000 and 3600 cal BP, the highland precipitation supplying runoff to the south coast may have diminished sufficiently to restrict human occupation of ephemeral streams. North of ∼10°S, sea surface temperatures were warmer and seasonal precipitation may have been greater from prior to 13 000 to 5800 cal BP. From 9000 to 5800 cal BP, El Niño was absent or very infrequent. After 5800 cal BP, the northern coastal waters cooled and El Niño was present but with a lower frequency than seen in the historical and instrumental record. El Niño frequencies in the modern and historical range were established ∼3000 cal BP. Cultural changes correlate temporally with these major climatic transitions.

Dust in the Earth system: the biogeochemical linking of land, air and sea.

Understanding the response of the Earth's climate system to anthropogenic perturbation has been a pressing priority for society since the late 1980s. However, recent years have seen a major paradigm shift in how such an understanding can be reached. Climate change demands analysis within an integrated 'Earth-system' framework, taken to encompass the suite of interacting physical, chemical, biological and human processes that, in transporting and transforming materials and energy, jointly determine the conditions for life on the whole planet. This is a highly complex system, characterized by multiple nonlinear responses and thresholds, with linkages often between apparently disparate components. The interconnected nature of the Earth system is wonderfully illustrated by the diverse roles played by atmospheric transport of mineral 'dust', particularly in its capacity as a key pathway for the delivery of nutrients essential to plant growth, not only on land, but perhaps more importantly, in the ocean. Dust therefore biogeochemically links land, air and sea. This paper reviews the biogeochemical role of mineral dust in the Earth system and its interaction with climate, and, in particular, the potential importance of both past and possible future changes in aeolian delivery of the micro-nutrient iron to the ocean. For instance, if, in the future, there was to be a widespread stabilization of soils for the purpose of carbon sequestration on land, a reduction in aeolian iron supply to the open ocean would occur. The resultant weakening of the oceanic carbon sink could potentially offset much of the carbon sequestered on land. In contrast, during glacial times, enhanced dust supply to the ocean could have 'fertilized' the biota and driven atmospheric CO(2) lower. Dust might even play an active role in driving climatic change; since changes in dust supply may affect climate, and changes in climate, in turn, influence dust, a 'feedback loop' is formed. Possible feedback mechanisms are identified, recognition of whose operation could be crucial to our understanding of major climatic transitions over the past few million years.

The late Miocene to Pleistocene ice‐rafting history of southeast Greenland

Analysis of a Miocene‐Pleistocene ice‐rafted debris (IRD) record from the western Irminger Basin provides evidence for the initiation and long‐term behavior of the SE portion of the Greenland Ice Sheet. In the late Miocene (˜7.3 Ma), IRD supply to Ocean Drilling Program site 918 increased significantly indicating that glaciers large enough to reach sea level were present in SE Greenland long before the onset of widespread Northern Hemisphere glaciation. IRD accumulated at this site throughout the Pliocene and Pleistocene, supporting the hypothesis that SE Greenland was a key nucleation area for the formation of the Greenland Ice Sheet. Since glacial onset, the western Irminger Basin IRD record is characterized by a succession of episodes with high IRD mass accumulation rates (MARs). The site 918 IRD record indicates that greatest iceberg production in SE Greenland occurred during major climatic transitions (e.g. widespread Northern Hemisphere glacial expansion at 2.7 Ma and the mid‐Pleistocene climate shift at 0.9 Ma), and that SE Greenland sometimes also led the northern North Atlantic region in glacial response to climatic forcing (e.g. glacial intensification at ˜4.8 and, along with NE Greenland, at ˜3.5 Ma).

