The Medieval Climate Anomaly and Little Ice Age in Chesapeake Bay and the North Atlantic Ocean

This paper is not subject to U.S. copyright. The definitive version was published in Palaeogeography, Palaeoclimatology, Palaeoecology 297 (2010): 299-310, doi:10.1016/j.palaeo.2010.08.009.A new 2400-year paleoclimate reconstruction from Chesapeake Bay (CB) (eastern US) was compared to other paleoclimate records in the North Atlantic region to evaluate climate variability during the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA). Using Mg/Ca ratios from ostracodes and oxygen isotopes from benthic foraminifera as proxies for temperature and precipitation-driven estuarine hydrography, results show that warmest temperatures in CB reached 16–17 °C between 600 and 950 CE (Common Era), centuries before the classic European Medieval Warm Period (950–1100 CE) and peak warming in the Nordic Seas (1000–1400 CE). A series of centennial warm/cool cycles began about 1000 CE with temperature minima of ~ 8 to 9 °C about 1150, 1350, and 1650–1800 CE, and intervening warm periods (14–15 °C) centered at 1200, 1400, 1500 and 1600 CE. Precipitation variability in the eastern US included multiple dry intervals from 600 to 1200 CE, which contrasts with wet medieval conditions in the Caribbean. The eastern US experienced a wet LIA between 1650 and 1800 CE when the Caribbean was relatively dry. Comparison of the CB record with other records shows that the MCA and LIA were characterized by regionally asynchronous warming and complex spatial patterns of precipitation, possibly related to ocean–atmosphere processes

The effects of El Nino Events on Coccolithophore assemblages in the Santa Barbara Basin (California)

El Ni\uf1o (EN) is a disruption of the ocean-atmosphere system in the tropical Pacific that has important consequences for global weather patterns. EN events are marked by years with an increase of sea-surface temperature, sea-level barometric pressure and rain fall along the California Margin. A laminated box core from the Santa Barbara anoxic Basin is used in this work to study the EN effect on primary productivity, in particular on coccolithophores in the California Borderland during the last 60 years. The EN events were defined by two indexes: Multivariate ENSO Index (MEI) from 1996 to 1950 and Southern Oscillation Index (SOI) from 1950 to 1939. In the box-core samples we can observe an increase of coccolithophore carbonate flux during the years following EN events (positive MEI and negative SOI). In particular, Gepyrocapsa oceanica flux abundance shows a strong positive correlation with strong EN events, like in 1965-66, 1982-83 and in 1997-98 (for 1997-98 from sediment trap experiment in the center of SBB). Significant sea-cliff erosion and storm damage occurred along the central coast of California during the 1982-83 and 1997-98 El Nino winters. We will discuss the coccolithophore response to El Nino events, also considering the minor chemical elemental changes in the surface water column. In our study, it appears that the main oceanographic parameter controlling the distribution of coccolithophore assemblages is temperature, since nitrate and phosphate are relatively high throughout the study period

Coccolithophore response to the 1997-1998 El Niño in Santa Barbara Basin (California)

The response of coccolithophore export production to non-El Niño and El Niño conditions was monitored during a two year period (26 March 1996-3 April 1998) in the centre of the Santa Barbara Basin (34°14′ N; 120°02′ W), California borderland. During the 1997-1998 El Niño the seasonal cycle of the surface water conditions was altered by the presence of a relatively warm, low-nutrient and low-salinity water mass. Throughout the studied period, the total mass flux is dominated by lithogenic components with terrigenous input being highest during El Niño period. Although the annual biogenic sediment fluxes were largely dominated by silica, with diatoms as the major contributor to the opal flux, the coccolith flux was high during the entire studied period. During winter 1998 carbonate fluxes were unusually high for this time of the year. Moreover, a strong positive correlation between organic carbon and opal flux indicates that siliceous phytoplankton production is the most important factor controlling organic carbon flux in the Santa Barbara Basin. A positive correlation between carbonate and organic carbon fluxes existed indeed during the final phase of El Niño, when the correlation between organic carbon and opal fluxes is poor. The sinking coccolithophore assemblage was dominated by Emiliania huxleyi, followed by Florisphaera profunda, Umbilicosphaera sibogae, Gephyrocapsa oceanica, Helicosphaera carteri, Gephyrocapsa muellerae and Calcidiscus leptoporus. The intensification of El Niño 1997-1998 conditions altered the typical hydrographic structure of the Santa Barbara Basin weakening the spring upwelling as reflected by reduced diatom fluxes. In contrast, the coccolith flux was relatively high during El Niño, particularly for the tropical species G. oceanica, that showed a pronounced increase in its flux when the spring upwelling was reduced under El Niño conditions. In addition, we have found that this species has a strong correlation with nitrite, phosphate and iron availability in surface water. © 2005 Elsevier B.V. All rights reserved

