Passage:
In 1988, marine geologist Hartmut Heinrich published an analysis of deep-sea sediment core V23-81 recovered from the Dreizack seamount in the eastern North Atlantic basin. Heinrich identified six discrete, anomalous stratigraphic intervals deposited between 70,000 and 14,000 years ago during the last glacial epoch. In contrast to ambient pelagic sedimentation characterized by fine-grained biogenic calcite from the shells of planktonic foraminifera, these episodic layers—subsequently designated Heinrich events H1 through H6—were overwhelmingly dominated by coarse lithic detritus (>150 micrometers). Because clasts of such mass cannot be transported across oceanic distances by aeolian or surface-current processes, Heinrich postulated that they were deposited through ice-rafting: debris-laden armadas of icebergs calved en masse from continental ice sheets, drifting across the subpolar gyre and depositing their terrigenous load upon melting.
Subsequent geochemical and isotopic provenancing by Gerard Bond and Steven Hemming confirmed the glaciological origins and spatial heterogeneity of these ice-rafting episodes. Utilizing argon-40/argon-39 radiogenic dating of detrital hornblende grains and neodymium isotopic ratios (), researchers mapped the mineralogical signature of the detrital carbonate limestone and dolostone that constitute the bulk of the lithic material in layers H1, H2, H4, and H5. These analyses demonstrated that the icebergs responsible for these four episodes originated almost exclusively from the Hudson Strait outlet of the Laurentide Ice Sheet, which scoured the Paleozoic carbonate bedrock underlying Hudson Bay and the Churchill Province of the Canadian Shield. In contrast, sedimentological profiling of layers H3 and H6 revealed a radically different mineral assemblage dominated by basaltic glass and quartzose siltstone lacking detrital carbonate. Isotopic fingerprinting traced the detritus in H3 and H6 to ice surges originating from the Fennoscandian, British-Irish, and Icelandic ice caps.
Beyond establishing the provenance of the ice-rafted debris, researchers determined that the colossal influx of low-salinity meltwater accompanying Heinrich events severely lowered surface-water density, precipitating the near-total shutdown of North Atlantic Deep Water formation and the Atlantic Meridional Overturning Circulation.
Statement:
Based on the passage, the ice-rafted sediment layers associated with Heinrich events H3 and H6 are distinguished from those associated with events H1, H2, H4, and H5 by their primary composition of lithic material derived from Scandinavian, British-Irish, and Icelandic ice sheets rather than Canadian carbonate bedrock.
Answer: Answer