Salinity-driven density stratification of the upper Arctic Ocean isolates sea-ice cover and cold, nutrient-poor surface waters from underlying warmer, nutrient-rich waters. Recently, stratification has strengthened in the western Arctic but has weakened in the eastern Arctic; it is unknown if these trends will continue. Here we present foraminifera-bound nitrogen isotopes from Arctic Ocean sediments since 35,000 years ago to reconstruct past changes in nutrient sources and the degree of nutrient consumption in surface waters, the latter reflecting stratification. During the last ice age and early deglaciation, the Arctic was dominated by Atlantic-sourced nitrate and incomplete nitrate consumption, indicating weaker stratification. Starting at 11,000 years ago in the western Arctic, there is a clear isotopic signal of Pacific-sourced nitrate and complete nitrate consumption associated with the flooding of the Bering Strait. These changes reveal that the strong stratification of the western Arctic relies on low-salinity inflow through the Bering Strait. In the central Arctic, nitrate consumption was complete during the early Holocene, then declined after 5,000 years ago as summer insolation decreased. This sequence suggests that precipitation and riverine freshwater fluxes control the stratification of the central Arctic Ocean. Based on these findings, ongoing warming will cause strong stratification to expand into the central Arctic, slowing the nutrient supply to surface waters and thus limiting future phytoplankton productivity.
The cyclic growth and decay of continental ice sheets can be reconstructed from the history of global sea level. Sea level is relatively well constrained for the Last Glacial Maximum (LGM, 26,500 to 19,000 y ago, 26.5 to 19 ka) and the ensuing deglaciation. However, sea-level estimates for the period of ice-sheet growth before the LGM vary by > 60 m, an uncertainty comparable to the sea-level equivalent of the contemporary Antarctic Ice Sheet. Here, we constrain sea level prior to the LGM by reconstructing the flooding history of the shallow Bering Strait since 46 ka. Using a geochemical proxy of Pacific nutrient input to the Arctic Ocean, we find that the Bering Strait was flooded from the beginning of our records at 46 ka until 35.7 - 2.4 + 3.3 ka. To match this flooding history, our sea-level model requires an ice history in which over 50% of the LGM’s global peak ice volume grew after 46 ka. This finding implies that global ice volume and climate were not linearly coupled during the last ice age, with implications for the controls on each. Moreover, our results shorten the time window between the opening of the Bering Land Bridge and the arrival of humans in the Americas.
<p>Recent temperature history of the Juneau Icefield</p><p>&#160;</p><p>Mass loss from Alaskan glaciers makes a significant contribution to current sea-level rise. The Juneau Icefield (JIF) of southeast Alaska is one of the world largest, and longest-studied, ice fields, and is currently in a documented state of thinning, retreat, and negative mass balance. The climatological context of this glacier change is critical to understanding its causes, the future of the region, and perhaps that of similar mountain glaciers. Do these changes primarily reflect changes in accumulation or ablation? Are mean air temperatures in the region increasing? If so, during which season, ablation or accumulation, are the changes strongest?<br><br>Here we investigate the recent temperature history of the Juneau Icefield, using a combination of reanalysis data and in situ temperature observations from the Juneau Icefield Research Program.&#160; On the whole, we find a significant trend in annual average temperature since the 1950&#8217;s of 0.19&#176;C per decade. Interestingly, this warming is entirely a winter-season signal. We find no significant trend in summer-season temperatures, but a winter time trend of nearly 0.5&#176;C per decade, over twice that of the annual average. This pattern is consistent between the reanalysis products and the local temperature observations across the icefield. Using the in situ measurements from stations across the icefield, we find that the magnitude of the winter-season warming (and that of the annual mean warming) depends strongly on surface elevation: the higher the surface elevation the larger the trend in warming. These results have implications for the cause of recent glacier changes. While there is little evidence for a change in ablation-season temperatures, these results point toward changes in both the length of the ablation season and perhaps the phase of winter precipitation. The elevation-dependence of these trends may have further implications for the future stability of the JIF.</p>
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