Increased dissolved terrestrial input to the coastal ocean during the last deglaciation

Here we present the first downcore results for a new paleoproxy, the Mn/Ca ratio of foraminiferal calcite, applied to sediment accumulated in the extreme Eastern Tropical North Pacific (ETNP) over the last 30,000 years. The Mn/Ca results are compared to oxygen isotopes and sea surface temperature calculated from Mg/Ca. We determined metal ratios using flow‐through time‐resolved analysis to minimize the effects of secondary mineralization. The foraminiferal species used for this study calcify at different depths. Core top ratios of these variant species change in proportion to the concentration of dissolved manganese in the water column at the depth of calcification. Since terrestrial input and oxidation reduction reactions control the levels of dissolved Mn in the oceans today, it therefore should be possible to use the Mn/Ca ratios of foraminifera as a proxy for these processes in the past. Mn/Ca of a mixed‐layer species (G. ruber) suggest that dissolved terrestrial input to the surface waters of the ETNP during the last glacial maximum was lower than today but began to increase with initial sea level rise and reached a maximum at 15 ka B.P. before coming down to present‐day levels at the end of sea level rise in the mid‐Holocene (7–5 ka). Ratios of a deeper calcifying species (N. dutertrei) mimic those of G. ruber over this same time period, consistent with shoaling of the 18°C thermocline. Mn/Ca of a benthic species (U. peregrina) does not show a maximum at 15 ka, suggesting that Mn was efficiently remineralized in the water column during deglaciation. Assuming that the period from the last glacial until the mid‐Holocene was a time of increased productivity, as elevated Mn might imply, the oxygen minimum zone (OMZ) was at least as well developed during deglaciation as it is today. Expansion of the OMZ may have contributed to the Mn/Ca trends we observe through time.Keywords: manganese, paleoprox

Linking the 8.2 ka Event and its Freshwater Forcing in the Labrador Sea

The 8.2 ka event was the last deglacial abrupt climate event. A reduction in the Atlantic meridional overturning circulation (AMOC) attributed to the drainage of glacial Lake Agassiz may have caused the event, but the freshwater signature of Lake Agassiz discharge has yet to be identified in (delta)18O of foraminiferal calcite records from the Labrador Sea, calling into question the connection between freshwater discharge to the North Atlantic and AMOC strength. Using Mg/Ca-paleothermometry, we demonstrate that approx. 3 C of near-surface ocean cooling masked an 1.0 % decrease in western Labrador Sea (delta)18O of seawater concurrent with Lake Agassiz drainage. Comparison with North Atlantic (delta)18O of seawater records shows that the freshwater discharge was transported to regions of deep-water formation where it could perturb AMOC and force the 8.2 ka event

Barium cycling in the North Pacific: Implications for the utility of Ba as a paleoproductivity and paleoalkalinity proxy

Benthic incubation chambers have been deployed in a variety of geochemical environments that provide a comprehensive geochemical framework from which to address issues related to Ba geochemistry and the use of Ba as a paleoproxy. First order budgets for barium show that in the equatorial Pacific, present rates of Ba rain and benthic remobilization are nearly in balance, indicating that the rate of net accumulation is negligible and is clearly much less than the average for the Holocene; thus any paleoproxy algorithms built on the assumption of steady state are questionable. In contrast, budgets for sediments in the southern California Borderland indicate much higher burial efficiencies, in the range of 50–80%. The Ba:alkalinity (Alk) flux ratio is found to be remarkably constant throughout the environments studied and is indistinguishable from the deep water ratio used for paleoceanographic reconstructions. However, the Ba:organic carbon remobilization ratio is not constant. Combined, these results do not indicate a simple, first‐order direct link between Ba and alkalinity remobilization via organic carbon oxidation; however, the similarities in the Ba and alkalinity source functions conspire to maintain the Ba:Alk ratio near the global water column average. This latter observation provides promise for the use of the Ba:Ca ratio in benthic foraminifera as a paleocirculation tracer

