Sea surface temperature reconstruction of ODP Site 175-1087

Successful application of the alkenone palaeothermometer, the UK'37 index, relies upon the assumption that fossil alkenone synthesisers responded to growth-temperature changes in a similar manner to the modern producers, chiefly the coccolithophores Emiliania huxleyi and Gephyrocapsa oceanica. We compare coccolith and UK'37 data from ODP Site 1087 in the south-east Atlantic between 1500 and 500 ka, and show that evolutionary events and changes in species dominance within the coccolithophore populations had little impact on the UK'37 record. The relative abundances of the C37 and C38 alkenones also closely resembled those found in modern populations, and suggest a similar temperature sensitivity of UK'37 during the early and mid-Pleistocene to that found at present. These results support the application of the UK'37 index to reconstruct sea-surface temperatures (SSTs) throughout the Quaternary. The UK'37 record at ODP Site 1087 contains an SST signal that documents the emergence of the 100-kyr cycles that characterise the late Quaternary ice volume records. This is preceded by significant cooling at ODP Site 1087, marked by a negative shift in SSTs and a positive shift in the planktonic delta18O some 250-kyr earlier, at ca 1150-1000 ka. This results in a permanent fall in average SSTs of around 1.5 °C. The predicted increase in aridity onshore as a result of this cooling can be identified in a number of published records from southern Africa, and may have played a role in some important evolutionary events of the mid-Pleistocene.

Sediment composition and stable isotopic ratio of Neogloboquadrina pachyderma from DSDP Site 94-609_Site, North Altlantic

Digitized records of optical desnity in many North Atlantic cores exihibt rapid changes from lighter to darker extrems, typically within less than 200 years, at the 5d/5e, 5b/5c and 4/5 boundaries. In cores from DSDP site 609 the changes from lighter to darker color coincide with increasing in relative abundance of Neogloboquadrina pachyderma (l.c.), with increases in abundances of lithic grains and with decreasing in carbonate content. The rapid changes to dark color, therefore, are climate-driven and correspond to a lowering of seas surface temperatures and to increases in amounts of ice rafted debris relative to biogenic carbonate. At the 5d&4c boundary, delta18O in N. pachyderma (l.c.) increases abruptly with the change to darker sediments as expected for cooler sea surface temperatures. At the 4/5 boundary, however, delta18O decreases with the change to darker sediment and cooler sea surface temperatures, suggesting that a layer of fresh surface water was present in the North Atlantic at that time.

Planktonic foraminifera and stable isotope record of ODP Site 162-980

Detailed faunal, isotopic, and lithic marine records provide new insight into the stability and climate progression of the last interglacial period, Marine Isotope Stage (MIS) 5, which peaked approximately 125,000 years ago. In the eastern subpolar North Atlantic, at the latitude of Ireland, interglacial warmth of the ice volume minimum of substage 5e (MIS 5e) lasted ~10,000 years (10 ka) and its demise occurred in two cooling steps. The first cooling step marked the end of the climatic optimum, which was 2-3 ka long. Minor ice rafting accompanied each cooling step; the second, larger, step encompassing cold events C26 and C25 was previously identified in the northwestern Atlantic. Approximately 4 °C of cooling occurred between peak interglacial warmth and C25, and the region experienced an additional temporary cooling of at least 1-2 °C during C24, a cooling event associated with widespread ice rafting in the North Atlantic. Beginning with C24, MIS 5 was characterized by oscillations of at least 1-2 °C superimposed on a generally cool baseline. The results of this study imply that the marine climatic optimum of the last interglacial was shorter than previously thought. The finding that the eastern subpolar North Atlantic cooled significantly before C24 reconciles terrestrial evidence for progressive climate deterioration at similar and lower latitudes with marine conditions. Our results also demonstrate a close association between modest ice rafting, cooling, and deep ocean circulation even during the peak of MIS 5e and in the earliest stages of ice growth.