Response of Lake Baikal ecosystem to climate forcing and pCO 2 change over the last glacial/interglacial transition

The δ 13C record of total organic carbon in the Lake Baikal core BDP-93-2 over the last glacial/interglacial transition shows remarkable correlation with atmospheric CO 2 fluctuations and the negative isotopic shift of 3.5–4‰ in response to the global increase in paleo- pCO 2 level. With rapid evolution of the Lake Baikal carbon cycle from full glacial to full interglacial modes, however, the pCO 2 signal in the Baikal δ 13C sedimentary organic matter is inundated by the basin-wide productivity signal, in the Holocene. The Baikal diatom abundance record, as a paleotemperature proxy, lags behind the GISP2 ice core δ 18O temperature proxy record by ca. 1 ka, possibly reflecting the response time of the Baikal ecosystem to major climatic transitions. There is no lag associated with the pronounced Younger Dryas cooling, however, suggesting a different forcing mechanism, with a short response time, for this abrupt climatic event in continental interior Asia. The phase difference observed between the diatom abundance and δ 13C proxies in Lake Baikal core BDP-93-2 suggests their independent responses to different forcing mechanisms, further justifying their importance for paleoclimate reconstructions from Lake Baikal records.

Composite sedimentary record of falling stages of Pleistocene glacio-eustatic cycles in a shelf setting (Crotone basin, south Italy)

Abstract A thick Pleistocene shelf and nearshore cyclical succession was deposited in the S. Mauro sub-basin of the Crotone basin (southern Italy). The regressive units of the cycles are mostly represented by coastal siliciclastic and bioclastic prograding wedges showing a clinoform geometry. These are separated by blanket-like deposits of high lateral persistence recording major transgressive episodes. The aim of this paper is (1) to describe facies patterns and depositional setting of two prograding wedges, particularly focussing on their polycyclic internal architecture, (2) to analyze these units within a sequence-stratigraphic framework, and (3) to speculate on the possible origin of the small-scale cyclicity. The two wedges analyzed in this paper consist of a number of shingles. Individual shingles consist of two physically connected units: (1) a relatively thin package of sigmoid clinoforms, grading into (2) a volumetrically dominant package of oblique-tangential clinoforms with toplap terminations. The shingles are bounded by seaward-dipping surfaces with sigmoid clinoform geometry, which are ravinement surfaces updip, passing into conformable flooding surfaces downdip. The wedges are thus organized into high-frequency, small-scale sequences, each comprising transgressive, highstand and falling-stage systems tracts. As a whole, individual prograding wedges are interpreted as forced-regressive units, as the shoreline was subject to an overall shift basinwards and downwards along a low-angle trajectory, in spite of the repeated minor relative sea-level rises. Tectonic subsidence, and particularly the syndepositional growth of gentle synclines, are thought to have been the key factors allowing the preservation of these forced-regressive units. Progradation of the wedges took place in a high-energy wave climate characterized by high frequency of storms and very efficient alongshore redistribution of sediments. Recurrent, storm-driven, offshore currents led to intense reworking of sediments on the topset platform and gravity spreading on the foreset slope of the prograding wedges. Well-oxygenated conditions over the shelf due to intensified storm activity during glacial periods may have enhanced the rate of production of skeletal, foramol-type carbonates. It can reasonably be assumed that progradation took place from a line source and that the sand bodies are to be regarded as coastal prograding bodies. In spite of active syndepositional tectonics, the cycles can be correlated to Pleistocene high-amplitude sea-level oscillations. The older of the two wedges can be correlated, through bio-magnetostratigraphy, to the major climatic transition which occurred from the marine oxygen-isotope stage 25 to 24–22 ( Rio et al., 1996 ). The younger probably developed during the sea-level fall that ended with substage 18.2, as suggested by sequence- and bio-stratigraphic data. The prograding wedges are thus interpreted to record long-lived sea-level falls of fourth-order cycles. Due to the particular depositional setting, we are inclined to exclude authigenic mechanisms in the origin of small-scale cyclicity. Although the concomitance and interaction of different controlling factors may be taken into account, it is tempting to ascribe this cyclicity to minor eustatic changes punctuating long-lived, erratic falling stages, possibly accompanied by climate-driven fluctuations of sediment supply. Shelf-perched and shelf-edge prograding units consisting of foramol-type carbonates are apparently a common falling-stage to lowstand depositional feature in the Mediterranean area during the Late Pliocene and Pleistocene.

