Nacionalismo, fascismo y tradicionalismo en la Argentina moderna. Una historia, de Fernando Devoto, Buenos Aires, Siglo Veintiuno de Argentina editores, 2002, 306 pp.

Fil: Kahan, Emmanuel Nicolás. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.

Sea surface temperature estimation for the last 70 ka from IODP Site 306-U1313

During the six Heinrich Events of the last 70 ka episodic calving from the circum-Atlantic ice sheets released large numbers of icebergs into the North Atlantic. These icebergs and associated melt-water flux are hypothesized to have led to a shutdown of Atlantic Meridional Overturning Circulation (AMOC) and severe cooling in large parts of the Northern Hemisphere. However, due to the limited availability of high-resolution records the magnitude sea surface temperature (SST) changes related to the impact of Heinrich Events on the mid-latitude North Atlantic is poorly constrained. Here we present a record of UK37'-based SSTs derived from sediments of Integrated Ocean Drilling Project (IODP) Site U1313, located at the southern end of the ice-rafted debris (IRD)-belt in the mid-latitude North Atlantic (41°N). We demonstrate that all six Heinrich Events are associated with a rapid warming of surface waters by 2 to 4°C in a few thousand years. The presence of IRD leaves no doubt about the simultaneous timing and correlation between rapid surface water warming and Heinrich Events. We argue that this warming in the mid-latitude North Atlantic is related to a northward expansion of the subtropical gyre during Heinrich Events. As a wide-range of studies demonstrated that in the central IRD-belt Heinrich Events are associated with low SSTs, these results thus identify an anti-phased (seesaw) pattern in SSTs during Heinrich Events between the mid-latitude (warm) and northern North Atlantic (cold). This highlights the complex response of surface water characteristics in the North Atlantic to Heinrich Events that is poorly reproduced by fresh water hosing experiments and challenges the widely accepted view that within the IRD-belt of the North Atlantic Heinrich Events coincide with periods of low SSTs.

Depth tie points and ages of IODP Site 306-U1314

Integrated Ocean Drilling Program (IODP) Site U1314 of the North Atlantic is a critical sedimentary archive record of subpolar deep water from the southern Gardar Drift for which we derived an age model of orbital resolution for the last 1.8 Ma. This chronology combined with high-resolution (cm scale) X-ray fluorescence core scanning measurements of major elements allows tracking changes in terrigenous provenance during the last 1.1 Ma. Low Potassium to Titanium (K/Ti) ratios reflect enhanced transport of basalt-derived titanomagnetites during warm climate intervals, while high K/Ti ratios indicate a dominance of acidic sediment sources typical for glacial and stadial events. Changes in K/Ti and magnetic concentration at Site 1314 are coeval with fluctuations in smectite content and grain size data from nearby piston cores, suggesting that the provenance changes are mainly controlled by variable flow of the Iceland-Scotland Overflow Water, an important branch of North Atlantic Deep Water. Furthermore, K/Ti variations on orbital time scales show a striking similarity to the deep sea d13C record from ODP Site 607. Pervasive features of the K/Ti time series during and after the Mid-Pleistocene Transition are suborbital changes similar to Dansgaard/Oeschger and Bond oscillations that appear to be strongly amplified during ice growth phases when global benthic d18O was within the range of ~4.1-4.6 per mil. The strong increase in variability of sediment provenance and subsequently deep hydrography at benthic d18O values below ~4.1 suggests that the extent of glaciations and, therefore, sea level corresponding to this value constitutes an important physical threshold that was persistent at least for the last 1.1 Ma.

Mid-Pleistocene (800-400 ka) records of benthic and planktic foraminfer d18O and d13C, planktic foraminifer based SST, and IRD of IODP Site 306-U1314

