Long-term, sustained reduction in ocean productivity initiated at 4.6-4.4 Ma

<p>The late Miocene to early Pliocene was a time of global cooling, albeit in a warmer-than present climate state. Increased marine primary productivity characterizes this interval, often referred to as the late Miocene-early Pliocene biogenic bloom (~9-3.5 Ma). To explain its manifestation, paleoceanographers often involve ocean gateway or monsoon-related mechanisms, formulating hypotheses of increased or redistributed nutrients in the ocean. However, the exact cause-and-effect chains remain obscure, since important diachronicity is observed across ocean basins for the main phase and the termination of this event. Here, we compile proxy data for late Miocene to Pliocene paleoproductivity from all major ocean basins, including calcareous and siliceous plankton groups. By systematically evaluating the age-depth model accuracies of previously published records we demonstrate that a globally synchronous and long-sustained reduction in primary productivity was initiated with a sharp decline between 4.6 and 4.4 Ma. Our compilation supports that relatively rapid processes (~200 kyr) influenced nutrient availability towards the end of the biogenic bloom. By evaluating different mechanisms influencing the ocean nutrient budget on such time scales, we propose orbital forcing as an important candidate to have tipped the balance towards a less productive ocean. We show that this decline in productivity coincided with a prolonged period of low orbital eccentricity and a shift towards lower-amplitude obliquity. This specific astronomical configuration prevents the development of extreme seasonal contrasts which could lead to reduced nutrient supply to the ocean due to decreased riverine influx.</p>

Atmospheric iron supply and marine productivity in the glacial North Pacific Ocean

Iron (Fe) is a key element in the Earth climate system, as it can enhance marine primary productivity in the high-nutrient low-chlorophyll (HNLC) regions where, despite a high concentration of major nutrients, chlorophyll production is low due to iron limitation. Eolian mineral dust represents one of the main Fe sources to the oceans; thus, quantifying its variability over the last glacial cycle is crucial to evaluate its role in strengthening the biological carbon pump. Polar ice cores, which preserve detailed climate records in their stratigraphy, provide a sensitive and continuous archive for reconstructing past eolian Fe fluxes. Here, we show the Northern Hemisphere Fe record retrieved from the NEEM ice core (Greenland), which offers a unique opportunity to reconstruct the past Fe fluxes in a portion of the Arctic over the last 108 kyr. Holocene Fe fluxes (0.042–11.7 ka, 0.5 mg m−2 yr−1) at the NEEM site were 4 times lower than the average recorded over the last glacial period (11.7–108 ka, 2.0 mg m−2 yr−1), whereas they were greater during the Last Glacial Maximum (LGM; 14.5–26.5 ka, 3.6 mg m−2 yr−1) and Marine Isotope Stage 4 (MIS 4; 60–71 ka, 5.8 mg m−2 yr−1). Comparing the NEEM Fe record with paleoceanographic records retrieved from the HNLC North Pacific, we found that the coldest periods, characterized by the highest Fe fluxes, were distinguished by low marine primary productivity in the subarctic Pacific Ocean, likely due to the greater sea ice extent and the absence of major nutrients upwelling. This supports the hypothesis that Fe fertilization during colder and dustier periods (i.e., LGM and MIS 4) was more effective in other regions, such as the midlatitude North Pacific, where a closer relationship between marine productivity and the NEEM Fe fluxes was observed.

Active methanogenesis during the melting of Marinoan snowball Earth

Geological evidence indicates that the deglaciation of Marinoan snowball Earth ice age (~635 Myr ago) was associated with intense continental weathering, recovery of primary productivity, transient marine euxinia, and potentially extensive CH4 emission. It is proposed that the deglacial CH4 emissions may have provided positive feedbacks for ice melting and global warming. However, the origin of CH4 remains unclear. Here we report Ni isotopes (δ60Ni) and Yttrium-rare earth element (YREE) compositions of syndepositional pyrites from the upper most Nantuo Formation (equivalent deposits of the Marinoan glaciation), South China. The Nantuo pyrite displays anti-correlations between Ni concentration and δ60Ni, and between Ni concentration and Sm/Yb ratio, suggesting mixing between Ni in seawater and Ni from methanogens. Our study indicates active methanogenesis during the termination of Marinoan snowball Earth. This suggests that methanogenesis was fueled by methyl sulfides produced in sulfidic seawater during the deglacial recovery of marine primary productivity.

Appraisal of sedimentary alkenones for the quantitative reconstruction of phytoplankton biomass

Marine primary productivity (PP) is the driving factor in the global marine carbon cycle. Its reconstruction in past climates relies on biogeochemical proxies that are not considered to provide an unequivocal signal. These are often based on the water column flux of biogenic components to sediments (organic carbon, biogenic opal, biomarkers), although other factors than productivity are posited to control the sedimentary contents of the components, and their flux is related to the fraction of export production buried in sediments. Moreover, most flux proxies have not been globally appraised. Here, we assess a proxy to quantify past phytoplankton biomass by correlating the concentration of C37 alkenones in a global suite of core-top sediments with sea surface chlorophyll-a (SSchla) estimates over the last 20 y. SSchla is the central metric to calculate phytoplankton biomass and is directly related to PP. We show that the global spatial distribution of sedimentary alkenones is primarily correlated to SSchla rather than diagenetic factors such as the oxygen concentration in bottom waters, which challenges previous assumptions on the role of preservation on driving concentrations of sedimentary organic compounds. Moreover, our results suggest that the rate of global carbon export to sediments is not regionally constrained, and that alkenones producers play a dominant role in the global export of carbon buried in the seafloor. This study shows the potential of using sedimentary alkenones to estimate past phytoplankton biomass, which in turn can be used to infer past PP in the global ocean.