Evidence of a dissolution effect on benthic foraminiferal shell chemistry: δ¹³C, Cd/Ca, Ba/Ca, and Sr/Ca results from the Ontong Java Plateau

Core-top benthic foraminifera (Cibicidoides wuellerstorfi) from a depth transect of Soutar box cores from the Ontong-Java Plateau (1.6 – 4.4 km) were analyzed for cadmium, barium, and strontium (Cd/Ca, Ba/Ca, and Sr/Ca) and for their stable isotopic composition (δ¹³C and δ¹⁸O). We also measure bottom water δ¹³C, Cd, and Ba at these sites. Foraminiferal δ¹³C values remain roughly constant over the entire depth range while bottom water δ¹³C values increase slightly, such that the δ¹³C difference between C. wuellerstorfi and bottom water ranges from about +0.2 ‰ in cores above 2.5 km to about -0.2 ‰ in cores below 4 km. This apparent depth dependence has not been previously reported, but this range in Δδ¹³C values is comparable to the uncertainty in published δ¹³C calibration studies We observe strong decreases in foraminiferal Cd/Ca, Ba/Ca, and Sr/Ca ratios (50, 25, and 15 percent, respectively) at water depths greater than about 2.5 km. These decreases are substantially larger than the corresponding changes in bottom water trace element concentrations, and they are not correlated with variations in pore water Cd and Ba concentrations at these sites. Together, the foraminiferal and bottom water Cd/Ca, Ba/Ca, and Sr/Ca data yield decreases in the apparent distribution coefficients for these metals into calcite with increasing water depth, again a pattern which has not been previously reported. These results when combined with the data from published core-top calibration studies suggest that a preferential loss of Cd, Ba, and Sr occurs during the dissolution of benthic foraminiferal calcite on the sea floor and raise the possibility of a dissolution-driven decrease in benthic foraminiferal δ¹³C values

Seawater intrusion through the oceanic crust and carbonate sediment in the Equatorial Pacific: Lithium abundance and isotopic evidence

Large-scale lateral advection of seawater within the oceanic crust is thought to be the cause of low heat flow in the Equatorial Pacific. Until now supportive evidence is limited to reversals of B and δ¹¹B, Ca, Mg, SO₄⁻², Sr and ⁸⁷Sr/⁸⁶Sr in sediment pore waters. Here we report new evidence from a detailed study of Li and its isotopes in sediment cores from ODP Sites 844 and 851. Carbonates at these sites were significantly recrystallized leading to large variations in Li and δ⁷Li in the sediments and associated pore waters. In addition to diagenetic effects, distinct reversals in lithium concentration and isotopic ratio toward modern seawater composition are observed in waters near the basaltic basement, lending further support to the seawater intrusion hypothesis. Three endmembers are identified in the pore waters: seawater, a diagenetically altered component, and evolved crustal fluid. The new lithium isotopic results underscore the importance of diagenetic artifacts in carbonate sediments

The geochemistry of Atlantic hydrothermal particles

Particles were collected from the dilute portion of neutrally buoyant hydrothermal plumes from four Mid-Atlantic Ridge sites (MARK, 23°N; TAG, 26°N; Broken Spur, 29°N; Lucky Strike, 37°N). Comparison of data from proximal portions of the TAG (Atlantic) [German et al., 1991; this study] and North Cleft (Pacific) [Freely et al., 1994] plumes show that oxyanion (e.g., V) is more efficient at TAG, possibly due to a higher proportion of Fe removed as sulfides at North Cleft and/or the more vigorous mixing in the high energy TAG buoyant plume. Chalcophile elements (e.g., Cu) show two stage removal. They are precipitated as sulfides during initial mixing of vent fluids with seawater and are sedimented from the buoyant plume. In the dilute plume they are scavenged from seawater by Fe oxyhydroxides. The REE show continued scavenging in the neutrally buoyant plume and lower levels in 1993 samples compared to 1988 samples [German et al., 1990] suggesting that the amount of scavenging is related to particle recycling.Copyrighted by American Geophysical Union

