Glacial Water Mass Research Articles

Deeper and stronger North Atlantic Gyre during the Last Glacial Maximum

Subtropical gyre (STG) depth and strength are controlled by wind stress curl and surface buoyancy forcing1,2. Modern hydrographic data reveal that the STG extends to a depth of about 1 km in the Northwest Atlantic, with its maximum depth defined by the base of the subtropical thermocline. Despite the likelihood of greater wind stress curl and surface buoyancy loss during the Last Glacial Maximum (LGM)3, previous work suggests minimal change in the depth of the glacial STG4. Here we show a sharp glacial water mass boundary between 33° N and 36° N extending down to between 2.0 and 2.5 km—approximately 1 km deeper than today. Our findings arise from benthic foraminiferal δ18O profiles from sediment cores in two depth transects at Cape Hatteras (36–39° N) and Blake Outer Ridge (29–34° N) in the Northwest Atlantic. This result suggests that the STG, including the Gulf Stream, was deeper and stronger during the LGM than at present, which we attribute to increased glacial wind stress curl, as supported by climate model simulations, as well as greater glacial production of denser subtropical mode waters (STMWs). Our data suggest (1) that subtropical waters probably contributed to the geochemical signature of what is conventionally identified as Glacial North Atlantic Intermediate Water (GNAIW)5–7 and (2) the STG helped sustain continued buoyancy loss, water mass conversion and northwards meridional heat transport (MHT) in the glacial North Atlantic.

The first appearance of Glacial North Atlantic Intermediate Water (GNAIW) during the Mid-Pleistocene transition

The Mid-Pleistocene Transition (MPT) is arguably the most prominent shift in the Earth's climate system during the entire Pleistocene. It is characterized by a shift from a ∼ 40 kyr to a ∼ 100 kyr glacial cyclicity, going in hand with considerable growth of glacial continental ice-sheets. In search for potential drivers of these changes in the cryosphere, a slow-down of deep ocean circulation has been proposed as an important factor that might have reduced the atmospheric pCO2 level and thus created boundary conditions favourable for pertaining large ice-sheets. Additionally, as argued for the Last Glacial Maximum, enhanced production of intermediate waters might enhance intermediate to deep oceanic stratification and thus suppress CO2 release from the deep ocean and thereby reduce atmospheric greenhouse gas concentrations. To investigate potential changes in the North Atlantic intermediate water-mass configuration across the MPT we utilize δ18O and δ13C records obtained on the deep thermocline-dwelling foraminifera Globorotalia crassaformis from mid-latitude International Ocean Discovery Program (IODP) Site U1313. In conjunction with benthic and planktic δ18O and δ13C data from reference sites for Labrador Sea Water (IODP Site U1305) and intermediate water formation in the north-eastern North Atlantic (Ocean Drilling Program Site 982) our data show that major changes in the production and dispersal of intermediate water masses took place across the MPT. In particular, we note a shoaling of glacial intermediate waters commencing with Marine Isotope Stage (MIS) 22, which we interpret as the first occurrence of Glacial North Atlantic Intermediate Water, a prominent constituent of Late Pleistocene glacial water masses in the Atlantic Ocean. We further infer that the amount and vertical position of the warm and saline Mediterranean Outflow Water (MOW) had a major impact on the spatial configuration of North Atlantic intermediate water masses during the MPT, due to its capacity to modulate subsurface heat transport and isopycnal configuration. Our new findings thus provide evidence for a so far underestimated role of intermediate water circulation in shaping oceanic and potentially atmospheric changes during the MPT.