Seasonal and interannual export production of coccolithophores in Cariaco Basin (Venezuela): tools for climatic variability

Cariaco Basin shows marked seasonal and interannual variations in hydrographic properties and primary production. Partially isolated from the open Caribbean Sea by a series of shallow sills, Cariaco Basin is anoxic below \u2dc250m water depth because of restricted water exchange and high oxygen demand created by productive conditions associated with seasonal upwelling along the southern Caribbean margin. Moreover, the strength of the Northeast trade winds varies seasonally, because of the annual migration of the Intertropical Convergence Zone (ITCZ), causing a significant seasonal change in the surface waters of the Cariaco Basin. The combination of high biogeochemical input and the lack of bioturbation at the bottom allow the deposition of laminated sediments that exhibit high sedimentation rates. In this study we track changes in coccolith export production and ecology in sediment trap samples (10,30_N; 64,40_W; from November 1996 to October 1999) and laminated core (CAR7-1) samples from Cariaco Basin, in order to monitor and understand climate and ocean variability in the southern Caribbean region during the last 1000 years. During a three-year sediment trapping project in Cariaco Basin significant changes in coccolithophore fluxes occurred in response to changes in hydrography. Seasonal coccolith fluxes are observed with flux maxima in late-fall and winter, when the hydrographic conditions are relatively stable. In particular, coccolithophore patterns suggest that these trends are driven by both annual and interannual changes in the upper water column associated with El Ni\uf1o conditions. The coccolith flux recorded in core CAR7-1 was dominated by E. huxleyi, G. oceanica and F. profunda, the same species that dominated the sediment trap samples.We will show the significant changes occuring down-core, reflecting hydrographic and climatic variation in ITCZ position over the Caribbean region. Moreover, C. leptoporus in this basin may indicate an increase in cool water due to intensification in trade winds and upwelling intensity. Total coccolith fluxes show evidence of centennial scale variability, suggesting solar modulatio

Coccolith carbonate production in the Santa Barbara Basin (California borderland), linked to interannual and interdecadal climate changes

Coccolithophore ecology and the transformation of living biocoenoses into coccolith carbonate at the ocean floor are fundamental for documenting climatic effects and for estimating the global budget of coccolith carbonate. This understanding is also needed to quantitatively assess the information preserved in the sediment record, and to use coccolithophores as biotic proxies of climate change. In the Santa Barbara Basin (SBB), the combination of high biogeochemical input and the lack of bioturbation at the bottom allow the deposition of varved sediments. These laminated sediments are used here to trace the response of primary productivity, in particular on coccolithophore, to El Ni\uf1o (EN) events and their climatic forcing. We use a sediment trap anchored in the center of the basin (34\uf0b014\u2019N, 120\uf0b002\u2019W; ~590m water depth, about 50m off the bottom) to trace the seasonal trend during non-EN conditions. In addition, a laminated box-corer (SABA 9610) recovered from the center of SBB (34\ub0 19.004\u2019 N; 120\ub0 02.003\u2019W) was used to characterized the effect of EN events on coccolithophore during the last 60 years. The EN events were defined by the Southern Oscillation Index (SOI). The fine fraction (<32um) flux varied between 1.453 gr/m2/day and 0.370 gr/m2/day, with a maximum contribution to the total flux in the spring and summer (average 0.996 gr/m2/day) and minor in winter (average 0.624 gr/m2/day). During this year, in the SBB the coccolithophores are a minor contributor to total carbonate flux during the spring when the upwelling is present (being only 2-3% of the total carbonate), while they increment (5-7% of the total carbonate) during the fall when the coccolith bloom was recorded. The sinking coccolithophore assemblage was dominated by Emiliania huxleyi (varied between 142.51 107 m-2 day-1and 1.844 107 m-2 day-1). Follow by Florisphaera profunda, Helicosphaera carteri, Gephyrocapsa muellerae and Calcidiscus leptoporus. In the box-core samples we observe an increment of coccolithophore flux during the year following EN event (negative SOI). In contrast, during the year following La Nina conditions (positive SOI) the flux of coccolithophore was reduced