Stable carbon and oxygen isotopes from benthic and planktic foraminifers, planktic foraminifer assemblages and ice rafted debris from the North Atlantic Site U1314 (Integrated Ocean Drilling Program Expedition 306) were examined to investigate orbital and millennial-scale climate variability in the North Atlantic and its impact on global circulation focusing on the development of glacial periods during the mid-Pleistocene (ca 800-400 ka). Glacial initiations were characterized by a rapid cooling (6-10 °C in less than 7 kyr) in the mean annual sea surface temperature (SST), increasing benthic d18O values and high benthic d13C values. The continuous increase in benthic d18O suggests a continuous ice sheet growth whereas the positive benthic d13C values indicate that the flow of the Iceland Scotland Overflow water (ISOW) was vigorous. Strong deep water formation in the Norwegian Greenland Sea promoted a high transfer of freshwater from the ocean to the continents. However, low SSTs at Site U1314 suggest a subpolar gyre cooling and freshening that may have reduced deep water formation in the Labrador Sea during glacial initiations. Once the 3.5 per mil threshold in the benthic d18O record was exceeded, ice rafting started and ice sheet growth was punctuated by millennial-scale waning events which returned to the ocean part of the freshwater accumulated on the continents. Ice-rafting events were associated with a rapid reduction in the ISOW (benthic d13C values dropped 0.5-1 per mil) and followed by millennial-scale warmings. The first two millennial-scale warm intervals of each glacial period reached interglacial temperatures and were particularly abrupt (6-10 °C in ~3 kyr). Subsequent millennial-scale warm events were cooler probably because the AMOC was rather reduced as suggested by the low benthic d13C values. These two abrupt warming events that occurred at early glacial periods were also observed in the Antarctic temperature and CO2 records, suggesting a close correlation between both Hemispheres. The comparison of the sea surface proxies with the benthic d18O record (as the Southern sign) indicates the presence of a millennial-scale seesaw pattern similar to that seen during the Last Glacial period.

Sea surface temperature estimation for MIS 16 of IODP Site 306-U1313

Hudson Strait (HS) Heinrich Events, ice-rafting events in the North Atlantic originating from the Laurentide ice sheet (LIS), are among the most dramatic examples of millennial-scale climate variability and have a large influence on global climate. However, it is debated as to whether the occurrence of HS Heinrich Events in the (eastern) North Atlantic in the geological record depends on greater ice discharge, or simply from the longer survival of icebergs in cold waters. Using sediments from Integrated Ocean Drilling Program (IODP) Site U1313 in the North Atlantic spanning the period between 960 and 320 ka, we show that sea surface temperatures (SSTs) did not control the first occurrence of HS Heinrich(-like) Events in the sedimentary record. Using mineralogy and organic geochemistry to determine the characteristics of ice-rafting debris (IRD), we detect the first HS Heinrich(-like) Event in our record around 643 ka (Marine Isotope Stage (MIS) 16), which is similar as previously reported for Site U1308. However, the accompanying high-resolution alkenone-based SST record demonstrates that the first HS Heinrich(-like) Event did not coincide with low SSTs. Thus, the HS Heinrich(-like) Events do indicate enhanced ice discharge from the LIS at the end of the Mid-Pleistocene Transition, not simply the survivability of icebergs due to cold conditions in the North Atlantic.

Biomarker and XRD results for the last 3.4 Ma from IODP Site 306-U1313

Various types of abrupt/millennial-scale climate variability such as Dansgaard/Oeschger and Heinrich Events characterized the last glacial period. Over the last decade, a number of studies demonstrated that such millennial-scale climate variability was not limited to the last glacial but inherent to Quaternary climate. Here we review the occurrence and origin of millennial ice-rafting events in the North Atlantic during the late Pliocene and Pleistocene (last 3.4 Ma) with a special focus on North Atlantic Hudson Strait (HS) Heinrich(-like) Events. Besides a clear biomarker signature, we show that Heinrich Layers 5, 4, 2, and 1 in marine sediment cores from across the North Atlantic all bear the organic geochemical fingerprint of the Hudson area. Using this framework and combining previously published results, detailed investigations into the organic and inorganic chemistry of ice-rafted debris (IRD) found across the North Atlantic demonstrate that prior to MIS 16 (~ 650 ka) IRD in the North Atlantic did not originate from the Hudson area of northern Canada. The signature of this early IRD is distinctly different compared to that of HS Heinrich Layers. Rather ice-rafting events during the late Pliocene and early Pleistocene predominantly emanated from the calving of the Greenland and Fennoscandian ice sheets and possibly minor contributions from local ice streams from the North American and British ice sheets. Compared to North Atlantic HS Heinrich Events, these early Pleistocene IRD-events had a limited impact on surface water characteristics in the North Atlantic. North Atlantic HS Heinrich(-like) Events first occurred during MIS 16. At the same time, the dominant frequency in silicate-rich IRD accumulation shifted from the obliquity (41-ka) to a 100-ka frequency across the North Atlantic. Iceberg survivability or a change in iceberg trajectory likely did not control this change in IRD-regime. These results lend further support for the existing hypothesis that an increase in size (thickness) of the Laurentide ice sheet controls the occurrence of North Atlantic HS Heinrich Events, favoring an internal dynamic mechanism for their occurrence.