Heinrich Events

Session 3: Oceanographic and Atmospheric Processes at Low Latitudes

Session Description

This session will investigate marine, terrestrial and atmospheric processes at low latitudes. We will question whether we can reliably attribute chronological relationships between signals recorded at both low and high latitudes, and if we are able to decipher processes from these offsets.

Session Talks

Hydrological and thermal imprints of Heinrich events in the low latitude: Insights from West African monsoon and equatorial Atlantic

Syee Weldeab

Syee Weldeab

University of California, Santa Barbara, California, USA

Numerous paleoclimate records reveal that the tropics responded very sensitively to the millennial-scale northern high latitude ice sheet instabilities during the last glacial and marine isotope stage 3. Well-dated and highly resolved δ18O records from cave and marine deposits across the northern hemisphere monsoon systems indicate a rapid decline of precipitation during Heinrich events and Dansgaard-Oeschger stadials.

Notwithstanding the increasing number of past monsoon records, several key questions remain open. The lack of a quantitative proxy for past rainfall reconstruction prevents a more accurate assessment of the severity of monsoon weakening that occurred parallel to Heinrich events. Our understanding of the mechanism that facilitated a close link between the low and high latitude climates remains also tenuous. Furthermore, much of our current understanding of tropical ocean thermal response to Heinrich events is largely derived from records of the Young Dryas (YD) and Heinrich event 1 (H1). The YD and H1 events and ensuing rapid temperature changes occurred, however, in the backdrop of a rising atmospheric CO2 concentration. Consequently, the contribution of meltwater-induced perturbation to the Tropical Atlantic SST changes during the YD and H1 events is difficult to isolate from those related to an increase of atmospheric greenhouse gasses.

Here we present centennial-scale thermal and hydrological records of eastern equatorial Atlantic (EEA) and West African Monsoon over the last 75,000 years. Our record indicates that West African monsoon was severely weakened during Heinrich events. Rapid drops in monsoon precipitation were accompanied by a weak seasonal contrast and strong shift in δ18O of precipitation toward heavier values, suggesting changes in the source, pathway and recycling of moisture. This observation has an implication for the assessment of monsoon changes based solely on δ18Ocalcite record. We find also evidence that during the YD precipitation decline over equatorial West Africa was not accompanied by large-scale shift of the ITZC.

Our Mg/Ca-based SST estimates show that the EEA was highly sensitive to Heinrich events and responded very differently than upwelling-dominated low latitude continental margins. EEA SST rises between 0.8°C to 2°C concomitant with the onset of Heinrich events. Reduced heat export from the Tropical Atlantic to the northern high latitude, as suggested by modeling studies, may provide a partial explanation for the EEA surface water warmth. The persistence of elevated SST after the abrupt termination of the Heinrich events and the spatial heterogeneity pertaining the direction, magnitude, and duration of thermal changes across the Equatorial Atlantic demands, however, a considerable modulation of Equatorial Atlantic SST by regional processes. We hypothesize that changes of wind-induced low latitude zonal surface currents have significantly contributed to the heterogeneous patterns of Equatorial Atlantic SST.

Temperature and productivity records of Termination 1 in the Gulf of California: evidence for a subtropical response to Heinrich events?

Erin McClymont

Erin L. McClymont1, Raja S. Ganeshram2, Laetitia E. Pichevin2, Alan M. Haywood3, Joy S. Singarayer4 and Paul J. Valdes4

1Department of Geography, University of Durham, Durham, DH1 3LE, U.K.

2School of GeoSciences, University of Edinburgh, Edinburgh, EH9 3JW, U.K.

3School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, U.K.

4School of Geographical Sciences, University of Bristol, Bristol, BS8 1SS, U.K.

The impact of Heinrich events beyond the North Atlantic region continues to be debated, and in particular whether the tropical Pacific shows a response via sea-surface temperature change, ITCZ migration, and/or changing intermediate water properties. Here we present centennial-scale records of sea-surface temperature, opal content and terrigenous inputs to the Gulf of California (GOC) during the Last Glacial Maximum and Termination 1 (c. 25 - 6 ka) to test for evidence for a response to North Atlantic ice-rafting events. IMAGES site MD02-2515 was recovered from Guaymas Basin and is comprised of largely laminated sediments allowing high resolution sampling (< 100 years) to be undertaken. Through the application of two organic geochemistry proxies, the UK37 -index and the TEXH86 index, we present evidence for rapid, stepped changes in temperatures during deglaciation which may be related to North Atlantic ice-rafting events. These occur in both temperature proxies at 13 ka (~3°C increase in 270 yr), 10.0 ka (~2°C decrease over ~250 yr) and at 8.2 ka (3°C increase in <200 yr). An additional rapid warming step is also observed in TEXH86 at 11.5 ka. The onset of the deglacial warming from 17-18 ka is comparable to a "southern hemisphere" signal, but two pronounced decreases in opal content occur in synchrony with Heinrich event 1 and the Younger Dryas. By combining this new palaeo-data with climate model simulations, we consider the potential effects of migrating inter-tropical convergence zone (ITCZ) position, variations in the strength of the Subtropical High, and changing nutrient inputs to GOC in driving these patterns. We find that neither the modern seasonal cycle nor El Niño/Southern Oscillation (ENSO) patterns provide valid analogues for the trends we observe. These results thus challenge some hypotheses to explain the links between Heinrich events and millennial-scale climate oscillations in the tropical Pacific.

Dr. Jennifer D. Stanford, Geography & Environment, University of Southampton