caused by burrowing animals Flux in planktonic isotopic make-up (changes in δ13C, decreased δ18O) Pollen indications of cold-loving pines replacing oaks on the North American mainland (Grimm et al. 1993) Decreased foramaniferal abundance – which due to the pristine nature of many samples cannot be attributed to preservational bias and has been related to reduced salinity Increased terrigenous runoff from the continents, measured near the mouth of the Amazon River Increased grain size in wind-blown loess in China, suggesting stronger winds Changes in relative Thorium-230 abundance, reflecting variations in ocean current velocity Increased deposition rates in the northern Atlantic, reflected by an increase in continentally derived sediments (lithics) relative to background sedimentation Expansion of grass and shrubland across large areas of Europe The global extent of these records illustrates the dramatic impact of Heinrich events. Unusual Heinrich events H3 and H6 do not share such a convincing suite of Heinrich event symptoms as events H1, H2, H4, and H5, which has led some researchers to suggest that they are not true Heinrich events. That would make Gerard C. Bond's suggestion of Heinrich events fitting into a 7,000-year cycle ("Bond events") suspect. Several lines of evidence suggest that H3 and H6 were somehow different from the other events. Lithic peaks: a far smaller proportion of lithics (3,000 vs. 6,000 grains per gram) is observed in H3 and H6, which means that the role of the continents in providing sediments to the oceans was relatively lower. Foram dissolution: Foraminifera tests appear to be more eroded during H3 and H6 (Gwiazda et al., 1996). That may indicate an influx of nutrient-rich, hence corrosive, Antarctic Bottom Water by a reconfiguration of oceanic circulation patterns. Ice provenance: Icebergs in H1, H2, H4, and H5 are relatively enriched in Paleozoic "detrital carbonate" originating from the Hudson Strait region; while H3 and H6 icebergs carried less of this distinctive material Ice rafted debris distribution: Sediment transported by ice does not extend as far East during H3/6. Hence some researchers have been moved to suggest a European origin for at least some H3/6 clasts: America and Europe were originally adjacent to one another; hence, the rocks on each continent are difficult to distinguish, and the source is open to interpretation. Causes As with so many climate related issues, the system is far too complex to be confidently assigned to a single cause. There are several possible drivers, which