We crossed the Arctic Circle, 66° 33’ North, southbound on Friday afternoon, June 20, an exquisite, cloudless day. The snowy, black-rock mountains of Cape Dezhneva lay away to starboard, to port Cape Prince of Wales, geologic brothers separated by 45 miles of saltwater and international enmity. We’re approaching the two Diomedes fine on the starboard bow, home to the Yup’ic people since the retreat of the last Ice Age, though neither, vertical sided, naked ice-cleaved rock, looks congenial to human habitation. Big Diomede is Russian, Little Diomede U.S territory. The International Dateline as well as the border passes between them; it’s tomorrow in Russia, while we sail in today. Healy’s making 16 knots over the bottom, but you’d never know she was moving at all if you didn’t see the dial or look over the side, Hotel Healy. I’ll miss her. I’m curious about her behavior in a real seaway, but the old timers tell me she’s a gentle giant. Chief Engineer Tom Lowry has the “flume tank” up and operating. It directs water ballast from one side of the ship to the other opposing the direction of roll, thus dampening it, but if the forecast is correct, we won’t need it. Now, as to what we’ve learned: Let’s begin with the ice itself, pausing to repeat that most fundamental fact of nature—plankton bloom when light and nutrients are simultaneously present. The logical hypothesis resulting from ICESCAPE, yet untested, was that light reached the water column through those skylight-like melt ponds. We, however, have seen numerous large leads and country-lake-sized patches of open water, the result not of melting, but of the dynamics within the pack, floes crashing into one another and splitting apart. Plenty of light was reaching the water. So, never mind melt ponds for now, why didn’t we witness blooms as a result of all that light shining on open water? Because the air was so cold new, “grease ice” was constantly forming. When new ice forms, the process of brine extraction begins, whereby the salt leeches of out the bottom of the ice, making the surface water denser. This causes “convective mixing” as the heavy water displaces lighter water below. However, this same mixing acted to forestall the bloom, because as soon as any phytoplankton were carried up into light near the surface where, all things being equal, they would have bloomed, convection drove them back down out of the conducive light toward the bottom. This up-down, down-up cycle, the very action that spread the nutrients throughout the entire water column, also prevented the plankton from remaining in optimum conditions long enough to bloom robustly. In the shallow Chukchi, often no more than 50 meters deep, convective mixing reaches the sea floor and brings bottom sediment up into contact with the ice. We’ve seen a lot of “dirty” ice. When bottom sediment becomes entrained in the ice, it blocks light and therefore retards plankton blooms. Further, there was a consistent layer of brown algae attached to the keel of the ice that also prohibited light penetration. However, when we made that three-day stop, the system was thoroughly preconditioned for melting, and the algae was relinquishing its hold on the bottom of the floes as the ice softened and began to fall away. Then it snowed, barely an inch. Hitherto, no one expected that a slight late-season snowfall could raise the albedo high enough to forestall the formation of melt ponds. So we learned considerable new lessons about the penetration of light through the pack ice before the onset of melt ponds. Why melt ponds form in the first place was among the questions this expedition set out to answer. That this was unknown came as a surprise to me, and maybe I’m not alone in that. After all, we came looking for a plankton bloom—maybe even a mega bloom similar to the one that surprised everyone on ICESCAPE—fed presumably by light spread through melt ponds, yet the reason for their very existence remained obscure. Sea ice tends to be more or less porous due to enlarged grain size as temperatures warm or to pockets of salt trapped within an otherwise solid structure. So why doesn’t the fresh water that pools on the surface of the ice, above sea level, simply drain through the pores into the saltwater below? The answer turns out to be elegantly simple, as Ken and Chris concluded through repeated experimentation on the ice using fresh- and saltwater, and food dye to gauge the extent of ice permeability. Their conclusion: Because freshwater has a higher freezing point than saltwater, the freshwater (from snowmelt) quickly freezes at the bottom and sides of the pond where it contacts the ice and thereby forms a seal that prevents the freshwater from flowing out through the otherwise permeable sea ice below. Using those repeated CTD casts to measure temperature and salinity, Bob mapped and, as a result, redefined some of the oceanographic pathways by which nutrient-rich winter water from the Bering Strait spreads across the Chukchi shelf. It was important to learn that some of the nutrient-bearing water does not immediately exit the shelf to the north, but meanders its way back and forth via a maze of smaller filaments. This means that the nutrients remain longer on the shelf than previously supposed, able to support primary production locally in the Chukchi Sea rather than in the deep basin. It’s quite reasonable to ask whether the conditions we witnessed were typical or were they aberrations? We might ask the same question about the under-ice mega-bloom witnessed by these same scientists during ICESCAPE. We can’t know for sure because of the dearth of observations this time of year, understandable given the difficulty, the expense, and limited number of ships able to operate in pack ice. It’s sort of reflexive in this technological era, when we’ve been to the moon and unmanned vehicles have landed on Mars, when we can photograph from space every square meter of Earth’s surface, to think we know more than we actually do about the nature’s systems, including but not limited to our own climate. This expedition has taught me, and I reckon others of us, that our knowledge, despite the best efforts of keen and honest minds, is sorely limited. It’s a humbling thought, and yet exciting, too, the possibilities and opportunities for new knowledge. It has been my privilege to witness the exploration. Leave a Reply Cancel Reply Your email address will not be published. Name* Email* Website Comment Notify me of follow-up comments by email. Notify me of new posts by email.