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

Abstract

[1] 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 is therefore incompletely utilized during moderate winter upwelling events. Modern data, however, suggest that during intense upwelling episodes, silicic acid is preferentially utilized relative to nitrate by the biota, which we suggest reflects transient iron limitation. Our new d 30 Si record from the Guaymas Basin shows dramatic variations at millennial timescales. Low d 30 Si values synchronous with Heinrich events are interpreted as resulting from the decline in Si(OH) 4 utilization at times of decreased upwelling strength, while nearly complete Si(OH) 4 utilization 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 utilization 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 timescales.

Local and Remote Forcing of Denitrification in the Northeast Pacific for the Last 2,000 Years

“…Coherent TOC and Si/Ti maxima are observed (e.g., 1130 and 1300 CE), yet negative correlations between Si/Ti and TOC are shown during intervals of low Si/Ti, during which TOC remains relatively stable (Figures b and e). Anticorrelations between TOC and biogenic silica have also been recorded in the Gulf of California, where biogenic silica is the primary biogenic sediment component (Pichevin et al, ; Thunell, ). Previous studies have related this inverse relationship to either Fe limitation (Firme et al, ) and/or enhanced silica preservation during strong coastal upwelling events due to silica supersaturation in porewaters (Pichevin et al, ).…”

Wintertime enrichment of inorganic nutrients in the Ballenas Channel, Gulf of California

“…Estos valores de las razones N:P y Si:N se encuentran en el intervalo de valores documentados previamente para el GC. Por ejemplo, Torres-Valdés (2000) estimó razones N:P que variaron de 12.7 en invierno a 9.3 en verano, mientras que Pichevin et al (2012) registraron una razón Si:N >1.8 y la atribuyeron como característica del GC.…”

“…These values are within the range of values previously reported for the GC. For example, Torres-Valdés (2000) estimated N:P ratios varying between 12.7 in winter and 9.3 in summer, while Pichevin et al (2012) reported a Si:N ratio of >1.8 as characteristic for the GC. zona continental.…”

Persistent millennial-scale link between Greenland climate and northern Pacific Oxygen Minimum Zone under interglacial conditions

“…Intense productivity could have led to total Fe utilisation during upwelling events thus to transient Fe limitation at the end of an upwelling event (Hutchins and Bruland, 1998;Firme et al, 2003). Iron limitation can lead to reduced cellular organic matter production and increased silicification (high cellular Si : C and Si / N ratio) of diatoms (Baines et al, 2010Brzezinski et al, 2011), high Si:C ratio in sinking biogenic particles, high opal flux (Hutchins and Bruland, 1998;Firme et al, 2003;Pichevin et al, 2012), and better frustule preservation in sediments. This scenario is consistent with the unvarying TOC contents in MD08 core across stadial/interstadials DO intervals of MIS5 (Fig.…”