Coccolithophore productivity at the western Iberian Margin during the Middle Pleistocene (310–455 ka) – evidence from coccolith Sr∕Ca data

Coccolithophores contribute significantly to marine primary productivity and play a unique role in ocean biogeochemistry by using carbon for photosynthesis (soft-tissue pump) and for calcification (carbonate counter pump). Despite the importance of including coccolithophores in Earth system models to allow better predictions of the climate system's responses to planetary change, the reconstruction of coccolithophore productivity mostly relied on proxies dependent on accumulation and sedimentation rates and preservation conditions. In this study we used an independent proxy, based on the coccolith fraction (CF) Sr∕Ca ratio, to reconstruct coccolithophore productivity. We studied the marine sediment core MD03-2699 from the western Iberian margin (IbM), concentrating on glacial–interglacial cycles of Marine Isotopic Stage (MIS) 12 to MIS 9. We found that IbM coccolithophore productivity was controlled by changes in the oceanographic conditions, such as in sea surface temperature (SST) and nutrient availability, and by competition with other phytoplankton groups. Long-term coccolithophore productivity was primarily affected by variations in the dominant surface water mass. Polar and subpolar surface waters during glacial substages were associated with decreased coccolithophore productivity, with the strongest productivity minima concomitant with Heinrich-type events (HtEs). Subtropical, nutrient-poorer waters, increased terrigenous input, and moderate to strong upwelling during the deglaciation and early MIS11 are hypothesized to have attributed a competitive advantage to diatoms to the detriment of coccolithophores, resulting in intermediate coccolithophore productivity levels. During the progression towards full glacial conditions an increasing presence of nutrient-richer waters, related to the growing influence of transitional surface waters and/or intensified upwelling, probably stimulated coccolithophore productivity to maxima following the rapid depletion of silica by diatoms. We present conceptual models of the carbon and carbonate cycle components for the IbM in different time slices that might serve as a basis for further investigation and modelling experiments.

2000 years of marine primary productivity in the Eastern Tropical North Pacific

<p>Changes in marine primary productivity (MPP) over the 21st century are expected to occur under the prevailing climate change scenario. For better understanding of past climate variability, we reconstructed MPP at high resolution (~1-2 years) for the past 2000 years analyzing biogenic silica and total organic carbon (TOC %) on a sediment core collected from Soledad Basin (25°N, 112°W), Baja  California, Mexico. Located in the Eastern Tropical North Pacific, this suboxic basin is ideal for palaeoceanographic reconstructions due to its high sedimentation rate (2 mm/year), which allow us to reconstruct past changes in the ocean and climate at high resolution. Our results show an increasing trend in the variability of MPP for the past 2000 years: biogenic silica content does not show a well-defined trend, but rather it is dominated by strong multidecadal and prominent centennial-scale cycles while TOC (%) shows a slight increasing trend towards the present, starting at least 2000 years ago. Spectral analysis confirms the presence of multidecadal to centennial cycles. These results will be discussed in the context of the Anthropocene and natural climate variability.</p>

Year-to-year meridional shifts of the Great Whirl driven by oceanic internal instabilities

<p>The Great Whirl (GW) is a quasi-permanent anticyclonic eddy that forms off the horn of Africa in the western Arabian Sea. It generally appears in June, peaks in July-August, and dissipates in September. While the annual cycle of the GW has been described by past literature, its year-to-year variability has not yet been thoroughly explored. Satellite sea-level observations reveal that the leading mode of interannual variability (half of the interannual summer variance in the GW region) is associated with a typically ~100-km GW northward or southward shift. This meridional shift is associated with coherent sea surface temperature (SST) and surface chlorophyll signals, with warmer SST and reduced marine primary productivity in regions with positive sea level anomalies (and vice versa). Eddy-resolving (~10-km resolution) simulations with an ocean general circulation model capture those observed patterns reasonably well, even in the absence of interannual variations in the surface forcing. Interannual surface forcing variations enhance the GW interannual variability, but do not constrain its phase. Our results hence indicate that year-to-year variations in the Somalia upwelling SST and productivity associated with the GW are thus not a deterministic response to surface forcing, but largely arise from oceanic internal instabilities.</p>

Atmospheric dust stimulated marine primary productivity during Earth’s penultimate icehouse

The importance of dust as a source of iron (Fe) for primary production in modern oceans is well studied but remains poorly explored for deep time. Vast dust deposits are well recognized from the late Paleozoic and provisionally implicated in primary production through Fe fertilization. Here, we document dust impacts on marine primary productivity in Moscovian (Pennsylvanian, ca. 307 Ma) and Asselian (Permian, ca. 295 Ma) carbonate strata from peri-Gondwanan terranes of Iran. Autotrophic contents of samples, detected by both point-count and lipid-biomarker analyses, track concentrations of highly reactive Fe, consistent with the hypothesis that dust stimulated primary productivity, also promoting carbonate precipitation. Additionally, highly reactive Fe tracks the fine-dust fraction. Dust-borne Fe fertilization increased organic and inorganic carbon cycling in low- and mid-latitude regions of Pangaea, maintaining low pCO2.