Cyanobacteria and Cyanobacterial Lichens from Inselbergs of the Ivory Coast, Africa

Abstract:This study focuses on the saxicolous lichens and cyanobacteria of the open, exposed rock surface of inselbergs. Twenty‐three species of cyanobacteria and 17 cyanobacterial lichen species (“cyanolichens”) from several inselbergs and other rocky outcrops of three major climatic regions, savanna, transition zone and rain forest, are reported from the Ivory Coast. Inselbergs are isolated and frequently mountains consisting of Precambrian granites or gneisses that abruptly rise from the surrounding plains. Cyanobacteria were found to be the dominating organisms on all rock surfaces. The lichens found mainly belong to the family Peltulaceae and a few were present from the family Lichinaceae. Nine species of the cyanolichens and most of the cyanobacteria are new for the Ivory Coast. A gradient in total species number (cyanolichens and cyanobacteria) occurs from the savanna to the rain forest, with a decrease in species number towards the rain forest. Saxicolous cyanobacterial lichens reached a higher species number in the savanna type ecosystem (11) than on inselbergs in the rain forest (7). The cyanolichens and cyanobacteria found are characteristic for larger, light‐exposed rock surfaces and species like P. congregate, P. lingulata, P. tortuosa and P. umbilicata preferentially occur on the granite or sandstone of inselbergs.

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.

The GISP2 δ18O Climate Record of the Past 16,500 Years and the Role of the Sun, Ocean, and Volcanoes

Measured 18O/16O ratios from the Greenland Ice Sheet Project 2 (GISP2) ice core extending back to 16,500 cal yr B.P. provide a continuous record of climate change since the last glaciation. High-resolution annual 18O/16O results were obtained for most of the current millennium (A.D. 818-1985) and record the Medieval Warm Period, the Little Ice Age, and a distinct 11-yr 18O/16O cycle. Volcanic aerosols depress central Greenland annual temperature (∼1.5°C maximally) and annual 18O/16O for about 4 yr after each major eruptive event. On a bidecadal to millennial time scale, the contribution of solar variability to Holocene Greenlandic temperature change is ∼0.4°C. The role of thermohaline circulation change on climate, problematic during the Holocene, is more distinct for the 16,500-10,000 cal yr B.P. interval. (Analogous to 14C age calibration terminology, we express time in calibrated (cal) yr B.P. (A.D. 1950 = 0 cal yr B.P.)). The Oldest Dryas/Bølling/Older Dryas/Allerød/Younger Dryas sequence appears in great detail. Bidecadal variance in 18O/16O, but not necessarily in temperature, is enhanced during the last phase of lateglacial time and the Younger Dryas interval, suggesting switches of air mass transport between jet stream branches. The branched system is nearly instantaneously replaced at the beginning of the Bølling and Holocene (at ∼14,670 and ∼11,650 cal yr B.P., respectively) by an atmospheric circulation system in which 18O/16O and annual accumulation initially track each other closely. Thermodynamic considerations of the accumulation rate-temperature relationship can be used to evaluate the 18 O/16O-temperature relationship. The GISP2 ice-layer-count years of major GISP2 climate transitions also support the use of coral 14C ages for age calibration.

