Records of ice-rafted detritus (IRD) concentration in deep-sea cores from the southeast Atlantic Ocean reveal millennial-scale pulses of IRD delivery between 20,000 and 74,000 years ago. Prominent IRD layers correlate across the Polar Frontal Zone, suggesting episodes of Antarctic Ice Sheet instability. Carbon isotopes (delta(13)C) of benthic foraminifers, a proxy of deepwater circulation, reveal that South Atlantic IRD events coincided with strong increases in North Atlantic Deep Water (NADW) production and inferred warming (interstadials) in the high-latitude North Atlantic. Sea level rise or increased NADW production associated with strong interstadials may have resulted in destabilization of grounded ice shelves and possible surging in the Weddell Sea region of West Antarctica.

Quaternary Research 56 (2001) 191-198. doi:10.1006/qres.2001.2252Received by publisher: 2000-09-19Harvest Date: 2016-01-04 12:20:38DOI: 10.1006/qres.2001.2252Page Range: 191-19

Quaternary Research 52 (1999) 369-380. doi:10.1006/qres.1999.2072Received by publisher: 1999-05-14Harvest Date: 2016-01-04 12:20:38DOI: 10.1006/qres.1999.2072Page Range: 369-38

[1] The last glacial to interglacial transition was studied using down core records of stable isotopes in diatoms and foraminifera as well as surface water temperature, sea ice extent, and ice-rafted debris (IRD) concentrations from a piston core retrieved from the Atlantic sector of the Southern Ocean. Sea ice is the first variable to change during the last deglaciation, followed by nutrient proxies and sea surface temperature. This sequence of events is independent of the age model adopted for the core. The comparison of the marine records to Antarctic ice CO 2 variation depends on the age model as 14 C determinations cannot be obtained for the time interval of 29.5-14.5 ka. Assuming a constant sedimentation rate for this interval, our data suggest that sea ice and nutrient changes at about 19 ka B.P. lead the increase in atmospheric pCO 2 by approximately 2000 years. Our diatom-based sea ice record is in phase with the sodium record of the Vostok ice core, which is related to sea ice cover and similarly leads the increase in atmospheric CO 2 . If gas exchange played a major role in determining glacial to interglacial CO 2 variations, then a delay mechanism of a few thousand years is needed to explain the observed sequence of events. Otherwise, the main cause of atmospheric pCO 2 change must be sought elsewhere, rather than in the Southern Ocean.

No abstract

The Adirondack Mountain region is an extensive geographic area (26,305 km(2)) in upstate New York where acid deposition has negatively affected water resources for decades and caused the extirpation of local fish populations. The water quality decline and loss of an established brook trout (Salvelinus fontinalis [Mitchill]) population in Brooktrout Lake were reconstructed from historical information dating back to the late 1880s. Water quality and biotic recovery were documented in Brooktrout Lake in response to reductions of S deposition during the 1980s, 1990s, and 2000s and provided a unique scientific opportunity to re-introduce fish in 2005 and examine their critical role in the recovery of food webs affected by acid deposition. Using C and N isotope analysis of fish collagen and state hatchery feed as well as Bayesian assignment tests of microsatellite genotypes, we document in situ brook trout reproduction, which is the initial phase in the restoration of a preacidification food web structure in Brooktrout Lake. Combined with sulfur dioxide emissions reductions promulgated by the 1990 Clean Air Act Amendments, our results suggest that other acid-affected Adirondack waters could benefit from careful fish re-introduction protocols to initiate the ecosystem reconstruction of important components of food web dimensionality and functionality.

Fourth Lake is a drainage lake at 43°N, 74°W, from which a 37-cm long mud-water interface core was recovered. 210 Pb dating indicates the core spans %340 years, from the Little Ice Age through modern global warming. Diatom accumulation responds to anthropogenic watershed disturbances, declining slightly up-core until a peak in the late-1800s attributable to sediment and nutrient influx from logging and enlargement of the outlet dam. A dramatic decrease occurs %1900 as logging and lake filling ceased, and a smaller peak %1960 accompanies residential development. Similar changes occur in organic carbon accumulation, which ranges from 0.0038-0.024 mg cm À2 year À1 , with generally decreasing values upcore, punctuated by maximum values in the late-1800s. Expressing diatoms as concentration, however, reveals a doubling up-core that positively correlates with changes extending beyond the watershed, including Northern Hemisphere temperature, atmospheric CO 2 concentration and solar irradiance (R = 0.627, 0.675 and 0.400, respectively). A >50% increase in % organic carbon, from 3.8% to 5.9%, also positively correlates with these larger-scale environmental conditions (R = 0.828, 0.830 and 0.832), while negative correlations with the extrabasinal records are exhibited by magnetic susceptibility (R = À0.654, À0.496, and À0.660) and clay (R = À0.770, À0.762, and À0.737). These changes are consistent with decreased sediment influx and reduced dilution of biogenous sedimentary components. In contrast to total diatoms, the accumulation of planktonic genus Asterionella displays a long-term increase up-core. Potential explanations include increasing duration of the ice-free season or a shift in the timing of the spring bloom and a mismatch with abundance of predator(s). Asterionella also increases as a percentage of total diatoms, being positively correlated with extrabasinal conditions (R = 0.827, 0.774 and 0.674). This change occurs at the expense of many benthic genera and, over the past century, at the expense of tychoplanktonic genus, Aulacosiera. Heavily silicified, Aulacosiera requires strong mixing to remain within the epilimnion. Thus, its decline might result from increasing stratification caused by warming.

Previous study identified 6-7 millennial-scale episodes of South Atlantic ice-rafted sediment deposition (SA events) during the glaciation. Questions remain, however, regarding their origin, significance for sea-ice and/or Antarctic ice-sheet dynamics, and relationship to climate. Here I correlate sediment core (TTN057-21) SA events to Greenland and Antarctic ice using two independent methods, stable isotopes and geomagnetic paleointensity, placing SA events in the context of polar climate change in both hemispheres. Marine isotopic stage (MIS) 3 SA events generally coincided with Greenland interstadials and with cooling following Antarctic warm events (A1-A4). This anti-phase behavior is best illustrated when SA0 coincided with both the Antarctic Cold Reversal and Bolling-Allerod warming in Greenland. Moreover, SA events coincide with sea-level rises during the deglaciation (mwp1A) and MIS 3 (30.4, 38.3, 43.7, 51.5 ka), implying unpinning of grounded Weddell Sea region ice masses discharged debrisladen bergs that had a chilling effect on South Atlantic surface temperatures.