Orbital and suborbital variations of productivity and sea surface conditions in the Gulf of Alaska during the past 54,000 years: Impact of iron fertilization by icebergs and meltwater

Abstract

As a high-nutrient and low-chlorophyll region, the modern Gulf of Alaska (GoA) is strongly impacted by the limitation of iron. Paleostudies along the Alaskan slope have mainly focused on reconstructing environmental conditions over the past 18 ka. Based on micropaleontological, biogeochemical and sedimentological parameters, we explore a sediment record covering the past 54 ka at Integrated Ocean Drilling Program Site U1419 to understand the impact of orbital and suborbital-scale climate variability on productivity and sea-surface conditions. Close to the Cordilleran Ice Sheet (CIS), Site U1419 is ideally located to elucidate how the evolution of a large ice mass and glacial processes affected orbital-and suborbital-scale changes in nutrients-(e.g., iron) supply. Meltwater discharge from the northern CIS impacted sea surface dynamics of GoA coastal waters. The corresponding increase in bulk biogenic concentrations during Marine Isotope Stage (MIS) 3 and MIS 2 (54 -17.3 ka) suggests a direct impact from iron fertilization. Cooling of surface waters played no primary role in the occurrence of primary producers. The inundation of the subaerially exposed continental shelf during the last deglacial (17.3-10 ka) warming could have served as a major micronutrients source. Low productivity after the last deglaciation suggests reduced iron availability. Our multiproxy approach reveals a more complete picture of late Quaternary productivity variations compared to earlier studies along the Alaskan margin. The impact of tidewater glaciers and meltwater discharge on past marine productivity and nutrient budget dynamics of high-latitude coastal regions is discussed.

Ice and ocean constraints on early human migrations into North America along the Pacific coast

“…These data confirm and improve on previous chronologies of sea ice based on the IP 25 index, sea-ice diatoms, and dinocyst assemblages in the North Pacific and Bering Sea ( 22 , 31 , 42 , 45 – 47 ). Collectively, these multiproxy records indicate that winter sea ice was extensive and persistent in the Bering Sea and Gulf of Alaska between ~26 and 15 ka and was present intermittently in the Gulf of Alaska during late Marine Isotope Stage 3 (42 to 27 ka).…”

“… 20 ) in the Gulf of Alaska; ( C ) Percent benthic foraminifera indicative of meltwater from site U1421 ( 42 ); ( D ) Percent of sea-ice diatoms in core U1421 ( 42 ); ( E ) Alkenone %C 37:4 records from various Gulf of Alaska cores as a proxy for sea ice: data from U1419 are from ref. 31 , all other data are from this paper; ( F ) PbIP 25 sea-ice index from site SO202-27-6 in the Gulf of Alaska ( 22 ); >0.75 = extended sea ice, 0.5 to 0.75 = marginal sea ice, 0.1 to 0.5 = variable sea ice <0.1 = ice free). Blue shaded bars indicate Siku Events (SE1 & SE2); pink shaded bars indicate warm periods with little sea ice (Bølling-Allerød and early Holocene); yellow shaded bars indicate the inter-Siku time intervals in which winter sea ice was present in the Gulf of Alaska.…”

“…It is likely that sea ice was present in winter but mostly melted during summer in the Gulf of Alaska, even during the last glacial interval. Values of %C 37:4 between ~10 and 50% in the Gulf of Alaska records during the LGM and early deglacial period would correspond to ~20 to 60% mean annual sea ice concentrations based on modern observed correlations ( 36 ), indicating sea ice was present during only the coldest months of the year, and the IP 25 , diatom-, and dinocyst-based sea ice records show no indication of perennial sea ice during these times ( 22 , 31 , 45 – 47 ). SST reconstructions from the Bering Sea and Gulf of Alaska also indicate ocean temperatures remained above freezing during summer months throughout the LGM, implying that extensive winter sea ice would have given way to open water conditions during summer ( 22 , 31 ).…”

“…Values of %C 37:4 between ~10 and 50% in the Gulf of Alaska records during the LGM and early deglacial period would correspond to ~20 to 60% mean annual sea ice concentrations based on modern observed correlations ( 36 ), indicating sea ice was present during only the coldest months of the year, and the IP 25 , diatom-, and dinocyst-based sea ice records show no indication of perennial sea ice during these times ( 22 , 31 , 45 – 47 ). SST reconstructions from the Bering Sea and Gulf of Alaska also indicate ocean temperatures remained above freezing during summer months throughout the LGM, implying that extensive winter sea ice would have given way to open water conditions during summer ( 22 , 31 ). These data suggest that seasonal fluctuations between winter sea ice and summer kelp forest and other near-shore habitats persisted for millennia around the margins of the North Pacific, both conditions to which successful coastal people would have had to adapt.…”

“… 20 ). Sea ice formed in the subarctic North Pacific through much of this interval ( 22 , 31 ), which may have impacted boat transit and altered marine resource composition and availability during certain months of the year. Today, land-fast sea ice provides a relatively unobstructed and flat surface as a platform for travel between otherwise inaccessible high Arctic communities, typically in winter or spring ( 32 , 33 ).…”