Changing sea-surface and deep-water conditions in the southern Cape Verde Basin during the mid-Pleistocene to Holocene

Reconstruction of the Atlantic Meridional Overturning Circulation (AMOC) based on long-term records spanning multiple glacial-interglacial transitions is essential for understanding the interplay between ocean dynamics and global climate change. However, despite valuable previous studies, many uncertainties remain, especially regarding the distribution of glacial water masses in the Atlantic and the AMOC intensity. Such long-term reconstructions are highly constrained by sedimentary geochemical records. Here, we present isotope, geochemical, and micropaleontological records from the core ANS-33047 retrieved from the southern part of the Cape Verde Basin at a water depth of 4027 m. The data reflect the glacial–interglacial changes in surface and near-bottom conditions in the area over the last 513 kyr. Within the upper ocean layer, enhanced bioproductivity associated with northward ITCZ migration has been determined during MIS 13/12, early MIS 11, 9, 7, and MIS 1. A gradual deepening of the mixed layer is inferred from MIS 5 to the present. Stronger deep-water upwelling and shoaling of the mixed layer over the study site preceded these intervals. In the deep-water environment, we propose three different regimes within the interglacials: a “classical model” and “local productivity effect” associated with a stronger advection of paleo-NADW into the study area (MIS 11, 5, 1, warm early phases of MIS 9 and 7, MIS 13/12 boundary), as well as “amplified paleo-AABW influence” (late MIS 13 and 7). The bottom-water chemistry was affected by organic matter degradation coinciding with intervals of northward ITCZ migrations and enhanced surface bioproductivity. The most prominent signal of southern-sourced deep water was reconstructed during the late MIS 12, MIS 8, and at the MIS 10/9 boundary. During MIS 3 and sub-stage 12b, the study area was bathed by a mixture of paleo-NADW and paleo-AABW. A comparison of the carbon isotope records from the northeastern and southeastern equatorial Atlantic demonstrates a larger difference in the deep-water properties within glacials in contrast to interglacials. • ITCZ shifts northwards and bioproductivity is enhanced during interglacial intervals. • Increased surface bioproductivity affects δ 13 C composition of paleo-NADW. • Three deep-water regimes occurrs within interglacials. • Stronger meridional variability of deep-water circulation in cold climatic phases.

Assessing the potential capability of reconstructing glacial Atlantic water masses and AMOC using multiple proxies in CESM

Reconstructing the Atlantic Meridional Overturning Circulation (AMOC) during the Last Glacial Maximum (LGM) is essential for understanding glacial-interglacial climate change and the carbon cycle. However, despite many previous studies, uncertainties remain regarding the glacial water mass distributions in the Atlantic and the AMOC intensity. Here we use an isotope enabled ocean model with multiple geotracers (δ13C, εNd, 231Pa/230Th, δ18O and Δ14C) and idealized water tracers to study the potential constraints on LGM ocean circulation from multiple proxies. Our model suggests that the glacial Atlantic water mass distribution can be accurately constrained by the air-sea gas exchange signature of water masses (δ13CAS), but εNd might overestimate the North Atlantic Deep Water (NADW) percentage in the deep Atlantic probably because of the boundary source of Nd. A sensitivity experiment with an AMOC of similar geometry but much weaker strength suggests that the correct AMOC geometry is more important than the AMOC strength for simulating the observed glacial δ13C and εNd distributions. The kinematic tracer 231Pa/230Th is sensitive to AMOC intensity, but the interpretation might be complicated by the AMOC geometry and AABW transport changes during the LGM. δ18O in the benthic foraminifera (δ18Oc) from the Florida Straits provides a consistent measure of the upper ocean boundary current in the model, which potentially provides an unambiguous method to reconstruct glacial AMOC intensity. Finally, we propose that the moderate difference between AMOC intensity at LGM and PD, if any, is caused by the competition of the responses to CO2 forcing and continental ice sheet forcing.

