The sea-reach of the Thames stretched before us like the beginning of an interminable waterway. In the offing the sea and the sky were welded together without a joint . . . the very mist on the Essex marshes was like a gauzy and radiant fabric, hung from the wooded rises inland, and draping the low shores in diaphanous folds.
This famed description from the beginning of Conrad's Heart of Darkness, of the yawl Nellie waiting to set sail on the Thames, is as close an analogy for Mark Rothko's Brown and Grey paintings (1968-69) as literature provides. Conrad (unknowingly, of course) gives us a literary equivalent, expressing what it feels like to stand in the presence of these paintings even if, for Rothko, the images were remorselessly abstract. The sky welded to the sea and the gauzy mist might describe these sombre late works. Divided into two parts, each work has its upper section painted in a blackish brown acrylic, while the lower half - though the ratios differ from painting to painting - is made up of scrubbed, mudflat greys. What has been removed is the ingredient that made up Rothko's classic works of the 1950s: deep veils of colour. Everything has been reduced to subtle and barely visible variations of tone and brushstroke.
There has been a tendency to see these late paintings as intimations of Rothko's suicide (he slit open his veins at his New York studio in February 1970), but the powerful new retrospective of his work at Tate Modern reveals a more universal concern. As Rothko stated, "The exhilarated tragic experience is for me the only source of art." Elsewhere he wrote: "I'm not interested in the relationship of colour or form or anything else. I'm interested only in expressing basic human emotions: tragedy, ecstasy, doom . . ."
As in Conrad's novel, these late paintings suggest a psychological journey, a voyage into the unknown. Life and art are stripped to the bone as we, the viewers, are left staring into the inky void. Seeing is what all great art demands, but none more so than these late Rothkos - not a cursory "look", but a fully engaged relationship from the viewer. The artist appears to be saying that these dark washes are all there is; God is, indeed, dead.
Yet the paintings also seem to suggest a dualistic relationship between light and dark at the centre of the human psyche. Adopting the strategy of repetition and variation, not unlike that employed in Monet's 1890s haystacks, Rothko illustrates his belief that "if a thing is worth doing once, it is worth doing over and over again - exploring it, probing it, demanding by its repetition that the public look at it". If we stand long enough and strive to look until we see, we might ultimately come to know this place as though for the first time.
Rothko was born Marcus Rothkowitz in 1903 in Dvinsk, an important commercial town in the Russian Empire (now Daugavpils, Latvia) with a large Jewish population. His father, Jacob, was a pharmacist and socialist intellectual who provided his children with a secular upbringing. As a child, Rothko suffered anti-Semitism and witnessed some of the occasional attacks on Jews by Cossacks. In 1913, he emigrated to America, where he won a scholarship to Yale; he then abandoned his general studies and took up fine art. He was taught by the painter Max Weber, who helped introduce cubism to the States, and was a contemporary of Barnett Newman, another exemplar of abstract expressionism. Both Weber and Newman were also eastern European Jewish émigrés. Rothko has been called a spiritual and a religious painter, but he is a religious painter for a secular age, providing what the French philosopher Gaston Bachelard called "a space to daydream". It is a space in which we can contemplate not only the natural grandeur of the world, but the silence and stillness at the core of who we are.
The convoluted history of Rothko's Seagram murals has become one of the abiding myths of 20th-century art. In 1961, New York's Museum of Modern Art honoured the then 57-year-old Rothko with his first major retrospective. At the centre of it were the paintings that form the core of the Tate exhibition: ox-blood, melancholic yet muscular, originally commissioned to decorate the Four Seasons dining room of Mies van der Rohe's Seagram Building in New York. At first, Rothko seized on the project with enthusiasm, renting a former gymnasium that allowed him to simulate the restaurant's proportions, and completing the work by the summer of 1959, when, with his family, he set sail for Europe. Speculation has long been rife as to why, on his return, he withdrew the paintings. Some ascribe it to his liberal background (like his father, he was passionate about workers' rights) and to his apparently vituperative remark that he hoped "to paint something that will ruin the appetite of every son of a bitch who ever eats in that room".
The truth, more likely, is that he felt a mismatch between his client's wishes to decorate an upmarket dining room and his own desire to achieve much more. There is also evidence that he was exasperated with the general misinterpretation of his earlier, more lyrical and colourful works as decorative. Tragic grandeur was what mattered. His paintings had to create a "miraculous" psychological and spiritual empathy between artist and viewer.
In this new exhibition, for the first time, eight of the Tate's nine Seagram murals - Rothko bequeathed them to the gallery in November 1969 after years of negotiation, on condition that the gallery devote a room to them - are shown with a selection of those from the Kawamura Memorial Museum of Art in Sakura, Japan, and from the National Gallery of Art, Washington. Though a final scheme for the Four Seasons was never devised and the building could accommodate only seven paintings, Rothko made 30. The resonant surfaces resulted from his application of overlapping translucent and opaque paints. These were applied without ever losing the distinction between the various layers.
With their floating frames and portals, they have something atavistic about them. Rothko likened the effect they create to the claustrophobic atmosphere of Michelangelo's Laurentian Library in Florence. But, walking around the Tate, I kept thinking of Stonehenge, or the megalithic gateway at Mycenae. Even the colours, the deep maroons and blacks, conjure something ancient: the burnt pigments of cave painting.
Architecture was to play an important role in his next commission. After making the Seagram murals, Rothko was invited by the art patron Dominique de Ménil to create a set of paintings for a non-denominational, purpose-designed, octagonal chapel in Houston. These hard-edged, stripped-down compositions, which are not in the exhibition, share something of the qualities found in Rothko's 1964 series of so-called Black-Form paintings, which, with their lack of hovering fields and feathered edges, mark a complete break with his colour-field works of the 1950s.
At first glance, they seem totally black, and again, it is only through the process of engaged looking that the gradations of tone and texture are slowly revealed. Perceptions are challenged by complex layers that, rather than annihilating light, appear to radiate with an intense luminosity. Like some dark baptism, they surround the viewer, so that the experience becomes a form of sensual immersion. As with the Brown and Grey works on paper, they are a heroic re-evaluation of everything that Rothko had done.
In his final essay, published posthumously in On Late Style (2006), Edward Said considers late work by writers and musicians as examples of not "harmony and resolution but . . . intransigence, difficulty and contradiction", suggesting that they can reopen questions supposed to have been long resolved. At a point when an artist is fully in command of his medium he may choose to abandon communication with his established audience in a form of self-imposed exile. This, I would suggest, is exactly the territory of Rothko's late works.
Rothko formed a bridge between the Old and the New worlds, between the tragedies of war and genocide that had so recently coloured Europe and the optimism of mid-20th-century America. The next generation of American artists would abandon spiritual concerns and deconstruct the uniqueness of the art object: if a work of art could be reproduced endlessly, it no longer had a value as a "sacred" object (think of Andy Warhol's silk screens). Rothko was one of the last, great philosophical painters to put aesthetics before money and to believe in the redemptive power of art.
Sue Hubbard's new collection of short stories, "Rothko's Red", is published this month by Salt (£12.99)
"Rothko" is at Tate Modern, London SE1, until 1 February 2009. Details: www.tate.org.uk