Comparison of TEX₈₆ and U<super>K</super>₃₇ temperature proxies in sinking particles in the Cariaco Basin

The Cariaco Basin, a silled, permanently anoxic basin on the continental shelf of Venezuela with a dynamic chemocline (-240-350 m), has been subject of > 20 years of oceanographic observation and sediment trap studies. We evaluated U-37(K') and the TEX86 temperature proxies using sinking particles collected in shallow sediment trap samples at 275 m (Trap A) and 455 m (Trap B) (within and below the chemocline). The organic geochemical temperature proxies, U-37(K'). (based on coccolithophorid alkenone lipids) and TEX86 (based on archaeal glycerol dialkyl glycerol tetraether (GDGT) lipids), use observed relationships between the ratio of specific lipids and measured sea surface temperature to hindcast past sea surface temperatures. In this study, both U-37(K') and TEX86 temperature proxies record seasonal temperature variations, including the cooling associated with upwelling events. U-37(K')-based temperatures are colder than measured sea surface temperatures, and better correlated temperature at the chlorophyll maximum. In sediment trap material collected below the chemocline (Trap B),U-37(K') values are higher than those in Trap A. Warmer subchemocline U-37(K') based temperatures may be related to autooxidation of sinking particles, either by small amounts of available oxygen or by alternate electron acceptors concentrated in the biologically dynamic chemocline (e.g. intermediate sulfur compounds). The absolute flux weighted TEX86 temperature values measured in sinking particles from Trap A match the measured SST well. The differences in the TEX86 values between Traps A and B are small and reflect less impact of degradation. Overall, the TEX86 temperatures in sinking particles in the Cariaco Basin reflect annual SST

Silicic acid biogeochemistry in the Gulf of California: Insights from sedimentary Si isotopes

Iron is considered to play a large role in the cycling of Si in Fe limited regions of the ocean but little is known about its role in Si biogeochemistry outside these areas. Here, we present published sediment trap data, new nutrient profiles and high resolution sedimentary records (Si isotopes, Biogenic silica%, N% and C%) from the Gulf of California, a non-Fe limited region, to investigate the history of Si cycling in this highly productive basin. Modern nutrient profiles show that silicic acid in subsurface waters is in excess relative to nitrate and therefore incompletely utilised during moderate winter upwelling events. Modern data however suggest that during intense upwelling episodes, silicic acid is preferentially utilised relative to nitrate by the biota, which we suggest reflects transient iron limitation. Our new δ30Si record from the Guaymas Basin shows dramatic variations at millennial time-scales. Low δ30Si values synchronous with Heinrich events are interpreted as resulting from the decline in Si(OH)4 utilisation at times of decreased upwelling strength, while nearly complete Si(OH)4 utilisation was observed at times of invigorated upwelling and increased opal burial during the Holocene, the Bølling-Allerød and the last glacial period. We attribute the complete utilisation of Si(OH)4 to the occurrence of transient Fe limitation at these times. Our study highlights the importance of Fe limitation on Si and C cycling in coastal upwelling regions and suggests that upwelling dynamics, in combination with Fe availability, have the potential to modulate marine Si distribution and opal burial even at short time-scales