Response to comment on "Evidence of prehistoric human activity in the Falkland Islands".

Hamley et al. previously presented multiple lines of evidence that people were present in the Falkland Islands before Europeans and may have brought the now-extinct canid, Dusicyon australis. Stable isotope data reported by Clark et al. indicate that D. australis had a high-trophic, marine diet that terrestrialized following European arrival. This is consistent with our hypothesis of a human mutualism.

Evidence of prehistoric human activity in the Falkland Islands.

When Darwin visited the Falkland Islands in 1833, he noted the puzzling occurrence of the islands' sole terrestrial mammal, Dusicyon australis (or "warrah"). The warrah's origins have been debated, and prehistoric human transport was previously rejected because of a lack of evidence of pre-European human activity in the Falkland Islands. We report several lines of evidence indicating that humans were present in the Falkland Islands centuries before Europeans, including (i) an abrupt increase in fire activity, (ii) deposits of mixed marine vertebrates that predate European exploration by centuries, and (iii) a surface-find projectile point made of local quartzite. Dietary evidence from D. australis remains further supports a potential mutualism with humans. The findings from our study are consistent with the culture of the Yaghan (Yámana) people from Tierra del Fuego. If people reached the Falkland Islands centuries before European colonization, this reopens the possibility of human introduction of the warrah.

Younger Dryas deglaciation of Scotland driven by warming summers.

The Younger Dryas Stadial (YDS; ∼ 12,900-11,600 y ago) in the Northern Hemisphere is classically defined by abrupt cooling and renewed glaciation during the last glacial-interglacial transition. Although this event involved a global reorganization of atmospheric and oceanic circulation [Denton GH, Alley RB, Comer GC, Broecker WS (2005) Quat Sci Rev 24:1159-1182], the magnitude, seasonality, and geographical footprint of YDS cooling remain unresolved and pose a challenge to our understanding of abrupt climate change. Here, we present a deglacial chronology from Scotland, immediately downwind of the North Atlantic Ocean, indicating that the Scottish ice cap disintegrated during the first half of the YDS. We suggest that stratification of the North Atlantic Ocean resulted in amplified seasonality that, paradoxically, stimulated a severe wintertime climate while promoting warming summers through solar heating of the mixed layer. This latter process drove deglaciation of downwind landmasses to completion well before the end of the YDS.

Natural and anthropogenic variations in atmospheric mercury deposition during the Holocene near Quelccaya Ice Cap, Peru.

Mercury (Hg) is a toxic metal that is transported globally through the atmosphere. The emission of Hg from mineral reservoirs and subsequent recycling in surface reservoirs (i.e., soil/biomass, ocean, and atmosphere) are fundamental to the modern global Hg cycle, yet past emissions from anthropogenic and natural sources are not fully constrained. We use a sediment core from Yanacocha, a headwater lake in southeastern Peru, to study the anthropogenic and natural controls on atmospheric Hg deposition during the Holocene. From 12.3 to 3.5 ka, Hg fluxes in the record are relatively constant (mean ± 1σ: 1.4 ± 0.6 µg m-2 a-1, n = 189). Past Hg deposition does not correlate with changes in regional temperature and precipitation, inferred from nearby paleoclimate records, or with most large volcanic events that occurred regionally, in the Andean Central Volcanic Zone (~300-400 km from Yanacocha), and globally. In B.C. 1450 (3.4 ka), Hg fluxes abruptly increased and reached the Holocene-maximum flux (6.7 µg m-2 a-1) in B.C. 1200, concurrent with a ~100-year peak in Fe and chalcophile metals (As, Ag, Tl) and the presence of framboidal pyrite. Continuously elevated Hg fluxes from B.C. 1200-500 suggest a protracted mining-dust source near Yanacocha that is identical in timing to documented pre-Incan cinnabar mining in central Peru. During Incan and Colonial time (A.D. 1450-1650), Hg deposition remains elevated relative to background levels but lower relative to other Hg records from sediment cores in central Peru, indicating a limited spatial extent of preindustrial Hg emissions. Hg fluxes from A.D. 1980 to 2011 (4.0 ± 1.0 µg m-2 a-1, n = 5) are 3.0 ± 1.5 times greater than pre-anthropogenic fluxes and are similar to modern fluxes documented in remote lakes around the world.

Freshwater outburst from Lake Superior as a trigger for the cold event 9300 years ago.

Paleoclimate proxy records reveal a pervasive cooling event with a Northern Hemispheric extent approximately 9300 years ago. Coeval changes in the oceanic circulation of the North Atlantic imply freshwater forcing. However, the source, magnitude, and routing of meltwater have remained unknown. Located in central North America, Lake Superior is a key site for regulating the outflow of glacial meltwater to the oceans. Here, we show evidence for an approximately 45-meter rapid lake-level fall in this basin, centered on 9300 calibrated years before the present, due to the failure of a glacial drift dam on the southeast corner of the lake. We ascribe the widespread climate anomaly approximately 9300 years ago to this freshwater outburst delivered to the North Atlantic Ocean through the Lake Huron-North Bay-Ottawa River-St. Lawrence River valleys.

Comment on "Absence of cooling in New Zealand and the adjacent ocean during the Younger Dryas chronozone".

Barrows et al. (Reports, 5 October 2007, p. 86) presented cosmogenic exposure dates and data from an ocean sediment core that challenge evidence for glacier advance in New Zealand during the Younger Dryas event. We use modeling of geomorphic processes to argue that their cosmogenic exposure dates are inconclusive.

Near-synchronous interhemispheric termination of the last glacial maximum in mid-latitudes.

Isotopic records from polar ice cores imply globally asynchronous warming at the end of the last glaciation. However, 10Be exposure dates show that large-scale retreat of mid-latitude Last Glacial Maximum glaciers commenced at about the same time in both hemispheres. The timing of retreat is consistent with the onset of temperature and atmospheric CO2 increases in Antarctic ice cores. We suggest that a global trend of rising summer temperatures at the end of the Last Glacial Maximum was obscured in North Atlantic regions by hypercold winters associated with unusually extensive winter sea ice.

Regional insolation forcing of late Quaternary climate change in the Southern Hemisphere.

In agreement with the Milankovitch orbital forcing hypothesis it is often assumed that glacial-interglacial climate transitions occurred synchronously in the Northern and Southern hemispheres of the Earth. It is difficult to test this assumption, because of the paucity of long, continuous climate records from the Southern Hemisphere that have not been dated by tuning them to the presumed Northern Hemisphere signals. Here we present an independently dated terrestrial pollen record from a peat bog on South Island, New Zealand, to investigate global and local factors in Southern Hemisphere climate changes during the last two glacial-interglacial cycles. Our record largely corroborates the Milankovitch model of orbital forcing but also exhibits some differences: in particular, an earlier onset and longer duration of the Last Glacial Maximum. Our results suggest that Southern Hemisphere insolation may have been responsible for these differences in timing. Our findings question the validity of applying orbital tuning to Southern Hemisphere records and suggest an alternative mechanism to the bipolar seesaw for generating interhemispheric asynchrony in climate change.