Advected glacial seawater preserved in the subsurface of the Maldives carbonate edifice

Abstract Analysis of both conservative and reactive geochemical tracers in pore fluids extracted from sediment cores from the Maldives Inner Sea reveal extensive interstitial water masses whose origins lie in glacial intermediate water advected laterally through the Maldives carbonate edifice. These drillcores, recovered during Expedition 359 of the International Ocean Discovery Program, penetrate late Oligocene to Recent sediments including carbonate platform, slope, pelagic, and drift facies. Combined shipboard and shore-based analyses of the pore fluids show linearly co-varying changes in the concentration of chloride and δ 18 O and δ D values of 25 mM, 1.2‰, and 9‰, respectively. Additionally, pore fluid strontium concentrations and calcium isotope ratios are strongly anti-correlated with each other. Multiple reversals in these geochemical properties with depth indicate the existence of several distinct interstitial water masses. These variations are interpreted as representing the different ages and reactive histories of young (Pleistocene and Holocene) waters advecting through the much older sedimentary formations. In particular, the elevated chloride concentrations and water isotope ratios suggest that seawater from the Last Glacial Maximum is preserved in the subsurface, where it occupies over 400 m of the sediment column within Mid- to Late Miocene sediments. These pore fluids constrain the properties of an intermediate depth glacial water mass that had higher salinity and δ 18 O and δ D values than the average glacial ocean. This system represents a unique archive yielding the most direct observations of glacial seawater to date, as well as a demonstration of the long-lasting potential for water-rock interaction in carbonate platform systems.

Glacial water mass structure and rapid δ18O and δ13C changes during the last glacial termination in the Southwest Pacific

Changes in ocean circulation are thought to have contributed to lowering glacial atmospheric CO2 levels by enhancing deep ocean sequestration of carbon that was returned to the atmosphere during glacial terminations. High-resolution benthic foraminiferal δ13C and δ18O records from a depth transect of cores in the Southwest Pacific Ocean presented here provide evidence that both wind- and thermohaline-driven circulation drove CO2 from the ocean during the last deglaciation. Shallow geochemical stratification in the glacial Southern Ocean was followed by a short pulse of rapid δ13C enrichment to intermediate water depths during Heinrich Stadial 1, indicative of better-ventilated intermediate waters co-occurring with documented wind-driven upwelling in the Southern Ocean. Intermediate depth δ13C enrichment paused at the start of the Antarctic Cold Reversal (∼14.7 ka), implying a brief shallow restratification, while deeper layers were progressively flushed of δ13C-depleted and δ18O-enriched waters, likely caused by the increasing influence of deep waters sourced from the North Atlantic. The coincidence of atmospheric CO2 increases with these geochemical shifts in both shallow and deep cores suggests that shifts in both atmospheric and oceanic circulation contributed to the deglacial rise of CO2.

How much did Glacial North Atlantic Water shoal?

Observations of δ13C and Cd/Ca from benthic foraminifera have been interpreted to reflect a shoaling of northern source waters by about 1000 m during the Last Glacial Maximum, with the degree of shoaling being significant enough for the water mass to be renamed Glacial North Atlantic Intermediate Water. These nutrient tracers, however, may not solely reflect changes in water mass distributions. To quantify the distribution of Glacial North Atlantic Water, we perform a glacial water mass decomposition where the sparsity of data, geometrical constraints, and nonconservative tracer effects are taken into account, and the extrapolation for the unknown water mass end-members is guided by the modern-day circulation. Under the assumption that the glacial sources of remineralized material are similar to that of the modern day, we find a steady solution consistent with 241 δ13C, 87 Cd/Ca, and 174 δ18O observations and their respective uncertainties. The water mass decomposition indicates that the core of Glacial North Atlantic Water shoals and southern source water extends in greater quantities into the abyssal North Atlantic, as previously inferred. The depth of the deep northern-southern water mass interface and the volume of North Atlantic Water, however, are not grossly different from that of the modern day. Under this scenario, the vertical structure of glacial δ13C and Cd/Ca is primarily due to the greater accumulation of nutrients in lower North Atlantic Water, which may be a signal of the hoarding of excess carbon from the atmosphere by the glacial Atlantic.

