A fresh set of numbers is changing what scientists thought happens when a city‑sized slab of ice dissolves into the sea. The target was mega‑iceberg A‑68A, a floating platform roughly twice the area of Delaware that drifted toward South Georgia in early 2021.

“We think this is the first time measurements have been made so close to an iceberg,” said Natasha Lucas of the British Antarctic Survey, lead author of the new study.

For the first time, researchers steered robotic vehicles to within 14 mi of such a iceberg and kept them there long enough to watch meltwater, temperature, and life interact in real time. 

A‑68A calved from the Larsen C Ice Shelf back in 2017, then rode the currents of the Southern Ocean on a five‑year tour that ended near South Georgia.

Satellites could trace its general route, but ships kept their distance because the iceberg’s smaller rifts and spins defied prediction.

That standoff ended when a six‑foot‑long glider slipped off the deck of the research vessel RRS James Cook and began diving and climbing through a 0.6‑mile‑deep column of water.

Over 49 days it logged 265 vertical profiles, sending each packet of data to scientists piloting the craft from home offices locked down by COVID‑19 restrictions.

The adventure was not entirely smooth. One glider vanished in the melee of drifting growlers, and another spent 17 days wedged beneath the berg before currents spat it out, battery blinking but sensors intact.

Still, the mission mapped a zone of ocean that had never been observed so closely or for so long.

The glider recorded a frosty cap of freshwater spreading across the top 30 ft of the sea. Right below, the instruments found a band of cold winter water that had lost its usual punch, a sign that melt‑driven turbulence had chewed away the seasonal layer separating surface and depth.

As that barrier thinned, nutrient‑rich deep water began sliding upward, carrying iron and silica shaved from the iceberg’s base.

The upheaval altered local stratification, flipping the normal stack of density and temperature that keeps layers neatly in place during the Antarctic summer.

Salinity told the same story. At barely two miles from A‑68A, the profiles showed tongue‑shaped intrusions of water warmer and saltier than their neighbors, hallmarks of meltwater plumes that had raced up the iceberg’s base before fanning out sideways.

Plankton reacted almost immediately. Near the ice wall, the glider’s fluorometer logged low chlorophyll and high backscatter, evidence that meltwater diluted existing blooms while also dumping silt that clouded the light.

Farther away, a spike in chlorophyll flared about 36 hours after the iceberg’s passage, matching lab‑measured growth rates for the local phytoplankton community.

The burst hints that the same plume which first thinned the bloom later primed the water with micronutrients, a one‑two punch that temporarily shuffled who eats whom in the surface ocean.

Because upwelling also lifts dissolved carbon and heat, even short‑lived events like this matter to krill, fur seals, and the economy of South Georgia’s fisheries.

The carbon sink south of 35° S absorbs roughly 40 percent of humanity’s fossil‑fuel emissions. How that service will fare as calving accelerates remains uncertain.

The new findings show giant bergs can raise near‑surface stratification yet also stir the mid‑depths, two adjustments that tug carbon uptake in opposite directions.

Recent model work argues current estimates may already overshoot Southern Ocean uptake by up to 0.4 Pg C yr‑1. Field evidence like the A‑68A data set will help sort winners from losers in that accounting.

Flying a glider next to a iceberg the size of Los Angeles is an exercise in calculated gamble. Lucas and colleagues guided their craft from desks more than 7,500 miles away, relying on patchy satellite snapshots to dodge smaller ice.

When the glider went missing under the berg, frustration turned to nervous excitement, then relief as it resurfaced with a gold mine of temperature and salinity profiles.

That trove fixed the first real numbers on basal melt rate, pegging it at roughly 6.9 × 10^8 m³ per day, an estimate within a factor of two of satellite calculations derived from changes in the iceberg’s freeboard height.

A‑68A is gone now, splintered into shards tracked only by radar, but younger cousins are lining up. The trillion‑ton A‑23A grounded off South Georgia this March, parking just 43 miles from the shore.

Researchers expect more deep‑drafted icebergs to exit West Antarctica as warming currents nibble ice shelves.

Each one represents a floating lab, a hazard to shipping, and an agent of surprise for ecosystems that evolved around predictable cycles of light and nutrients.

The new study proves those labs are accessible with today’s tools. It also shows that a single iceberg can redraw local physics and biology in days, a reminder that climate change sends ripple effects far beyond melting ice.

The study is published in Nature Geoscience.

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