Age determination and stable isotope record of ODP Hole 162-980B

The conversion of surface water to deep water in the North Atlantic results in the release of heat from the ocean to the atmosphere, which may have amplified millennial-scale climate variability during glacial times (Broecker et al., 1990, doi:10.1029/PA005i004p00469) and could even have contributed to the past 11,700 years of relatively mild climate (known as the Holocene epoch) (Bond et al., 2001, doi:10.1126/science.1065680; Alley et al., 1997, doi:10.1130/0091-7613(1997)025<0483:HCIAPW>2.3.CO;2; Keigwin and Boyle, 2000, doi:10.1073/pnas.97.4.1343). Here we investigate changes in the carbon-isotope composition of benthic foraminifera throughout the Holocene and find that deep-water production varied on a centennial-millennial timescale. These variations may be linked to surface and atmospheric events that hint at a contribution to climate change over this period.

Aprubt climate change events in the Northeastern Atlantic Ocean

The western Iberian margin has been one of the key locations to study abrupt glacial climate change and associated interhemispheric linkages. The regional variability in the response to those events is being studied by combining a multitude of published and new records. Looking at the trend from Marine Isotope Stage (MIS) 10 to 2, the planktic foraminifer data, conform with the alkenone record of Martrat et al. [2007], shows that abrupt climate change events, especially the Heinrich events, became more frequent and their impacts in general stronger during the last glacial cycle. However, there were two older periods with strong impacts on the Atlantic meridional overturning circulation (AMOC): the Heinrich-type event associated with Termination (T) IV and the one occurring during MIS 8 (269 to 265 ka). During the Heinrich stadials of the last glacial cycle, the polar front reached the northern Iberian margin (ca. 41°N), while the arctic front was located in the vicinity of 39°N. During all the glacial periods studied, there existed a boundary at the latter latitude, either the arctic front during extreme cold events or the subarctic front during less strong coolings or warmer glacials. Along with these fronts sea surface temperatures (SST) increased southward by about 1°C per one degree of latitude leading to steep temperature gradients in the eastern North Atlantic and pointing to a close vicinity between subpolar and subtropical waters. The southern Iberian margin was always bathed by subtropical water masses - surface and/ or subsurface ones -, but there were periods when these waters also penetrated northward to 40.6°N. Glacial hydrographic conditions were similar during MIS 2 and 4, but much different during MIS 6. MIS 6 was a warmer glacial with the polar front being located further to the north allowing the subtropical surface and subsurface waters to reach at minimum as far north as 40.6°N and resulting in relative stable conditions on the southern margin. In the vertical structure, the Greenland-type climate oscillations during the last glacial cycle were recorded down to 2465 m during the Heinrich stadials, i.e. slightly deeper than in the western basin. This deeper boundary is related to the admixing of Mediterranean Outflow Water, which also explains the better ventilation of the intermediate-depth water column on the Iberian margin. This compilation revealed that latitudinal, longitudinal and vertical gradients existed in the waters along the Iberian margin, i.e. in a relative restricted area, but sufficient paleo-data exists now to validate regional climate models for abrupt climate change events in the northeastern North Atlantic Ocean.

Debris flow magnitudes: a global catalog

The episodic occurrence of debris flow events in response to stochastic precipitation and wildfire events makes hazard prediction challenging. Previous work has shown that frequency-magnitude distributions of non-fire-related debris flows follow a power law, but less is known about the distribution of post-fire debris flows. As a first step in parameterizing hazard models, we use frequency-magnitude distributions and cumulative distribution functions to compare volumes of post-fire debris flows to non-fire-related debris flows. Due to the large number of events required to parameterize frequency-magnitude distributions, and the relatively small number of post-fire event magnitudes recorded in the literature, we collected data on 73 recent post-fire events in the field. The resulting catalog of 988 debris flow events is presented as an appendix to this article. We found that the empirical cumulative distribution function of post-fire debris flow volumes is composed of smaller events than that of non-fire-related debris flows. In addition, the slope of the frequency-magnitude distribution of post-fire debris flows is steeper than that of non-fire-related debris flows, evidence that differences in the post-fire environment tend to produce a higher proportion of small events. We propose two possible explanations: 1) post-fire events occur on shorter return intervals than debris flows in similar basins that do not experience fire, causing their distribution to shift toward smaller events due to limitations in sediment supply, or 2) fire causes changes in resisting and driving forces on a package of sediment, such that a smaller perturbation of the system is required in order for a debris flow to occur, resulting in smaller event volumes.