Late Pleistocene and Holocene paleoenvironments of the North Pacific coast

Unlike the North Atlantic, the North Pacific Ocean probably remained free of sea ice during the last glacial maximum (LGM), 22,000 to 17,000 BP. Following a eustatic low in sea level of ca. −120 m at 19,000 BP, a marine transgression had flooded the Bering and Chukchi shelves by 10,000 BP. Post-glacial sea-level history varied widely in other parts of the North Pacific coastline according to the magnitude and timing of local tectonism and glacio-isostatic rebound. Glaciers covered much of the continental shelf between the Alaska Peninsula and British Columbia during the LGM. Maximum glacier extent during the LGM was out of phase between southern Alaska and southern British Columbia with northern glaciers reaching their outer limits earlier, between 23,000 and 16,000 BP, compared to 15,000–14,000 BP in the south. Glacier retreat was also time-transgressive, with glaciers retreating from the continental shelf of southern Alaska before 16,000 BP but not until 14,000–13,000 BP in southwestern British Columbia. Major climatic transitions occurred in the North Pacific at 24,000–22,000, 15,000–13,000 and 11,000–9000 BP. Rapid climate changes occurred within these intervals, including a possible Younger Dryas episode. An interval of climate warmer and drier than today occurred in the early Holocene. Cooler and wetter conditions accompanied widespread Neoglaciation, beginning in some mountain ranges as early as the middle Holocene, but reaching full development after 3000 BP.

Synchronized changes in regional water balance since the mid-holocene

Detailed records of past climatic changes, especially those related to water balance, can be used to study regional scale climatic changes associated with both natural and anthropogenic causes. Such ancient records are available from various locations around the globe. The four records presented here have sufficient time resolution to demonstrate short-lived global oscillations in parameters related to water balance during the last 7,000 y. The data indicate that sub-Milankovitch climatic events are not restricted to times of major climatic transitions but occurred throughout the Holocene. Most of the past changes due to natural variability are at least of the same order of magnitude as those predicted by various models to occur during the next century.

Climatic evolution of the Southern Ocean during the Pliocene epoch from 4.8 to 2.6 million years ago

We studied the paleoclimatic history of the Southern Ocean during the Pliocene, before the intensification of northern hemisphere glaciation at 2.4 Ma, from isotopic and sedimentologic data from ODP Leg 114 sites (subantarctic South Atlantic). Relatively warm climatic conditions prevailed in the Southern Ocean during the early early Pliocene (between ∼4.6 and 4.2 Ma) as evidenced by benthic and planktonic δ 18O values that were generally less than the Holocene. Despite this warmth, significant ice-rafting episodes are recorded at several drill sites and piston-core locations throughout the Southern Ocean. This interval correlates with a eustatic highstand and represents a climatic optimum for the Southern Ocean when low- and mid-latitude species migrated into the Antarctic and surface waters were warmer than at present. Although it is generally agreed that this interval was warm in the southern hemisphere, divergent opinions exist as to whether this warming led to the partial decay or growth of the Antarctic ice-sheet. A distinct mid-Pliocene cooling occurred between ∼3.5 and 3.2 Ma, as evidenced by an increase in δ 18O values of planktonic and benthic foraminifers in Hole 704A, and by faunal changes indicative of progressive cooling of Antarctic surface waters and development of biotic provinciality in the Southern Ocean. During the late Pliocene (3.25-2.6 Ma), planktonic and benthic δ 18O values commonly exceeded those of the Holocene and several distinct cooling and/or ice volume events occurred during the late Gauss Chron (most notably at 2.9 and 2.63 Ma). Faunal studies suggest several northward advances of the Polar Front Zone and expansion of sea ice in the Antarctic during this period. These events preceded the major climatic transition at ∼2.4 Ma when the PFZ moved to the north, sea ice expanded in the Antarctic, and glaciation intensified in both hemispheres.

A plausible model for the synchroneity or the phase shift between climatic transitions

A class of climate models describing the dynamics of the sea ice‐ocean surface temperature interactions is considered. The coupling mechanism between different spatial regions is analyzed, and the possibility of inhomogeneous phase shifted states is established. It is concluded that many major climatic transitions should occur in an asynchronous fashion. A new examination of the climatic record seems therefore to be necessary, in the light of this possibility.