Routing of western Canadian Plains runoff during the 8.2 ka cold event

The collapse of the Laurentide Ice Sheet over Hudson Bay ∼8.47 ka allowed the rapid drainage of glacial Lake Agassiz into the Labrador Sea, an event identified as causing a reduction in Atlantic meridional overturning circulation (AMOC) and the 8.2 ka cold event. Atmosphere‐ocean models simulations based on this forcing, however, fail to reproduce several characteristics of this event, particularly its duration. Here we use planktonic foraminifera U/Ca records to document the routing of western Canadian Plains runoff that accompanied ice‐sheet collapse. Geochemical modeling of the ∼7 nmol/mol increase in U/Ca at the opening of Hudson Bay indicates an increase in freshwater discharge of 0.13 ± 0.03 Sverdrups (106 m3 s−1) from routing, a sufficient magnitude to cause an AMOC reduction. We suggest that this routing event suppressed AMOC strength for several centuries after the drainage of Lake Agassiz, explaining multi‐centennial climate anomalies associated with the 8.2 ka cold event

Is methane venting at the seafloor recorded by δ13C of benthic foraminifera shells?

The isotopic composition of the dissolved inorganic carbon (DIC) collected at sites of active methane discharge on Hydrate Ridge, Oregon, reveals anaerobic methane oxidation mediated by bacteria, with δ13CDIC reaching values as low as –48‰ in the upper 4 cm of the sediment. In spite of the high sulfide levels in the discharging fluids, living specimens of the benthic foraminifera Uvigerina peregrina are abundant in the vents, probably owing to the rich bacterial food source. Although pore water δ13CDIC is extremely low (–6 to –48%), the δ13C values of living (Rose Bengal stained) foraminifera shells collected from active methane seeps are not significantly lower than those observed in nonventing pelagic sediments, and are within the range expected from local organic matter decomposition (0 to –4%). The apparent δ13C disequilibrium between biogenic calcite and DIC suggests that at seep localities, foraminifera calcify mostly during periods when there is little methane discharge or during intermittent episodes of seawater flow into the sediments. The isotopic composition and Mg/Ca ratios of fossil (unstained) foraminifera recovered at carbonate-rich sites on the northern Hydrate Ridge reveals overprinting of the biogenic record by inorganic calcite with high Mg/Ca and anomalously low d13C values. Thus overprinting of the original isotopic composition of foraminifera by overgrowths or recrystallization at or below the sediment surface, rather than primary calcification in contact with 13C depleted DIC, can explain extreme 13C depletion observed in fossil foraminifera recovered from sites of active methane discharge

Geochemical proxies of North American freshwater routing during the Younger Dryas cold event

The Younger Dryas cold interval represents a time when much of the Northern Hemisphere cooled from ≈12.9 to 11.5 kiloyears B.P. The cause of this event, which has long been viewed as the canonical example of abrupt climate change, was initially attributed to the routing of freshwater to the St. Lawrence River with an attendant reduction in Atlantic meridional overturning circulation. However, this mechanism has recently been questioned because current proxies and dating techniques have been unable to confirm that eastward routing with an increase in freshwater flux occurred during the Younger Dryas. Here we use new geochemical proxies (ΔMg/Ca, U/Ca, and ⁸⁷Sr/⁸⁶Sr) measured in planktonic foraminifera at the mouth of the St. Lawrence estuary as tracers of freshwater sources to further evaluate this question. Our proxies, combined with planktonic δ¹⁸Oseawater and δ¹³C, confirm that routing of runoff from western Canada to the St. Lawrence River occurred at the start of the Younger Dryas, with an attendant increase in freshwater flux of 0.06 ± 0.02 Sverdrup (1 Sverdrup = 10⁶ m³·s⁻¹). This base discharge increase is sufficient to have reduced Atlantic meridional overturning circulation and caused the Younger Dryas cold interval. In addition, our data indicate subsequent fluctuations in the freshwater flux to the St. Lawrence River of ≈0.06–0.12 Sverdrup, thus explaining the variability in the overturning circulation and climate during the Younger Dryas.Keywords: abrupt climate change, paleoclimate, Atlantic meridional overturning circulatio