Benthic foraminiferal B/Ca ratios reflect deep water carbonate saturation state

Boron/calcium ratios were measured in four benthic foraminiferal species (three calcitic: Cibicidoides wuellerstorfi, Cibicidoides mundulus, and Uvigerina spp., and one aragonitic: Hoeglundina elegans) from 108 core-top samples located globally. Comparison of coexisting species shows: B/Ca of C. wuellerstorfi > C. mundulus > H. elegans > Uvigerina spp., suggestive of strong “vital effects” on benthic foraminiferal B/Ca. A dissolution effect on benthic B/Ca is not observed. Core-top data show large intra-species variations (50–130 μmol/mol) in B/Ca. Within a single species, benthic foraminiferal B/Ca show a simple linear correlation with deep water Δ[CO 3 2−], providing a proxy for past deep water [CO 3 2−] reconstructions. Empirical sensitivities of Δ[CO 3 2−] on B/Ca have been established to be 1.14 ± 0.048 and 0.69 ± 0.072 μmol/mol per μmol/kg for C. wuellerstorfi and C. mundulus, respectively. The uncertainties associated with reconstructing bottom water Δ[CO 3 2−] using B/Ca in C. wuellerstorfi and C. mundulus are about ± 10 μmol/kg. A preliminary application shows that the Last Glacial Maximum (LGM) B/Ca ratios were increased by 12% at 1–2 km and decreased by 12% at 3.5–4.0 km relative to Holocene values in the North Atlantic Ocean. This implies that the LGM [CO 3 2−] was higher by ∼ 25–30 μmol/kg at intermediate depths and lower by ∼ 20 μmol/kg in deeper waters, consistent with glacial water mass reorganization in the North Atlantic Ocean inferred from other paleochemical proxies.

Glacial water mass geometry and the distribution of δ13C of ΣCO2 in the western Atlantic Ocean

Oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa Cibicidoides and Planulina from 25 new piston cores, gravity cores, and multicores from the Brazil margin. The cores span water depths from about 400 to 3000 m and intersect the major water masses in this region. These new data fill a critical gap in the South Atlantic Ocean and provide the motivation for updating the classic glacial western Atlantic δ13C transect of Duplessy et al. (1988). The distribution of δ13C of ΣCO2 requires the presence of three distinct water masses in the glacial Atlantic Ocean: a shallow (∼1000 m), southern source water mass with an end‐member δ13C value of about 0.3–0.5‰ VPDB, a middepth (∼1500 m), northern source water mass with an end‐member value of about 1.5‰, and a deep (>2000 m), southern source water with an end‐member value of less than −0.2‰, and perhaps as low as the −0.9‰ values observed in the South Atlantic sector of the Southern Ocean (Ninnemann and Charles, 2002). The origins of the water masses are supported by the meridional gradients in benthic foraminiferal δ18O. A revised glacial section of deep water δ13C documents the positions and gradients among these end‐member intermediate and deep water masses. The large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation.

Barium content of benthic foraminifera controlled by bottom-water composition

THE carbon isotope ratio (δ13;C) and cadmium content (Cd/Ca) of benthic foraminifera shells have been used to reconstruct deep-water circulation patterns of the glacial oceans1–7. These tracers co-vary with phosphorus in the modern ocean because they are nearly quantitatively regenerated from sinking biological debris in the upper water column. Hence they can be used to reconstruct the distribution of labile nutrients in glacial water masses. Independent constraints on glacial deep-ocean circulation patterns could be provided by a tracer of the distribution of silica and alkalinity, the deeply regenerated constituents of planktonic hard parts. Barium shares key aspects of its behaviour with these refractory nutrients because it is removed from solution in surface waters and incorporated into sinking particles which slowly dissolve deep in the water column and in the sediments8. The fractionation of Ba between deep-water masses of the major ocean basins is largely controlled by thermohaline circulation patterns, so Ba conforms to different boundary conditions from Cd andδ 13C. As Ba substitutes into trigonal carbonates9, it is a potential palaeoceano-graphic tracer if the Ba content of foraminifera shells reflects ambient dissolved Ba concentrations. Here we present data from Recent core-top benthic foraminifera which indicate that the Ba content of some recent calcitic benthic foraminifera does co-vary with bottom-water Ba.