Major element concentration and radionuclides of equatorial Pacific deep-sea sediments

The (231Pa/230Th)xs,0 records obtained from two cores from the western (MD97-2138; 1°25'S, 146°24'E, 1900 m) and eastern (ODP Leg 138 Site 849, 0°11.59'N, 110°31.18'W, 3851 m) equatorial Pacific display similar variability over the last 85000 years, i.e. from isotopic stages 1 to 5a, with systematically higher values during the Holocene, isotopic stage 3 and isotopic stage 5a, and lower values, approaching the production rate ratio of the two isotopes (0.093), during the colder periods corresponding to isotopic stages 2 and 4. We have also measured the 230Th-normalized biogenic preserved and terrigenous fluxes, as well as major and trace elements concentrations, in both cores. The (231Pa/230Th)xs,0 results combined with the changes in preserved carbonate and opal fluxes at the eastern site indicate lower productivity in the eastern equatorial Pacific during glacial periods. The (231Pa/230Th)xs,0 variations in the western equatorial Pacific (WEP) also seem to be controlled by productivity (carbonate and/or opal). The generally high (231Pa/230Th)xs,0 ratios (>0.093) of the profile could be due to opal and/or MnO2 in the sinking particles. The profiles of (231Pa/230Th)xs,0 and 230Th-normalized fluxes indicate a decrease in exported carbonate, and possibly opal, during isotopic stages 2 and 4 in MD97-2138. Using 230Th-normalized flux, we also show that sediments from the two cores were strongly affected by sediment redistribution by bottom currents suggesting a control of mass accumulation rates by sediment focusing variability.

Nd isotopes of sediments, foraminifera and fish debris of the western North Atlantic

Understanding changes in ocean circulation during the last deglaciation is crucial to unraveling the dynamics of glacial-interglacial and millennial climate shifts. We used neodymium isotope measurements on postdepositional iron-manganese oxide coatings precipitated on planktonic foraminifera to reconstruct changes in the bottom water source of the deep western North Atlantic at the Bermuda Rise. Comparison of our deep water source record with overturning strength proxies shows that both the deep water mass source and the overturning rate shifted rapidly and synchronously during the last deglacial transition. In contrast, any freshwater perturbation caused by Heinrich event 1 could have only affected shallow overturning. These findings show how changes in upper-ocean overturning associated with millennial-scale events differ from those associated with whole-ocean deglacial climate events.

Glacial-interglacial sediment record from the North Atlantic

Studies from the subtropical western and eastern Atlantic Ocean, using the 231Pa/230Th ratio as a kinematic proxy for deep water circulation, provided compelling evidence for a strong link between climate and the rate of meridional overturning circulation (MOC) over the last deglaciation. In this study, we present a compilation of existing and new sedimentary 231Pa/230Th records from North Atlantic cores between 1710 and 4550 m water depth. Comparing sedimentary 231Pa/230Th from different depths provides new insights into the evolution of the geometry and rate of deep water formation in the North Atlantic during the last 20,000 years. The 231Pa/230Th ratio measured in upper Holocene sediments indicates slow water renewal above ?2500 m and rapid flushing below, consistent with our understanding of modern circulation. In contrast, during the Last Glacial Maximum (LGM), Glacial North Atlantic Intermediate Water (GNAIW) drove a rapid overturning circulation to a depth of at least ?3000 m depth. Below ~4000 m, water renewal was much slower than today. At the onset of Heinrich event 1, transport by the overturning circulation declined at all depths. GNAIW shoaled above 3000 m and significantly weakened but did not totally shut down. During the Bølling-Allerød (BA) that followed, water renewal rates further decreased above 2000 m but increased below. Our results suggest for the first time that ocean circulation during that period was quite distinct from the modern circulation mode, with a comparatively higher renewal rate above 3000 m and a lower renewal rate below in a pattern similar to the LGM but less accentuated. MOC during the Younger Dryas appears very similar to BA down to 2000 m and slightly slower below.