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Its hydrogen atoms and lone pairs of electrons are 20. Teeter, M. M. Water structure of a hydrophobic protein at atomic resolution: Pentagon rings of water molecules in crystals of crambin. Proc. Natl Acad. Sci. USA 81, 6014–6018 (1984). directed towards these neighbours, thus facilitating the formation of four 21. Baker, E. N. Structure of actinidin, after refinement at 1.7 A˚ resoution. J. Mol. Biol. 141, 441–484 hydrogen bonds per water molecule, with each molecule acting as a double (1980). 22. Liljas, A. et al. Crystal structure of human carbonic anhydrase C. Nature New Biol. 235, 131–137 donor as well as a double acceptor. In each case, the lattice consists of two- (1972). dimensional layers of hexagonal rings in the chair conformation. These puckered 23. Quiocho, F. A., Wilson, D. K. & Vyas, N. K. Substrate specificity and affinity of a protein modulated by sheets are stacked on one another, and are interconnected by the formation of bound water molecules. Nature 340, 404–407 (1989). hexagonal rings composed of three molecules each form adjacent layers. The Supplementary information is available on Nature’s World-Wide Web site (http://www.nature.com) or as paper copy from the London editorial office of Nature. two forms of ice I differ only in the stacking sequence of the puckered layers relative to one another, such that the interconnecting rings are in the boat Acknowledgements. We thank C. Barnes for assistance with the unit cell and space group determination. This work was supported by the US NSF. conformation in ice Ih, while they assume the chair conformation in ice Ic. Correspondence and requests for materials should be addressed to J.L.A. (e-mail: chemja@mizzou1. missouri.edu). techniques has afforded a relatively clear understanding of the structure of water in the solid state, and 12 polymorphic forms of ice, denoted as Ih,Ic and II–XI, have been characterized to date. Effect of the formation Phases Ih and Ic are the two most common forms of ice and are strikingly similar in structure (Fig. 4). Extended water aggregates are of the Isthmus of Panama also encountered in multi-component crystals and well-known examples include biological macromolecules15 and clathrate on Atlantic Ocean hydrates16. We also note that a recent study has produced a solid- state structure which is stabilized by an infinite two-dimensional thermohaline circulation 17 ice-like framework intercalated between CdNi(CN)4 layers . An improved understanding of the three-dimensional structural Gerald H. Haug* & Ralf Tiedemann aspects of water has important implications in the area of structural GEOMAR, Forschungszentrum fu¨r Marine Geowissenschaften, Universita¨t Kiel, biology. Indeed, investigations of the forces at play in macromole- Wischhofstrasse 1-3, D-24148 Kiel, Germany cular interactions18 have demonstrated the importance of water . structuring, and this point has been illustrated by several examples The Late Cenozoic closure of the seaway between the North including the structures of Scapharca dimeric haemoglobin19, and South American continents is thought to have caused exten- crambin20, actinidin21 and carbonic anhydrase C (ref. 22). There is sive changes in ocean circulation and Northern Hemisphere much evidence that alludes to the presence of ordered water clusters climate1–2. But the timing and consequences of the emergence of in the active clefts of these proteins, but unambiguous positional the Isthmus of Panama, which closed the seaway, remain contro- information is still rare. It is thought that water molecules con- versial1–5. Here we present stable-isotope and carbonate sand- tribute to the complex stability by mediating hydrogen bonds fraction records from Caribbean sediments which, when compared between the functional groups of the protein and the ligands, and to Atlantic and Pacific palaeoceanographic records, indicate that by filling potential voids or holes inside the binding site23. the closure caused a marked reorganization of ocean circulation The structure reported here demonstrates that the ice Ic arrange- starting 4.6 million years ago. Shallowing of the seaway intensified ment can be a favourable conformation for a water cluster in a the Gulf Stream and introduced warm and saline water masses to mixed-component system, even at room temperature. It also shows high northern latitudes. These changes strengthened deep-water how the cluster and its surroundings assume a complementary formation in the Labrador Sea over the next million years—as relationship, resulting in the void available to the cluster being indicated by an increased deep-water ventilation and carbonate optimally occupied both in terms of packing efficiency and the preservation in the Caribbean Sea—and favoured early Pliocene maximization of intermolecular interactions. M warming of the Northern Hemisphere. The evaporative cooling of surface waters during North Atlantic Deep Water formation Received 5 February; accepted 6 April 1998. would have introduced moisture to the Northern Hemisphere. 1. Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspectives (VCH, Weinheim, 1995). 2. Lehn, J.-M. Supramolecular chemistry. Science 260, 1762–1763 (1993). 3. Seto, C. T. & Whitesides, G. M. Molecular self-assembly through hydrogen bonding: Supramolecular * Present address: Department of Earth Sciences, University of Southern California. Los Angeles, aggregates based on the cyanuric acid ⋅ melamine lattice. J. Am. Chem. Soc. 115, 905–916 (1993). California 90089-0740, USA. Nature © Macmillan Publishers Ltd 1998 NATURE | VOL 393 | 18 JUNE 1998 673 letters to nature Although the pronounced intensification of Northern Hemi- intensification of Northern Hemisphere ice-sheet growth. However, sphere glaciation between 3.1 and 2.5 million years ago substan- the identification of the particular step in the closure of the tially lagged the full development of North Atlantic Deep Water Panamanian gateway that acted as a critical threshold for profound formation, we propose that the increased atmospheric moisture changes in deep-ocean circulation and climate remained qualitative content was a necessary precondition for ice-sheet growth, which and speculative9. Here we present proxy data which demonstrate was then triggered by the incremental changes in the Earth’s that the closure has affected deep ocean circulation since 4.6 Myr orbital obliquity. ago. 13 The gradual closing of the Isthmus of Panama lasted from 13 to Today, a mixture of nutrient-enriched, low-d C Antarctic Inter- 3–5 13 1.9 Myr ago (all originally published ages were adjusted to the mediate Water (AAIW) and nutrient-depleted, high-d C Upper new astronomically dated timescale6). Most evidence for restricted North Atlantic Deep Water (UNADW) and Mediterranean Over- 8 water-mass exchange through the Panama strait is based on sedi- flow Water cross the Atlantic-Caribbean sills at 1,600–1,900 m ment records from Caribbean and Pacific Deep Sea Drilling Pro- (Windward Passage, Anegada-Jungfern Passage) and fill the deep gram (DSDP) sites 502 and 503.